THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, M.A., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., Fellow of Christ's College, Cambridge; University Lecturer on the Morphology of Invertebrates

VOLUME V

PERIPATUS

By Adam Sedgwick, M.A., F.R.S., Fellow and Lecturer of Trinity College, Cambridge

MYRIAPODS

By F. G. Sinclair, M.A., Trinity College, Cambridge

INSECTS

PART I. Introduction, Aptera, Orthoptera, Neuroptera, and a portion of Hymenoptera (Sessiliventres and Parasitica)

By David Sharp, M.A. (Cantab.), M.B. (Edinb.), F.R.S.

London
MACMILLAN AND CO.
AND NEW YORK
1895

All rights reserved

"Creavit in cœlo Angelos, in terra vermiculos: non superior in illis, non inferior in istis. Sicut enim nulla manus Angelum, ita nulla posset creare vermiculum."—Saint Augustine, Liber soliloquiorum animae ad Deum, Caput IX.

CONTENTS

SCHEME OF THE CLASSIFICATION (RECENT FORMS) ADOPTED IN THIS BOOK

MYRIAPODA
Order.		Family.
CHILOGNATHA (= DIPLOPODA)		Polyxenidae (p. [43]). Glomeridae (p. [43]). Sphaerotheriidae (p. [43]). Julidae (p. [43]). Blanjulidae (p. [44]). Chordeumidae (p. [44]). Polydesmidae (p. [44]). Polyzoniidae (p. [44]).
CHILOPODA		Lithobiidae (p. [45]). Scolopendridae (p. [45]). Notophilidae (p. [45]). Geophilidae (p. [46]).
SCHIZOTARSIA		Cermatiidae (= Scutigeridae) (p. [46]).
SYMPHYLA.		Scolopendrellidae (p. [46]).
PAUROPODA		Pauropidae (p. [47]).

INSECTA
Order.		Division, Series, or Sub-Order.		Family.		Tribe or Sub-Family.		Group.
APTERA (p. [180])		Thysanura (p. [182])		Campodeidae (p. [183]). Japygidae (p. [184]). Machilidae (p. [184]). Lepismidae (p. [185]).
		Collembola (p. [189])		Lipuridae (p. [190]). Poduridae (p. [190]). Smynthuridae (p. [191]).
ORTHOPTERA (p. [198])		Orthoptera cursoria		Forficulidae (p. [202]). Hemimeridae (p. [217]).
				Blattidae (p. [220])		Ectobiides. Phyllodromiides. Nyctiborides. Epilamprides. Periplanetides. Panchlorides. Blaberides. Corydiides. Oxyhaloides. Perisphaeriides. Panesthiides. ? Geoscapheusides.
				Mantidae (p. [242])		Amorphoscelides. Orthoderides. Mantides. Harpagides. Vatides. Empusides.
				Phasmidae (p. [260])		Lonchodides. Bacunculides. Bacteriides. Necroscides. Clitumnides. Acrophyllides. Cladomorphides. Anisomorphides. Phasmides. Aschipasmides. Bacillides. Phylliides.
		Orthoptera saltatoria		Acridiidae (p. [279])		Tettigides. Pneumorides. Mastacides. Proscopiides. Tryxalides. Oedipodides. Pyrgomorphides. Pamphagides. Acridiides.
				Locustidae (p. [311])		Phaneropterides. Meconemides. Mecopodides. Prochilides. Pseudophyllides. Conocephalides. Tympanophorides. Sagides. Locustides. Decticides. Callimenides. Ephippigerides. Hetrodides. Gryllacrides. Stenopelmatides.
				Gryllidae (p. [330])		Tridactylides. Gryllotalpides. Myrmecophilides. Gryllides. Oecanthides. Trigonidiides. Eneopterides.
NEUROPTERA (p. [341])		Mallophaga (p. [345])				Leiotheides. Philopterides.
		Pseudoneuroptera		Embiidae (p. [351]). Termitidae (p. [356]). Psocidae (p. [390]).
		Neuroptera Amphibiotica		Perlidae (p. [398]).
				Odonata (p. [409])		Anisopterides		Gomphinae. Cordulegasterinae. Aeschninae. Corduliinae. Libellulinae.
						Zygopterides		Calepteryginae. Agrioninae.
				Ephemeridae (p. [429]).
		Neuroptera planipennia		Sialidae (p. [444])		Sialides. Raphidiides.
				Panorpidae (p. [449]).
				Hemerobiidae (p. [453])		Myrmeleonides (p. [454]).
						Ascalaphides (p. [459])		Holophthalmi. Schizophthalmi.
						Nemopterides (p. [462]). Mantispides (p. [463]).
						Hemerobiides (p. [465])		Dilarina. Nymphidina. Osmylina. Hemerobiina.
						Chrysopides (p. [469]). Coniopterygides (p. [471]).
		Trichoptera		Phryganeidae (p. [473])		Phryganeides (p. [480]). Limnophilides (p. [481]). Sericostomatides (p. [482]). Leptocerides (p. [482]). Hydropsychides (p. [482]). Rhyacophilides (p. [483]). Hydroptilides (p. [484]).
HYMENOPTERA (p. [487])		Hymenoptera Sessiliventres		Cephidae (p. [504]). Oryssidae (p. [506]). Siricidae (p. [507]). Tenthredinidae (p. [510]).
		Hymenoptera Petiolata (part)		Cynipidae (p. [523]). Proctotrypidae (p. [533]). Chalcididae (p. [539]). Ichneumonidae (p. [551]). Braconidae (p. [558]). Stephanidae (p. [561]). Megalyridae (p. [562]). Evaniidae (p. [562]). Pelecinidae (p. [563]). Trigonalidae (p. [564]).
(To be continued in Vol. VI.)

PERIPATUS

BY

ADAM SEDGWICK, M.A., F.R.S.

Fellow of Trinity College, Cambridge.

CHAPTER I

PERIPATUS

INTRODUCTION–EXTERNAL FEATURES–HABITS–BREEDING–ANATOMY–ALIMENTARY CANAL–NERVOUS SYSTEM–THE BODY WALL–THE TRACHEAL SYSTEM–THE MUSCULAR SYSTEM–THE VASCULAR SYSTEM–THE BODY CAVITY–NEPHRIDIA–GENERATIVE ORGANS–DEVELOPMENT–SYNOPSIS OF THE SPECIES–SUMMARY OF DISTRIBUTION.

The genus Peripatus was established in 1826 by Guilding,[^[1]] who first obtained specimens of it from St. Vincent in the Antilles. He regarded it as a Mollusc, being no doubt deceived by the slug-like appearance given by the antennae. Specimens were subsequently obtained from other parts of the Neotropical region and from South Africa and Australia, and the animal was variously assigned by the zoologists of the day to the Annelida and Myriapoda. Its true place in the system, as a primitive member of the group Arthropoda, was first established in 1874 by Moseley,[^[2]] who discovered the tracheae. The genus has been monographed by Sedgwick,[^[3]] who has also written an account of the development of the Cape species.[^[4]] A bibliography will be found in Sedgwick's Monograph.

There can be no doubt that Peripatus is an Arthropod, for it possesses the following features, all characteristic of that group, and all of first-class morphological importance: (1) The presence of appendages modified as jaws; (2) the presence of paired lateral ostia perforating the wall of the heart and putting its cavity in communication with the pericardium; (3) the presence of a vascular body cavity and pericardium (haemocoelic body cavity); (4) absence of a perivisceral section of the coelom. Finally, the tracheae, though not characteristic of all the classes of the Arthropoda, are found nowhere outside that group, and constitute a very important additional reason for uniting Peripatus with it.

Peripatus, though indubitably an Arthropod, differs in such important respects from all the old-established Arthropod classes, that a special class, equivalent in rank to the others, and called Prototracheata, has had to be created for its sole occupancy. This unlikeness to other Arthropoda is mainly due to the Annelidan affinities which it presents, but in part to the presence of the following peculiar features: (1) The number and diffusion of the tracheal apertures; (2) the restriction of the jaws to a single pair; (3) the disposition of the generative organs; (4) the texture of the skin; and (5) the simplicity and similarity of all the segments of the body behind the head.

The Annelidan affinities are superficially indicated in so marked a manner by the thinness of the cuticle, the dermo-muscular body wall, the hollow appendages, that, as already stated, many of the earlier zoologists who examined Peripatus placed it amongst the segmented worms; and the discovery that there is some solid morphological basis for this determination constitutes one of the most interesting points of the recent work on the genus. The Annelidan features are: (1) The paired nephridia in every segment of the body behind the first two (Saenger, Balfour[^[5]]); (2) the presence of cilia in the generative tracts (Gaffron). It is true that neither of these features are absolutely distinctive of the Annelida, but when taken in conjunction with the Annelidan disposition of the chief systems of organs, viz. the central nervous system, and the main vascular trunk or heart, may be considered as indicating affinities in that direction. Peripatus, therefore, is zoologically of extreme interest from the fact that, though in the main Arthropodan, it possesses features which are possessed by no other Arthropod, and which connect it to the group to which the Arthropoda are in the general plan of their organisation most closely related. It must, therefore, according to our present lights, be regarded as a very primitive form; and this view of it is borne out by its extreme isolation at the present day. Peripatus stands absolutely alone as a kind of half-way animal between the Arthropoda and Annelida. There is no gradation of structure within the genus; the species are very limited in number, and in all of them the peculiar features above mentioned are equally sharply marked.

Peripatus, though a lowly organised animal, and of remarkable sluggishness, with but slight development of the higher organs of sense, with eyes the only function of which is to enable it to avoid the light—though related to those animals most repulsive to the aesthetic sense of man, animals which crawl upon their bellies and spit at, or poison, their prey—is yet, strange to say, an animal of striking beauty. The exquisite sensitiveness and constantly changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty. Of all the species which I have seen alive, the most beautiful are the dark green individuals of Capensis, and the species which I have called Balfouri. These animals, so far as skin is concerned, are not surpassed in the animal kingdom. I shall never forget my astonishment and delight when on bearing away the bark of a rotten tree-stump in the forest on Table Mountain, I first came upon one of these animals in its natural haunts, or when Mr. Trimen showed me in confinement at the South African Museum a fine fat, full-grown female, accompanied by her large family of thirty or more just-born but pretty young, some of which were luxuriously creeping about on the beautiful skin of their mother's back.

External Features.

The anterior part of the body may be called the head, though it is not sharply marked off from the rest of the body (Fig. 1).

Fig. 1.—Peripatus capensis, drawn from life. Life size. (After Sedgwick.)

Fig. 2.—Ventral view of hind-end of P. Novae-Zealandiae. (After Sedgwick.) g, Generative opening; a, anus.

Fig. 3.—Ventral view of the head of P. capensis. (After Sedgwick.) ant, Antennae; or.p, oral papillae; F.1, first leg; T, tongue.

The head carries three pairs of appendages, a pair of simple eyes, and a ventrally placed mouth. The body is elongated and vermiform; it bears a number of paired appendages, each terminating in a pair of claws, and all exactly alike. The number varies in the different species. The anus is always at the posterior end of the body, and the generative opening is on the ventral surface just in front of the anus; it may be between the legs of the last pair (Fig. 2), or it may be behind them. There is in most species a thin median white line extending the whole length of the dorsal surface of the body, on each side of which the skin pigment is darker than elsewhere. The colour varies considerably in the different species, and even in different individuals of the same species. The ventral surface is nearly always flesh-coloured, while the dorsal surface has a darker colour. In the South African species the colour of the dorsal surface varies from a dark green graduating to a bluish gray, to a brown varying to a red orange. The colour of the Australasian species varies in like manner, while that of the Neotropical species (S. American and W. Indian) is less variable. The skin is thrown into a number of transverse ridges, along which wart-like papillae are placed. The papillae, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. Each papilla carries at its extremity a well-marked spine.

The appendages of the head are the antennae, the jaws and the oral papillae.

The antennae, which are prolongations of the dorso-lateral parts of the head, are ringed, and taper slightly till near their termination, where they are slightly enlarged. The rings bear a number of spines, and the free end of the antennae is covered by a cap of spiniferous tissue like that of the rings.

Fig. 4.—Inner jaw-claw of P. capensis. (After Balfour.)

Fig. 5.—Outer jaw-claw of P. capensis. (After Balfour.)

The mouth is at the hinder end of a depression called the buccal cavity, and is surrounded by an annular tumid lip, raised into papilliform ridges and bearing a few spines (Fig. 3). Within the buccal cavity are the two jaws. They are short, stump-like, muscular structures, armed at their free extremities by a pair of cutting blades or claws, and are placed one on each side of the mouth. In the median line of the buccal cavity in front is placed a thick muscular protuberance, which may be called the tongue, though attached to the dorsal instead of to the ventral wall of the mouth (Fig. 3). The tongue bears a row of small chitinous teeth. The jaw-claws (Figs. 4 and 5), which resemble in all essential points the claws borne by the feet, and like these are thickenings of the cuticle, are sickle-shaped. They have their convex edge directed forwards and their concave or cutting edge turned backwards. The inner cutting plate (Fig. 4) usually bears a number of cutting teeth. The jaws appear to be used for tearing the food, to which the mouth adheres by means of the tumid suctorial lips. The oral papillae are placed at the sides of the head (Fig. 3). The ducts of the slime-glands open at their free end. They possess two main rings of projecting tissue, and their extremities bear papillae irregularly arranged.

The ambulatory appendages vary in number. There are seventeen pairs in P. capensis and eighteen in P. Balfouri, while in P. Edwardsii the number varies from twenty-nine to thirty-four pairs. They consist of two main divisions, which we may call the leg and the foot (Figs. 6 and 7). The leg (l) has the form of a truncated cone, the broad end of which is attached to the ventro-lateral wall of the body, of which it is a prolongation. It is marked by a number of rings of papillae placed transversely to its long axis, the dorsal of which are pigmented like the dorsal surface of the body, and the ventral like the ventral surface. At the narrow distal end of the leg there are on the ventral surface three spiniferous pads, each of which is continued dorsally into a row of papillae.

Fig. 6.—Ventral view of last leg of a male P. capensis. (After Sedgwick.) f, Foot; l, leg; p, spiniferous pads. The white papilla on the proximal part of this leg is characteristic of the male of this species.

Fig. 7.—Leg of P. capensis seen from the front. (After Sedgwick.) f, Foot; l, leg; p, spiniferous pads.

The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end. It bears two sickle-shaped claws and a few papillae. The part of the foot which carries the claws is especially retractile, and is generally found more or less telescoped into the proximal part. The legs of the fourth and fifth pairs differ from the others in the fact that the proximal pad is broken up into three, a small central and two larger lateral. The enlarged nephridia of these legs open on the small central division.

The males are generally rather smaller than the females. In those species in which the number of legs varies, the male has a smaller number of legs than the female.

Habits.

They live beneath the bark of rotten stumps of trees, in the crevices of rock, and beneath stones. They require a moist atmosphere, and are exceedingly susceptible to drought. They avoid light, and are therefore rarely seen. They move with great deliberation, picking their course by means of their antennae and eyes. It is by the former that they acquire a knowledge of the ground over which they are travelling, and by the latter that they avoid the light. The antennae are extraordinarily sensitive, and so delicate, indeed, that they seem to be able to perceive the nature of objects without actual contact. When irritated they eject with considerable force the contents of their slime reservoirs from the oral papillae. The force is supplied by the sudden contraction of the muscular body wall. They can squirt the slime to the distance of almost a foot. The slime, which appears to be perfectly harmless, is extremely sticky, but it easily comes away from the skin of the animal itself.

I have never seen them use this apparatus for the capture of prey, but Hutton describes the New Zealand species as using it for this purpose. So far as I can judge, it is used as a defensive weapon; but this of course will not exclude its offensive use. They will turn their heads to any part of the body which is being irritated and violently discharge their slime at the offending object. Locomotion is effected entirely by means of the legs, with the body fully extended.

Of their food in the natural state we know little; but it is probably mainly, if not entirely, animal. Hutton describes his specimens as sucking the juices of flies which they had stuck down with their slime, and those which I kept in captivity eagerly devoured the entrails of their fellows, and the developing young from the uterus. They also like raw sheep's liver. They move their mouths in a suctorial manner, tearing the food with their jaws. They have the power of extruding their jaws from the mouth, and of working them alternately backwards or forwards. This is readily observed in individuals immersed in water.

Breeding.

All species are viviparous. It has been lately stated that one of the Australian species is normally oviparous, but this has not been proved. The Australasian species come nearest to laying eggs, inasmuch as the eggs are large, full of yolk, and enclosed in a shell; but development normally takes place in the uterus, though, abnormally, incompletely developed eggs are extruded.

The young of P. capensis are born in April and May. They are almost colourless at birth, excepting the antennae, which are green, and their length is 10 to 15 mm. A large female will produce thirty to forty young in one year. The period of gestation is thirteen months, that is to say, the ova pass into the oviducts about one month before the young of the preceding year are born. They are born one by one, and it takes some time for a female to get rid of her whole stock of embryos; in fact, the embryos in any given female differ slightly in age, those next the oviduct being a little older (a few hours) than those next the vagina. The mother does not appear to pay any special attention to her young, which wander away and get their own food.

There does not appear to be any true copulation. The male deposits small, white, oval spermatophores, which consist of small bundles of spermatozoa cemented together by some glutinous substance, indiscriminately on any part of the body of the female. Such spermatophores are found on the bodies of both males and females from July to January, but they appear to be most numerous in our autumn. It seems probable that the spermatozoa make their way from the adherent spermatophore through the body wall into the body, and so by traversing the tissues reach the ovary. The testes are active from June to the following March. From March to June the vesiculae of the male are empty.

There are no other sexual differences except in some of the South African species, in which the last or penultimate leg of the male bears a small white papilla on its ventral surface (Fig. 6).

Whereas in the Cape species embryos in the same uterus are all practically of the same age (except in the month of April, when two broods overlap in P. capensis), and birth takes place at a fixed season; in the Neotropical species the uterus, which is always pregnant, contains embryos of different ages, and births probably take place all the year round.

In all species of Peripatus the young are fully formed at birth, and differ from the adults only in size and colour.

ANATOMY

The Alimentary Canal (Fig. 8).

Fig. 8.—Peripatus capensis dissected so as to show the alimentary canal, slime glands, and salivary glands. (After Balfour.) The dissection is viewed from the ventral side, and the lips (L) have been cut through in the middle line behind and pulled outwards so as to expose the jaws (j), which have been turned outwards, and the tongue (T) bearing a median row of chitinous teeth, which branches behind into two. The muscular pharynx, extending back into the space between the first and second pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S-shaped ventro-dorsal curve. A, Anus; at, antenna; F.1, F.2, first and second feet; j, jaws; L, lips; oe, oesophagus; or.p, oral papilla; ph, pharynx; R, rectum; s.d, salivary duct; s.g, salivary gland; sl.d, slime reservoir; sl.g, portion of tubules of slime gland; st, stomach; T, tongue in roof of mouth.

The buccal cavity, as explained above, is a secondary formation around the true mouth, which is at its dorsal posterior end. It contains the tongue and the jaws, which have already been described, and into the hind end of it there opens ventrally by a median opening the salivary glands (s.g). The mouth leads into a muscular pharynx (ph), which is connected by a short oesophagus (oe) with a stomach (st). The stomach forms by far the largest part of the alimentary canal. It is a dilated soft-walled tube, and leads behind into the short narrow rectum (R), which opens at the anus. There are no glands opening into the alimentary canal.

Nervous System.

The central nervous system consists of a pair of supra-oesophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-oesophageal ganglia (Fig. 9).

The ventral cords at first sight appear to be without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (Fig. 9). These enlargements may be regarded as imperfect ganglia. There are, therefore, as many pairs of ganglia as there are pairs of legs. There is in addition a ganglionic enlargement at the commencement of the oesophageal commissures, where the nerves to the oral papillae are given off (Fig. 9, or.g).

Fig. 9.—Brain and anterior part of the ventral nerve-cords of Peripatus capensis enlarged and viewed from the ventral surface. (After Balfour.) The paired appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic nerves (sy) arising from the ventral surface of the hinder part. From the commencement of the oesophageal commissures pass off on each side a pair of nerves to the jaws (Jn). The three anterior commissures between the ventral nerve-cords are placed close together; immediately behind them the nerve-cords are swollen, to form the ganglionic enlargements from which pass off to the oral papillae a pair of large nerves on each side (orn). Behind this the cords present a series of enlargements, one pair for each pair of feet, from which a pair of large nerves pass off on each side to the feet (pn). atn, Antennary nerves; co, commissures between ventral cords; d, ventral appendages of brain; E, eye; en, nerves passing outwards from ventral cord; F.g.1, ganglionic enlargements from which nerves to feet pass off; jn, nerves to jaws; org, ganglionic enlargement from which nerves to oral papillae pass off; orn, nerves to oral papillae; pc, posterior lobe of brain; pn, nerves to feet; sy, sympathetic nerves.

The ventral cords are placed each in the lateral compartments of the body cavity, immediately within the longitudinal layer of muscles. They are connected with each other, rather like the pedal nerves of Chiton and the lower Prosobranchiata, by a number of commissures. These commissures exhibit a fairly regular arrangement from the region included between the first and the last pair of true feet. There are nine or ten of them between each pair of feet. They pass along the ventral wall of the body, perforating the ventral mass of longitudinal muscles. On their way they give off nerves which innervate the skin.

Posteriorly the two nerve-cords nearly meet immediately in front of the generative aperture, and then, bending upwards, fall into each other dorsally to the rectum. They give off a series of nerves from their outer borders, which present throughout the trunk a fairly regular arrangement. From each ganglion two large nerves (pn) are given off, which, diverging somewhat from each other, pass into the feet.

From the oesophageal commissures, close to their junction with the supra-oesophageal ganglia, a nerve arises on each side which passes to the jaws, and a little in front of this, apparently from the supra-oesophageal ganglion itself, a second nerve to the jaws also takes its origin.

The supra-oesophageal ganglia (Fig. 9) are large, somewhat oval masses, broader in front than behind, completely fused in the middle, but free at their extremities. Each of them is prolonged anteriorly into an antennary nerve, and is continuous behind with one of the oesophageal commissures. On the ventral surface of each, rather behind the level of the eye, is placed a hollow protuberance (Fig. 9, d), of which I shall say more in dealing with the development. About one-third of the way back the two large optic nerves take their origin, arising laterally, but rather from the dorsal surface (Fig. 9). Each of them joins a large ganglionic mass placed immediately behind the retina.

The histology of the ventral cords and oesophageal commissures is very simple and uniform. They consist of a cord almost wholly formed of nerve-fibres placed dorsally, and of a ventral layer of ganglion cells.

The Body Wall.

The skin is formed of three layers.

(1) The cuticle.

(2) The epidermis or hypodermis.

(3) The dermis.

The cuticle is a thin layer. The spines, jaws, and claws are special developments of it. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary papillae, which in most instances bear at their free extremity a somewhat prominent spine. The whole surface of each of the secondary papillae just described is in its turn covered by numerous minute spinous tubercles.

The epidermis, placed immediately within the cuticle, is composed of a single layer of cells, which vary, however, a good deal in size in different regions of the body. The cells excrete the cuticle, and they stand in a very remarkable relation to the secondary papillae of the cuticle just described. Each epidermis cell is in fact placed within one of these secondary papillae, so that the cuticle of each secondary papilla is the product of a single epidermis cell. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the outer ends of the cells in the form of small granules.

At the apex of most, if not all, the primary wart-like papillae there are present oval aggregations, or masses of epidermis cells, each such mass being enclosed in a thickish capsule and bearing a long projecting spine. These structures are probably tactile organs. In certain regions of the body they are extremely numerous; more especially is this the case in the antennae, lips, and oral papillae. On the ventral surface of the peripheral rings of the thicker sections of the feet they are also very thickly set and fused together so as to form a kind of pad (Figs. 6 and 7). In the antennae they are thickly set side by side on the rings of skin which give such an Arthropodan appearance to these organs in Peripatus.

The Tracheal System.

The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which may be called the tracheal pit (Fig. 10), the walls of which are formed of epithelial cells bounded towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. The pits vary somewhat in depth; the pit figured was about 0.09 mm. It perforates the dermis and terminates in the subjacent muscular layer.

Internally it expands in the transverse plane and from the expanded portion the tracheal tubes arise in diverging bundles. Nuclei similar in character to those in the walls of the tracheal pit are placed between the tracheae, and similar but slightly more elongated nuclei are found along the bundles. The tracheae are minute tubes exhibiting a faint transverse striation which is probably the indication of a spiral fibre. They appear to branch, but only exceptionally. The tracheal apertures are diffused over the surface of the body, but are especially developed in certain regions.

Fig. 10.—Section through a tracheal pit and diverging bundles of tracheal tubes taken transversely to the long axis of the body. (After Balfour.) tr, Tracheae, showing rudimentary spiral fibre; tr.c, cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheae; tr.o, tracheal stigma; tr.p, tracheal pit.

The Muscular System.

The general muscular system consists of—(1) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract.

The muscular wall of the body is formed of—(1) an external layer of circular fibres; (2) an internal layer of longitudinal muscles.

The main muscles of the body are unstriated and divided into fibres, each invested by a delicate membrane. The muscles of the jaws alone are transversely striated.

The Vascular System.

The vascular system consists of a dorsal tubular heart with paired ostia leading into it from the pericardium, of the pericardium, and the various other divisions of the perivisceral cavity (Fig. 14, D). As in all Arthropoda, the perivisceral cavity is a haemocoele; i.e. it contains blood and forms part of the vascular system. The heart extends from close to the hind end of the body to the head.

The Body Cavity.

The body cavity is formed of four compartments—one central, two lateral, and a pericardial (Fig. 14, D). The former is by far the largest, and contains the alimentary tract, the generative organs, and the slime glands. It is lined by a delicate endothelial layer, and is not divided into compartments nor traversed by muscular fibres. The lateral divisions are much smaller than the central, and are shut off from it by the inner transverse band of muscles. They are almost entirely filled with the nerve-cord and salivary gland in front and with the nerve-cord alone behind, and their lumen is broken up by muscular bands. They further contain the nephridia. They are prolonged into the feet, as is the embryonic body cavity of most Arthropoda. The pericardium contains a peculiar cellular tissue, probably, as suggested by Moseley, equivalent to the fat-bodies of insects.

Nephridia.

In Peripatus capensis nephridia are present in all the legs. In all of them (except the first three) the following parts may be recognised (Fig. 11):—

(1) A vesicular portion opening to the exterior on the ventral surface of the legs by a narrow passage.

(2) A coiled portion, which is again subdivided into several sections.

(3) A section with closely packed nuclei ending by a somewhat enlarged opening.

(4) The terminal portion, which consists of a thin-walled vesicle.

The last twelve pairs of these organs are all constructed in a very similar manner, while the two pairs situated in the fourth and fifth pairs of legs are considerably larger than those behind, and are in some respects very differently constituted.

It will be convenient to commence with one of the hinder nephridia. Such a nephridium from the ninth pair of legs is represented in Fig. 11. The external opening is placed at the outer end of a transverse groove at the base of one of the legs, while the main portion of the organ lies in the body cavity in the base of the leg, and extends into the trunk to about the level of the outer edge of the nerve-cord of its side. The external opening (o.s) leads into a narrow tube (s.d), which gradually dilates into a large sac (s). The narrow part is lined by small epithelial cells, which are directly continuous with and perfectly similar to those of the epidermis. The sac itself, which forms a kind of bladder or collecting vesicle for the organ, is provided with an extremely thin wall, lined with very large flattened cells. The second section of the nephridium is formed by the coiled tube, the epithelial lining of which varies slightly in the different parts. The third section (s.o.t), constitutes the most distinct portion of the whole organ. Its walls are formed of columnar cells almost filled by oval nuclei, which absorb colouring matters with very great avidity, and thus render this part extremely conspicuous. The nuclei are arranged in several rows. It ends by opening into a vesicle (Fig. 14, D), the wall of which is so delicate that it is destroyed when the nephridium is removed from the body, and consequently is not shown in Fig. 11.

Fig. 11.—Nephridium from the 9th pair of legs of P. capensis. o.s, External opening of segmental organ; p.f, internal opening of nephridium into the body cavity (lateral compartment); s, vesicle of segmental organ; s.c.1, s.c.2, s.c.3, s.c.4, successive regions of coiled portion of nephridium; s.o.t, third portion of nephridium broken off at p.f from the internal vesicle, which is not shown.

The fourth and fifth pairs are very considerably larger than those behind, and are in other respects peculiar. The great mass of each organ is placed behind the leg on which the external opening is placed, immediately outside one of the lateral nerve-cords. The external opening, instead of being placed near the base of the leg, is placed on the ventral side of the third ring (counting from the outer end) of the thicker portion of the leg. It leads into a portion which clearly corresponds with the collecting vesicle of the hinder nephridia. This part is not, however, dilated into a vesicle. The three pairs of nephridia in the three foremost pairs of legs are rudimentary, consisting solely of a vesicle and duct. The salivary glands are the modified nephridia of the segment of the oral papillae.

Generative Organs.

Male.—The male organs (Fig. 12) consist of a pair of testes (te), a pair of vesicles (v), vasa deferentia (v.d), and accessory glandular tubules (f). All the above parts lie in the central compartment of the body cavity. In P. capensis the accessory glandular bodies or crural glands of the last (17th) pair of legs are enlarged and prolonged into an elongated tube placed in the lateral compartment of the body cavity (a.g).

Fig. 12.—Male generative organs of Peripatus capensis, viewed from the dorsal surface. (After Balfour.) a.g, Enlarged crural glands of last pair of legs; F.16, 17, last pairs of legs; f, small accessory glandular tubes; p, common duct into which the vasa deferentia open; te, testis; v, seminal vesicle; v.c, nerve-cord; v.d, vas deferens.

The right vas deferens passes under both nerve-cords to join the left, and form the enlarged tube (p), which, passing beneath the nerve-cord of its side, runs to the external orifice. The enlarged terminal portion possesses thick muscular walls, and possibly constitutes a spermatophore maker, as has been shown to be the case in P. N. Zealandiae, by Moseley. In some specimens a different arrangement obtains, in that the left vas deferens passes under both nerve-cords to join the right.

Female.—The ovaries consist of a pair of tubes closely applied together, and continued posteriorly into the oviducts. The oviducts, after a short course, become dilated into the uteruses, which join behind and open to the exterior by a median opening. The ovaries always contain spermatozoa, some of which project through the ovarian wall into the body cavity. Spermatozoa are not found in the uterus and oviducts, and it appears probable that they reach the ovary directly by boring through the skin and traversing the body cavity.[^[6]] In the neotropical species there is a globular receptaculum seminis opening by two short ducts close together into the oviduct, and there is a small receptaculum ovorum with extremely thin walls opening into the oviduct by a short duct just in front of the receptaculum seminis. The epithelium of the latter structure is clothed with actively moving cilia. In the New Zealand species there is a receptaculum seminis with two ducts, but the receptacula ovorum has not been seen.

There appear to be present in most, if not all, the legs some accessory glandular structures opening just externally to the nephridia. They are called the crural glands.

DEVELOPMENT.

As stated at the outset, Peripatus is found in three of the great regions, viz. in Africa, in Australasia, and in South America and the West Indies. It is a curious and remarkable fact that although the species found in these various localities are really closely similar, the principal differences relating to the structure of the female generative organs and to the number of the legs, they do differ in the most striking manner in the structure of the ovum and in the early development. In all the Australasian species the egg is large and heavily charged with food-yolk, and is surrounded by a tough membrane. In the Cape species the eggs are smaller, though still of considerable size; the yolk is much less developed, and the egg membrane is thinner though dense. In the neotropical species the egg is minute and almost entirely devoid of yolk. The unsegmented uterine ovum of P. Novae-Zealandiae measures 1.5 mm. in length by .8 mm. in breadth; that of P. capensis is .56 mm. in length; and that of P. Trinidadensis .04 mm. in diameter. In correspondence with these differences in the ovum there are differences in the early development, though the later stages are closely similar.

Fig. 13.—A series of embryos of P. capensis. The hind end of embryos B, C, D is uppermost in the figures, the primitive streak is the white patch behind the blastopore. (After Sedgwick.) A, Gastrula stage, ventral view, showing blastopore. B, Older gastrula stage, ventral view, showing elongated blastopore and primitive streak. C, Ventral view of embryo with three pairs of mesoblastic somites, dumb-bell-shaped blastopore and primitive streak. D, Ventral view of embryo, in which the blastopore has completely closed in its middle portion, and given rise to two openings, the embryonic mouth and anus. The anterior pair of somites have moved to the front end of the body, and the primitive groove has appeared on the primitive streak. E, Side view of embryo, in which the hind end of the body has begun to elongate in a spiral manner, and in which the appendages have begun. At, antenna; d, dorsal projection; p.s, preoral somite. F, Ventral view of head of embryo intermediate between E and G. The cerebral grooves are wide and shallow. The lips have appeared, and have extended behind the openings of the salivary glands, but have not yet joined in the middle line. At, antennae; c.g, cerebral groove; j, jaws; j.s, swelling at base of jaws; L, lips; M, mouth; or.p, oral papillae; o.s, opening of salivary gland. G, Side view of older embryo with the full number of appendages, to show the position in which the embryos lie in the uterus.

But unfortunately the development has only been fully worked out in one species, and to that species—P. capensis—the following description refers. The ova are apparently fertilised in the ovary, and they pass into the oviducts in April and May. In May the brood of the preceding year are born, and the new ova, which have meanwhile undergone cleavage, pass into the uterus. There are ten to twenty ova in each uterus. The segmentation is peculiar, and leads to the formation of a solid gastrula, consisting of a cortex of ectoderm nuclei surrounding a central endodermal mass, which consists of a much-vacuolated tissue with some irregularly-shaped nuclei. The endoderm mass is exposed at one point—the blastopore (gastrula mouth). The central vacuoles of the endoderm now unite and form the enteron of the embryo, and at the same time the embryo elongates into a markedly oval form, and an opacity—the primitive streak—appears at the hind end of the blastopore (Fig. 13, B). This elongation of the embryo is accompanied by an elongation of the blastopore, which soon becomes dumb-bell shaped (Fig. 13, C). At the same time the mesoblastic somites (embryonic segments of mesoderm) have made their appearance in pairs at the hind end, and gradually travel forward on each side of the blastopore to the front end, where the somites of the anterior pair soon meet in front of the blastopore (Fig. 13, D). Meanwhile the narrow middle part of the blastopore has closed by a fusion of its lips, so that the blastopore is represented by two openings, the future mouth and anus. A primitive groove makes its appearance behind the blastopore (Fig. 13, D). At this stage the hind end of the body becomes curved ventrally into a spiral (Fig. 13, E), and at the same time the appendages appear as hollow processes of the body wall, a mesoblastic somite being prolonged into each of them. The first to appear are the antennae, into which the praeoral somites are prolonged. The remainder appear from before backwards in regular order, viz. jaw, oral papillae, legs 1-17. The full number of somites and their appendages is not, however, completed until a later stage. The nervous system is formed as an annular thickening of ectoderm passing in front of the mouth and behind the anus, and lying on each side of the blastopore along the lines of the somites. The praeoral part of this thickening, which gives rise to the cerebral ganglia, becomes pitted inwards on each side (Fig. 13, F, c.g). These pits are eventually closed, and form the hollow ventral appendages of the supra-pharyngeal ganglia of the adult (Fig. 9, d). The lips are formed as folds of the side wall of the body, extending from the praeoral lobes to just behind the jaw (Fig. 13, F, L). They enclose the jaws (j) mouth (M), and opening of the salivary glands (o.s), and so give rise to the buccal cavity. The embryo has now lost its spiral curvature, and becomes completely doubled upon itself, the hind end being in contact with the mouth (Fig. 13, G). It remains in this position until birth. The just-born young are from 10-15 mm. in length and have green antennae, but the rest of the body is either quite white or of a reddish colour. This red colour differs from the colour of the adult in being soluble in spirit.

The mesoblastic somites are paired sacs formed from the anterior lateral portions of the primitive streak (Fig. 13, C). As they are formed they become placed in pairs on each side of the blastopore. The somites of the first pair eventually obtain a position entirely in front of the blastopore (Fig. 13, D). They form the somites of the praeoral lobes. The full complement of somites is acquired at about the stage of Fig. 13, E.

Fig. 14.—A series of diagrams of transverse sections through Peripatus embryos to show the relations of the coelom at successive stages. (After Sedgwick.) A, Early stage: 1, gut; 2, mesoblastic somite; no trace of the vascular space; endoderm and ectoderm in contact. B, Endoderm has separated from the dorsal and ventral ectoderm. The somite is represented as having divided on the left side into a dorsal and ventral portion: 1, gut; 2, somite; 3, haemocoele. C, The haemocoele (3) has become divided up into a number of spaces, the arrangement of which is unimportant. The dorsal part of each somite has travelled dorsalwards, and now constitutes a small space (triangular in section) just dorsal to the gut. The ventral portion (2′) has assumed a tubular character, and has acquired an external opening. The internal vesicle is already indicated, and is shown in the diagram by the thinner black line: 1, gut; 2′, nephridial part of coelom; 3, haemocoele; 3′, part of haemocoele which will form the heart—the part of the haemocoele on each side of this will form the pericardium; 4, nerve-cord. D represents the conditions at the time of birth; numbers as in C, except 5, slime glands. The coelom is represented as surrounded by a thick black line, except in the part which forms the internal vesicle of the nephridium.

The relations of the somites is shown in Fig. 14, A, which represents a transverse section taken between the mouth and anus of an embryo of the stage of Fig. 13, D. The history of these somites is an exceedingly interesting one, and may be described shortly as follows:—They divide into two parts—a ventral part, which extends into the appendage, and a dorsal part (Fig. 14, B). The ventral part acquires an opening to the exterior just outside the nerve-cord, and becomes entirely transformed into a nephridium (Fig. 14, D, 2′). The dorsal part shifts dorsalwards and diminishes relatively in size (Fig. 14, C). Its fate differs in the different parts of the body. In the anterior somites it dwindles and disappears, but in the posterior part it unites with the dorsal divisions of contiguous somites of the same side, and forms a tube—the generative tube (Fig. 14, D, 2). The last section of this tube retains its connexion with the ventral portion of the somite, and so acquires an external opening, which is at first lateral, but soon shifts to the middle line, and fuses with its fellow, to form the single generative opening. The praeoral somite develops the rudiment of a nephridium, but eventually entirely disappears. The jaw somite also disappears; the oral papilla somite forms ventrally the salivary glands, which are thus serially homologous with nephridia. The perivisceral cavity of Peripatus is, as in all Arthropoda, a haemocoele. Its various divisions develop as a series of spaces between the ectoderm and endoderm, and later in the mesoderm. The mesoderm seems to be formed entirely from the proliferation of the cells of the mesoblastic somites. It thus appears that in Peripatus the coelom does not develop a perivisceral portion, but gives rise only to the renal and reproductive organs.

Synopsis of the Species of Peripatus.

Peripatus, Guilding.

Soft-bodied vermiform animals, with one pair of ringed antennae, one pair of jaws, one pair of oral papillae, and a varying number of claw-bearing ambulatory legs. Dorsal surface arched and more darkly pigmented than the flat ventral surface. Skin transversely ridged and beset by wart-like spiniferous papillae. Mouth anterior, ventral; anus posterior, terminal. Generative opening single, median, ventral, and posterior. One pair of simple eyes. Brain large, with two ventral hollow appendages; ventral cords widely divaricated, without distinct ganglia. Alimentary canal simple, uncoiled. Segmentally arranged, paired nephridia are present. Body cavity is continuous with the vascular system, and does not communicate with the paired nephridia. Heart tubular, with paired ostia. Respiration by means of tracheae. Dioecious; males smaller and generally less numerous than females. Generative glands tubular, continuous with the ducts. Viviparous. Young born fully developed. They shun the light, and live in damp places beneath stones, leaves, and bark of rotten stumps. They eject when irritated a viscid fluid through openings at the apex of the oral papillae.

Distribution: South Africa, New Zealand, and Australia, South America and the West Indies [and in Sumatra?].

South African Species.

With three spinous pads on the legs and two primary papillae on the anterior side of the foot, and one accessory tooth on the outer blade of the jaw; with a white papilla on the ventral surface of the last fully developed leg of the male. Genital opening subterminal, behind the last pair of fully-developed legs. The terminal unpaired portion of vas deferens short. Ova of considerable size, but with only a small quantity of food-yolk. (Colour highly variable, number of legs constant in same species (?).)

P. capensis (Grube).—South African Peripatus, with seventeen pairs of claw-bearing ambulatory legs. Locality, Table Mountain.

P. Balfouri (Sedgwick).—South African Peripatus, with eighteen pairs of claw-bearing ambulatory legs, of which the last pair is rudimentary. With white papillae on the dorsal surface. Locality, Table Mountain.

P. brevis (De Blainville).—South African Peripatus, with fourteen pairs of ambulatory legs. Locality, Table Mountain. (I have not seen this species. Presumably it has the South African characters.)

P. Moseleyi (Wood Mason).—South African Peripatus, with twenty-one and twenty-two pairs of claw-bearing ambulatory legs. Locality, near Williamstown, Cape Colony; and Natal.[^[7]]

Doubtful Species.

(1) South African Peripatus, with twenty pairs of claw-bearing ambulatory legs (Sedgwick). Locality, Table Mountain. (Also Peters, locality not stated.)

(2) South African Peripatus, with nineteen pairs of ambulatory legs (Trimen). Locality, Plettenberg Bay, Cape Colony. (Also Peters, locality not stated.)

Australasian Species.

With fifteen pairs of claw-bearing ambulatory legs, with three spinous pads on the legs, and a primary papilla projecting from the median dorsal portion of the feet. Genital opening between the legs of the last pair. Receptacula seminis present. Unpaired portion of vas deferens long and complicated. Ova large and heavily charged with yolk. (Colour variable, number of legs constant in same species (?).)

P. Novae Zealandiae (Hutton).—Australasian Peripatus, without an accessory tooth on the outer blade of the jaw, and without a white papilla on the base of the last leg of the male. New Zealand.

P. Leuckarti (Saenger).—Australasian Peripatus, with an accessory tooth on the outer blade of the jaw, and a white papilla on the base of the last leg of the male. Queensland.

Neotropical Species.

With four spinous pads on the legs, and the generative aperture between the legs of the penultimate pair. Dorsal white line absent. Primary papillae divided into two portions. Inner blade of jaw with gap between the first minor tooth and the rest. Oviducts provided with receptacula ovorum and seminis. Unpaired part of vas deferens very long and complicated. Ova minute, without food-yolk. (Colour fairly constant, number of legs variable in same species (?).)

P. Edwardsii.[^[8]]—Neotropical Peripatus from Caracas, with a variable number of ambulatory legs (twenty-nine to thirty-four). Males with twenty-nine or thirty legs, and tubercles on a varying number of the posterior legs. The basal part or the primary papilla is cylindrical.

P. Trinidadensis (n. sp.).—Neotropical Peripatus from Trinidad, with twenty-eight to thirty-one pairs of ambulatory legs, and a large number of teeth on the inner blade of the jaw. The basal portion of the primary papillae is conical.

P. torquatus (Kennel).— Neotropical Peripatus from Trinidad, with forty-one to forty-two pairs of ambulatory legs. With a transversely placed bright yellow band on the dorsal surface behind the head.

Doubtful Species.

The above are probably distinct species. Of the remainder we do not know enough to say whether they are distinct species or not. The following is a list of these doubtful species, with localities and principal characters:—

P. juliformis (Guilding).—Neotropical Peripatus from St. Vincent, with thirty-three pairs of ambulatory legs.

P. Chiliensis (Gay).—Neotropical Peripatus from Chili, with nineteen pairs of ambulatory legs.

P. demeraranus (Sclater).—Neotropical Peripatus from Maccasseema, Demerara, with twenty-seven to thirty-one pairs of ambulatory legs and conical primary papillae.

Peripatus from Cayenne (Audouin and Milne-Edwards).—With thirty pairs of legs. Named P. Edwardsii by Blanchard.

Peripatus from Valentia Lake, Columbia (Wiegmann).—With thirty pairs of legs.

Peripatus from St. Thomas (Moritz).—No description.

Peripatus from Colonia Towar, Venezuela (Grube).—With twenty-nine to thirty-one pairs of ambulatory legs. Named P. Edwardsii by Grube.

Peripatus from Santo Domingo, Nicaragua (Belt).—With thirty-one pairs of ambulatory legs.

Peripatus from Dominica (Angas).—Neotropical Peripatus, with twenty-six to thirty (Pollard) pairs of ambulatory legs.

Peripatus from Jamaica (Gosse).—With thirty-one and thirty-seven pairs of ambulatory legs.

Peripatus from Santaram.—Neotropical Peripatus, with thirty-one pairs of ambulatory legs.

Peripatus from Cuba.—No details.

Peripatus from Hoorubea Creek, Demerara (Quelch).—With thirty pairs of legs.

Peripatus from Marajo (Branner).—No details.

Peripatus from Utuado, Porto Rico (Peters).—With twenty-seven, thirty, thirty-one, and thirty-two pairs of legs.

Peripatus from Surinam (Peters).—No details.

Peripatus from Puerto Cabello, Venezuela (Peters).—With thirty and thirty-two pairs of legs.

Peripatus from Laguayra, Venezuela (Peters).—No details.

Peripatus Quitensis (Schmarda).—From Quito, with thirty-six pairs of legs.

Peripatus from Sumatra (?).

P. Sumatranus (Horst).—Peripatus from Sumatra, with twenty-four pairs of ambulatory legs, and four spinous pads on the legs. The primary papillae of the neotropical character with conical bases. Generative opening between the legs of the penultimate pair. Feet with only two papillae.[^[9]]

Summary of Distribution

Distribution of the South African Species—

Slopes of Table Mountain, neighbourhood of Williamstown, Plettenberg Bay—Cape Colony—Natal.

Distribution of the Australasian Species—

Queensland—Australia.

North and South Islands—New Zealand.

Oriental Region (?)—

Sumatra.

Distribution of the Neotropical Species—

Nicaragua.

Valencia Lake, Caracas, Puerto Cabello, Laguayra, Colonia Towar—Venezuela.

Quito—Ecuador.

Maccasseema, Hoorubea Creek—Demerara.

Surinam (Peters).

Cayenne.

Santarem, Marajo, at the mouth of the Amazon—Brazil.

Chili.

And in the following West Indian Islands—Cuba, Dominica, Porto Rico (Peters), Jamaica, St. Thomas, St. Vincent, Trinidad.

MYRIAPODA

BY

F. G. SINCLAIR, M.A.
(FORMERLY F. G. HEATHCOTE)

Trinity College, Cambridge.

CHAPTER II

MYRIAPODA

INTRODUCTION–HABITS–CLASSIFICATION–STRUCTURE–CHILOGNATHA–CHILOPODA–SCHIZOTARSIA–SYMPHYLA–PAUROPODA–EMBRYOLOGY–PALAEONTOLOGY.

Tracheata with separated head and numerous, fairly similar segments. They have one pair of antennae, two or three pairs of mouth appendages, and numerous pairs of legs.

The Myriapoda are a class of animals which are widely distributed, and are represented in almost every part of the globe. Heat and cold alike seem to offer favourable conditions for their existence, and they flourish both in the most fertile and the most barren countries.

They have not attracted much notice until comparatively recent times. Compared with Insects they have been but little known. The reason of this is not hard to find. The Myriapods do not exercise so much direct influence on human affairs as do some other classes of animals; for instance, Insects. They include no species which is of direct use to man, like the silkworm or the cochineal insect, and they are of no use to him as food. It is true that they are injurious to his crops. For instance, the species of Millepede known as the "wire worm"[^[10]] is extremely harmful; but this has only attracted much notice in modern times, when land is of more value than formerly, and agriculture is pursued in a more scientific manner, and the constant endeavour to get the utmost amount of crop from the soil has caused a minute investigation into the various species of animals which are noxious to the growing crop. The species of Myriapoda best known to the ancients were those which were harmful to man on account of their poisonous bite.

Some writers have supposed that the word which is translated "mole" in the Bible (Lev. xi. 30) is really Scolopendra (a genus of Centipede), and, if this is so, it is the earliest mention of the Myriapods. They were rarely noticed in the classical times; almost the only mention of them is by Ælian, who says that the whole population of a town called Rhetium were driven out by a swarm of Scolopendras. Pliny tells us of a marine Scolopendra, but this was most probably a species of marine worm.

Linnaeus included Myriapods among the Insects; and the writers after him till the beginning of this century classed them with all sorts of Insects, with Spiders, Scorpions, and even among Serpents. It was Leach who first raised them to the importance of a separate class, and Latreille first gave them the name of Myriapoda, which they have retained ever since.

Myriapods are terrestrial animals, crawling or creeping on the ground or on logs of wood, or even under the bark of trees. There is, however, a partial exception to this; various naturalists have from time to time given descriptions of marine Centipedes. These are not found in the sea, but crawl about on the shore, where they are submerged by each tide. Professor F. Plateau has given an account of the two species of Myriapods that are found thus living a semi-aquatic life. They are named Geophilus maritimus and Geophilus submarinus, and Plateau found that they could exist in sea water from twelve to seventy hours, and in fresh water from six to ten days. They thus offer a striking example of the power that their class possess of existing under unfavourable circumstances.

With regard to their habits the different species differ very considerably. On the one hand we have the Chilopoda, or Centipedes, as they are called in this country, active, swift, and ferocious; living for the most part in dark and obscure places, beneath stones, logs of wood, and dried leaves, etc., and feeding on living animals. On the other hand, we have the Chilognatha, or Millepedes, distinguished by their slow movements and vegetable diet; inoffensive to man, except by the destruction they occasion to his crops, and having as a means of defence no formidable weapon like the large poison claws of the Centipedes, but only a peculiarly offensive liquid secreted by special glands known by the unpleasant though expressive name of "stink glands," or by the more euphonious Latin name of glandulae odoriferae.

As a general rule the larger species of Myriapods are found in the hotter climates, some of the tropical species being very large, and some, among the family of the Scolopendridae, extremely poisonous; and it is even said that their bite is fatal to man.

Fig. 15.—Scolopendra obscura. (From C. L. Koch, Die Myriapoden.)

If, however, the Centipede is sometimes fatal to man, it does not always have it its own way, for we read of man making food of Centipedes. It is hard to believe that any human being could under any circumstances eat Centipedes, which have been described by one naturalist as "a disgusting tribe loving the darkness." Nevertheless, Humboldt informs us that he has seen the Indian children drag out of the earth Centipedes eighteen inches long and more than half an inch wide and devour them.

Fig. 16.—Chordeuma sylvestre. (From C. L. Koch, Die Myriapoden.)

This, I believe, is the only account of human beings using the Myriapoda as food, if we except the accounts of the religious fanatics among the African Arabs, who are said to devour Centipedes alive; though this is not a case of eating for pleasure, for the Scolopendras are devoured in company with leaves of the prickly pear, broken glass, etc., as a test of the unpleasant things which may be eaten under the influence of religious excitement.

A cold climate, however, is not fatal to some fairly large species of Centipedes. A striking instance of this came under my own observation some years ago. In 1886 I was travelling in the island of Cyprus—the "Enchanted Island," as Mr. Mallock calls it in his book written about the same time—with the intention of observing its natural history. This island consists of a broad flat country crossed by two mountain ranges of considerable height, thus offering the contrast of a hot climate in the plains and a cold climate in the mountains. On the plain country I found among the Myriapoda that the most common species were a large Scolopendra and a large Lithobius. The Scolopendra was fairly common, living for the most part under large stones, and it was a pleasant task to search for them in a ruined garden near Larnaca.

This garden was made for the public, and is situated about a quarter of a mile from the old town of Larnaca. It has been suffered to fall into decay, and is now quite neglected. Mr. Mallock has described many beautiful scenes in his book, but I think he could have found few more beautiful than this old garden with its deserted gardener's house, now a heap of ruins, but overgrown with masses of luxuriant vegetation, with beautiful flowers peeping out here and there as if charitably endeavouring to hide the negligence of man, and to turn the desolation into a scene of beauty. I got several prizes in this garden, but found the Myriapods were principally represented by the species I have mentioned.

After leaving Larnaca I rode across the plain country through blazing heat, which was rapidly parching up the ground to a uniform brown colour. At every stopping-place I found the same species of Scolopendra and of Lithobius. After a few days I began to get up among the mountains of the northern range, and the burning heat of the treeless plain was gradually exchanged for the cool shade of the pine-trees and the fresh air of the mountains. As I ascended higher and higher the temperature grew cooler till I reached the top of Mount Troodos, the ancient Olympus. Here in the month of May the snow still lingered in white patches, and the air was clear and cold. I remained on the top of Troodos for a week, while I made a close examination of the fauna to be found there. I was much surprised to find the identical species of Scolopendra and Lithobius with which I had become acquainted in the heat of the low country, quite at home among the snow, and as common as in, what I should have imagined to be, the more congenial climate. Nor were they any the less lively. Far from exhibiting any sort of torpor from the cold, the first one which I triumphantly seized in my forceps wriggled himself loose and fastened on my finger with a vigour which made me as anxious to get rid of him as I had formerly been to secure him. However, he eventually went into my collecting box.

On the whole, we may say that the Chilopoda are most largely represented in the hotter climates, where they find a more abundant diet in the rich insect life of the tropical and semi-tropical countries. The more brightly-coloured Myriapods, too, are for the most part inhabitants of the warmer countries. The ease with which they are introduced into a country in the earth round plants, and in boxes of fruit, may account to a great extent for the wide distribution of the various species in different countries. Mr. Pocock, who examined the Myriapods brought back from the "Challenger" Expedition, informs us that of ten species brought from Bermuda, four had been introduced from the West Indies. There is no doubt that animals which can bear changes of temperature and deprivation of food, and even a short immersion in the water, are well calculated to be introduced into strange countries in many unexpected ways.

As might be expected from a class of animals so widely distributed, Myriapods show an almost infinite variety of size and colour. We find them so small that we can hardly see them with the naked eye, as in the case of the tiny Polyxenus, the Pauropidae, and the Scolopendrellidae. We also find them more than six inches in length, as the larger species of Scolopendridae. I am afraid we must dismiss as an exaggeration an account of Centipedes in Carthagena a yard in length, and more than six inches in breadth. The giver of this account—Ulloa—informs us that the bite of this gigantic serpent-like creature is mortal if a timely remedy be not applied. It is certainly extremely probable that the bite of a Centipede of this size would be fatal to any one. Some Centipedes are short and broad, and composed of few segments, as Glomeris; some are long and thin, with more than a hundred segments, as Geophilus. They may be beautifully coloured with brilliant streaks of colour, as in some of the Julidae or Polydesmidae, or may be of a dull and rusty iron colour, or quite black.

One of the strangest peculiarities found among Myriapods is that some of them (e.g. Geophilus electricus) are phosphorescent. As I was walking one summer evening near my home in Cambridgeshire I saw what I thought was a match burning. Looking more closely, I saw it move, and thinking it was a glow-worm I picked it up, and was surprised to find that it was a Geophilus shining with a brilliant phosphorescent light. I let it crawl over my hand, and it left a bright trail of light behind it, which lasted some time. I have been told that this species is common in Epping Forest; also in Cambridgeshire.[^[11]]

Besides G. electricus, G. phosphoreus has been described as a luminous species by Linnaeus, on the authority of a Swedish sea captain, who asserted that it dropped from the air, shining like a glow-worm, upon his ship when he was sailing in the Indian Ocean a hundred miles from land.

What the use of this phosphorescence may be is not known with any degree of certainty. It may be either a defence against enemies, or else a means of attracting the two sexes to one another.

The places which the Myriapods select for their habitation vary as much as their colour and size, though, with a few exceptions, they chose dark and obscure places. A curious species of Myriapod is Pseudotremia cavernarum (Cope), which is found in certain caves in America. The peculiar life it leads in these caves seems to have a great influence on its colour, and also affects the development of its eyes. Mr. Packard's account of them is worth quoting: "Four specimens which I collected in Little Wyandotte cave were exactly the same size as those from Great Wyandotte cave. They were white tinged, dusky on the head and fore part of the body. The eyes are black and the eye-patch of the same size and shape, while the antennae are the same.

"Six specimens from Bradford cave, Ind. (which is a small grotto formed by a vertical fissure in the rock, and only 300 to 400 yards deep), showed more variation than those from the two Wyandotte caves. They are of the same size and form, but slightly longer and a little slenderer.... The antennae are much whiter than in those from the Wyandotte caves, and the head and body are paler, more bleached out than most of the Wyandotte specimens.... It thus appears that the body is most bleached and the eyes the most rudimentary in the Bradford cave, the smallest and most accessible, and in which consequently there is the most variation in surroundings, temperature, access of light and changed condition of air. Under such circumstances as these we should naturally expect the most variation."[^[12]]

A strong contrast to these animals is afforded us by the Scutigeridae (Schizotarsia). They are unknown in this country, but abound in some of the Mediterranean countries and in parts of Africa. They remind one strongly of spiders, with their long legs and their peculiar way of running on stones and about the walls of houses.

Fig. 17.—Cermatia (Scutigera) variegata. (From C. L. Koch, Die Myriapoden.)

Some years ago I was in Malta, and I used to go and watch them on the slopes outside Valetta, where they were to be found in great numbers. They used to come out from beneath great stones and run about rapidly on the ground or on the stones and rubbish with which the ground was covered, now and again making a dart at some small insect which tempted them, and seemingly not minding the blazing sun at all. As might be expected from their habits, their eyes, far from being rudimentary, like those of the cave-living Pseudotremia, or absent like those of the Polydesmidae, or of our own Cryptops, are highly developed, and form the only example among the Myriapods of what are known as facetted eyes. The Scutigeridae are also remarkable among Myriapods for the possession of a peculiar sense-organ which is found in no other Myriapod.

The Myriapods most numerous in our own country are Lithobius and Julus. Lithobius, which will be described later on, may be found in almost any garden under dried leaves, stones, etc. Julus, the common wire-worm, is found crawling on plants and leaves and under the bark of trees, and does a good deal of damage in a garden. Polydesmus is also frequently found in great numbers, and usually a great many of them together. Glomeris is also found, though it is not so common as the first two mentioned animals. Geophilus is also common, and especially in the south of England. Scolopendridae are only represented by a single genus, Cryptops, which is not very common, though by no means rare. The best place to find them is in manure heaps. The animals of this species are small compared to most Scolopendras, and have the peculiarity of being without any eyes.

Scutigera is unrepresented in this country. One was found in Scotland some years ago by Mr. Gibson Carmichael, but was shown to have been imported, and not bred in the place.

The means of defence possessed by these animals also differ very much in the different species of Myriapods. In the Centipedes the animals are provided with a powerful weapon in the great poison claws which lie just beneath the mouth, and which are provided with large poison glands, which supply a fluid which runs through a canal in the hard substance of the claw and passes into the wound made by the latter. The effect of this fluid is instantaneous on the small animals which form the food of the Centipedes. I have myself watched Lithobius in this country creep up to a blue-bottle fly and seize it between the poison claws. One powerful nip and the blue-bottle was dead, as if struck by lightning. I have also seen them kill worms and also other Lithobius in the same way. When another Lithobius is wounded by the poison claws it seems to be paralysed behind the wound. The Millepedes, on the other hand, have no such offensive and defensive weapon. They rely for protection on the fluid secreted by the stigmata repugnatoria (or glandulae odoriferae) mentioned before. This fluid has been shown to contain prussic acid, and has a very unpleasant odour.

Fig. 18.—Polyxenus lagurus (From C. L. Koch, Die Myriapoden).

Most of the Millepedes are provided with these glands; but in the cave Myriapods mentioned before, the animals have not to contend against so many adversaries, and these glands almost disappear. Other Myriapods defend themselves by means of the long and stiff bristles with which they are provided, e.g. the little Polyxenus. This means of defence seems to have been more common among the fossil Myriapods than among those still living. Variations in the shape and size of the limbs are numerous, as might be expected in so large a class of animals. One of the most curious of such variations is found in a Centipede of the Scolopendra tribe, called Eucorybas, in which the last limbs are flattened out and provided with paddle-shaped lobes. The use of these is unknown, but it is probable that they are concerned in some way with the breeding habits of the animal. The habits of the Myriapods connected with their breeding are most interesting, but have been very insufficiently investigated. There is no doubt that a full inquiry into all such habits would be of great interest, and would help to answer some of the problems which are still unsolved in these forms. My own observations refer to two forms—Julus terrestris among the Millepedes, and Lithobius forficatus among the Centipedes. Julus terrestris is one of the most common of the English Millepedes, and can be easily obtained. I kept them in large shallow glass vessels with a layer of earth at the bottom, and thus was able easily to watch the whole process. They breed in the months of May, June, and July. The female Julus when about to lay her eggs sets to work to form a kind of nest or receptacle for her eggs. She burrows down into the earth, and at some distance below the surface begins the work. She moistens small bits of earth with the sticky fluid secreted by her salivary glands, which become extraordinarily active in the spring. She works up these bits of earth with her jaws and front legs till they are of a convenient size and shape, and places them together. When complete, the nest is shaped like a hollow sphere, the inside being smooth and even, while the outside is rough and shows the shape of the small knobs of earth of which it is composed. She leaves a small opening in the top. The size of the whole nest is about that of a small nut. When she is ready to lay her eggs she passes them through the hole in the top, and usually lays about 60 to 100 eggs at a time. The eggs, which are very small, are coated with a glutinous fluid which causes them to adhere together. When they are all laid she closes up the aperture with a piece of earth moistened with her saliva; and having thus hermetically sealed the nest, she leaves the whole to its fate. The eggs hatch in about twelve days.

A naturalist named Verloef has lately found that the males of some Julidae undergo certain changes in the form of the legs and other organs in autumn and spring. These changes are probably connected with the breeding of the animal, and remind us of the changes undergone during the breeding season by salmon among the fishes.

Julus breed very readily if carefully attended to and well supplied with food. If they cannot obtain the food they like they will not breed so well. I found that sliced apples with leaves and grass formed the best food for them.

The process in the case of Lithobius is much harder to watch. Lithobius is not so plentiful as Julus terrestris, and the animals are more impatient of captivity, more shy in their habits, and do not breed so readily.

In January 1889 I was given the use of a room in the New Museums at Cambridge, and was allowed to fit it up as I liked, so that I was able to try the effect of different degrees of light and darkness, and of different degrees of warmth. I succeeded in observing the whole process. The female Lithobius is furnished with two small movable hooks at the end of the under surface of the body close to the opening of the oviduct. These small hooks have been observed by many naturalists, but their use has, so far as I know, never been described before. They play an important part in the proceedings following the laying of the egg. The time of breeding in Lithobius is rather later than in Julus, and begins about June and continues till August. There are first of all some convulsive movements of the last segments of the body, and then in about ten minutes the egg appears at the entrance of the oviduct. The egg is a small sphere (about the size of a number five shot), rather larger than that of Julus, and is covered with a sticky slime secreted by the large glands inside the body, usually called the accessory glands. When the egg falls out it is received by the little hooks, and is firmly clasped by them. This is the critical moment in the existence of the Lithobius into which the egg is destined to develop. If a male Lithobius sees the egg he makes a rush at the female, seizes the egg, and at once devours it. All the subsequent proceedings of the female seem to be directed to the frustration of this act of cannibalism. As soon as the egg is firmly clasped in the little hooks she rushes off to a convenient place away from the male, and uses her hooks to roll the egg round and round until it is completely covered by earth, which sticks to it owing to the viscous material with which it is coated; she also employs her hind legs, which have glands on the thighs, to effect her purpose. When the operation is complete the egg resembles a small round ball of mud, and is indistinguishable from the surrounding soil. It is thus safe from the voracious appetite of the male, and she leaves it to its fate. The number of eggs laid is small when compared with the number laid by Julus.

The food in the case of Lithobius consisted of worms and blue-bottles, which were put alive into the glass vessel containing the Lithobius. I tried raw meat chopped up, but they did not thrive on it in the same way that they did on the living animals. I also put into their vessel bits of rotten wood containing larvae of insects, etc.

I have succeeded in bringing back some specimens of Polydesmus alive from Madeira, and in getting them to breed in this country—of course in artificial warmth—and their way of laying eggs and making a nest resembles that of Julus. Geophilus has one curious habit in connexion with the fertilisation of the female. The male spins a web and deposits in the middle of it a single spermatophore, and the female comes to the web to be fertilised. The Scolopendridae are said to bring forth their young alive, but I think the evidence for this is unsatisfactory. What have been taken for the young Scolopendrae are perhaps the large spermatophores of the male, which are not unlike a larval Myriapod in size and shape. I have never been able to observe the process of breeding in this family. I have had the spermatophores sent me from Gibraltar as "eggs," but a little examination soon showed me their real character.

The mode of progression in the Myriapods differs considerably, as might be expected in a class in which the number of legs varies to such an extent. The swiftest among them are the Scutigeridae with their long spider-like legs. The Scolopendridae are also able to move with considerable rapidity, and are also able to move tail forward almost as well as in the ordinary manner. Where there are such a number of legs it becomes a curious question as to the order in which the animal moves them; and though several people have endeavoured to find this out, the number of legs to be moved and the rapid movements have rendered accurate observation impossible.

Some years ago Professor E. Ray Lankester tried to study the order in which the legs of Centipedes moved, and came to the conclusion (recorded in an amusing letter in Nature, 23rd May 1889) that if the animal had to study the question itself, it would not get on at all. He finishes his letter with the following verses:—

A Centipede was happy quite

Until a toad in fun

Said, "Pray which leg moves after which?"

This raised her doubts to such a pitch,

She fell exhausted in the ditch,

Not knowing how to run.

The progression of Millepedes is much slower than that of the Centipedes, and it is remarkable that when the animal is in motion a sort of wave runs down the long fringe-like row of feet. I have endeavoured to make out this motion, but have never been able to understand it satisfactorily. My belief was that the feet were moved in sets of five.

This wave-like peculiarity of motion is described in a curious old book, An Essay towards a Natural History of Serpents. Charles Owen, D.D. London, 1742: "The Ambua, so the natives of Brazil call the Millepedes and the Centipedes, are serpents. Those reptiles of thousand legs bend as they crawl along, and are reckoned very poisonous. In these Multipedes the mechanism of the body is very curious; in their going it is observable that on each side of their bodies every leg has its motion, one regularly after another, so that their legs, being numerous, form a kind of undulation, and thereby communicate to the body a swifter progression than one could imagine where so many short feet are to take so many short steps, that follow one another rolling on like the waves of the sea."

Before proceeding to the classification of Myriapods, which will form the next part of this account, a few words on the common names for them may not be without interest.

In English we have the names Centipede and Millepede, and the Continental nations have similar names implying the possession of a hundred or a thousand legs, as the German "Tausendfüsse" and the French "Millepieds." Of course these are general words, simply implying the possession of a great number of legs. But we have also among the peasantry a name for Centipedes which conveys a much more accurate idea of the number. The people of the eastern counties (I daresay the term is more widely spread) call them "forty legs." This is not quite accurate, but as Lithobius has 17 legs on each side, and Scolopendra (Cryptops is the English species) has 21 on each side, it is a better approximation than Centipede. But another country has a still more accurate term. I found some Scolopendra in Beyrout, and asked my native servant what he called them. He gave them what I afterwards found was the common Arab name for them, "‘arba wál ‘arbarin," forty-four legs. Now the Scolopendras, which in hotter climates are the chief representatives of the Centipedes, have actually forty-two legs, or, if the poison claws are counted, forty-four. In looking up the Arab term for Centipede I came across a curious description given of them by Avicenna, the great Arabian physician: "This is an animal known for its habit of going into ears. For the most part it is a palm's length" [about four inches, which is the average length of many species]. "On each side of the body it has twenty-two feet, and moves equally well either backwards or forwards."

With regard to its alleged habit of going into ears, the learned Arabian has evidently made a false imputation on the character of our animal, and has probably relied too much on the stories told him. He has also exaggerated in stating that it goes equally well either backwards or forwards. Some Centipedes can go backwards very easily and well, though not so well as forwards. Perhaps he preferred examining dead specimens, which afford an easy opportunity of counting their legs, to experimenting with living animals, which might have resented liberties taken with them.

The Persians have several words for them, less accurate than the Arabs and more like our own terms. For instance, they call them "Hazarpa," or thousand feet, like our Millepedes; also "Sadpa," or hundred feet, equivalent to our Centipedes. Another term resembles our common term before mentioned, "Chehlpa," forty feet. A more figurative term is "tasbih dud," a worm resembling a rosary with a hundred beads; this word is translated in Richardson's Persian Dictionary as "a venomous insect having eight feet and a piked tail."

Classification of the Myriapoda.

Two of the principal writers on the classification of the Myriapods are Koch and Latzel, both of whom have classified the whole group. I do not wish for a moment to undervalue the many authors who have done excellent work on the classification of different groups and families, but I wish here to give an outline of a classification of the whole class, and I naturally have recourse to the authors who have treated the subject as a whole.

Koch's two works, the System der Myriapoden[^[13]] and Die Myriapoden,[^[14]] cover the whole range of the class, and his divisions are clearly marked out and are easily understood, but both works are comparatively old. He does not include the Scolopendrellidae or the Pauropidae, which are now included by all naturalists in the Myriapoda. Latzel is a more recent writer, and though his work is entitled The Myriapods of the Austro-Hungarian Empire,[^[15]] he gives much information about Myriapods not found in Europe, and his work is fairly entitled to be considered as embracing the whole class. He divides the Myriapods into four Orders, including the Scolopendrellidae and Pauropidae. On the whole, I think it will be better here to take the classification of Koch, and to add to it the two Orders before mentioned, viz. Symphyla containing one family the Scolopendrellidae, and Pauropoda with one family the Pauropidae.

The Orders are as follows:—

Order I. Chilognatha (= Diplopoda)

Antennae 7 joints, three anterior body rings with one pair of legs to each ring. Posterior rings with two pairs of legs to each. Genital organs opening ventrally on the anterior rings of the posterior part of the body, i.e. on one of the anterior of the segments bearing two pairs of legs; usually the 7th.

This Order is divided into eight families:—

Family 1. Polyxenidae.

Ten body rings, not counting the neck-plate. Thirteen pairs of limbs. Eyes hard to find, on the lateral corner of the head (Fig. 18, p. [37]).

Family 2. Glomeridae.

11 body rings. 17 pairs of legs. Eyes arranged in a row curved outwards.

Fig. 19.—Glomeris marginata. (From C. L. Koch, Die Myriapoden.)

Family 3. Sphaerotheriidae.

12 body rings. 19 pairs of legs. Eyes crowded together in a cluster.

Fig. 20.—Sphaerotherium grossum. (From C. L. Koch, Die Myriapoden.)

Family 4. Julidae.

Body cylindrical. More than 30 body rings. Many eyes crowded together in a cluster.

Fig. 21.—Julus nemorensis. (From C. L. Koch, Die Myriapoden.)

Family 5. Blanjulidae.

Thin cylindrical body with more than 30 body rings. Eyes either absent or in a simple row beneath the edge of the forehead.

Fig. 22.—Blanjulus guttulatus. (From C. L. Koch, Die Myriapoden.)

Family 6. Chordeumidae.

Resemble the Polydesmidae (Fam. 7), but the head is longer and less rounded in the forehead. The antennae are placed more at the side of the head. Eyes small and numerous, in a cluster. Body rings always 30 (Fig. 16).

Family 7. Polydesmidae.

Body cylindrical, with a lobe or keel on the posterior part of the upper surface of the body ring. Always 19 body rings. No eyes.

Fig. 23.—Polydesmus collaris. (From C. L. Koch, Die Myriapoden.)

Family 8. Polyzoniidae.

Body with varying number of rings arched transversely downwards and sharp at the sides. The anterior part of the ring somewhat hidden. The eyes in a simple row. The stigmata very small and placed near the lateral corner of the body ring. Head small in proportion to the body.

Fig. 24.—Polyzonium germanicum. (From C. L. Koch, Die Myriapoden.)

Order II. Chilopoda (or Syngnatha).

Antennae with many joints, at least 14. Only one pair of legs to each body ring. The genital opening on the last ring of the body. Bases of the legs widely separate.

There are four families in this Order:—

Family 1. Lithobiidae.

Body with 9 principal and 6 subsidiary rings. On both principal and subsidiary rings one pair of legs, except on the last ring of the body. Many eyes; the posterior ones large and kidney-shaped. The antennae with many rings.

Fig. 25.—Lithobius erythrocephalus. (From C. L. Koch, Die Myriapoden.)

Family 2. Scolopendridae.

Body with 21 or 23 rings, no intermediate rings. Every ring with one pair of legs. The last pair very long. Last pair at the point of the last ring. Four or no eyes. Antennae with 17 or 20 joints. (Fig. 15, p. [31]).

Family 3. Notophilidae.

Fig. 26.—Notophilus taeniatus. (From C. L. Koch, Die Myriapoden.)

Body very long, 200 to 350 rings; alternate principal and subsidiary rings. A pair of legs to each principal ring. No eyes. Maxillary palps very thick. Compact or very short limbs. The terminal point of the last limb without claws.

Family 4. Geophilidae.

Body long, 80 to 180 rings, principal and subsidiary. No eyes. The maxillary palps not compact, and with first joint large. Last joint of the last pair of legs with a sharp claw.

Fig. 27.—Geophilus longicornis. (From C. L. Koch, Die Myriapoden.)

Order III. Schizotarsia.

The tarsi of all the legs multiarticulate. The eyes facetted. Peculiar sense organ beneath the head.

Family 1. Cermatiidae (Scutigeridae)

Antennae with unequal number of joints. Body rings, each with one pair of legs. Dorsal scutes not so large as ventral. Limbs long and multiarticulate. (Fig. 17, p. [35]).

Order IV. Symphyla.

Myriapods resembling Thysanura. A pair of limbs to each segment. The antennae are simple and multiarticulate with unequal joints. Eyes few. Mandibles short. One pair of maxillae. No maxillipedes. Genital orifice in the last segment of the body. A single pair of tracheae. Two abdominal glands on the posterior part of the body. Two caudal appendages. Free dorsal scutes. Ventral scutes often with parapodia.

Family 1. Scolopendrellidae.

With the characters of the Order.

Order V. Pauropoda.

A pair of limbs to each segment. Antennae branched. Eyes few or none. Labrum and labium indistinct. Genital orifice at the base of the second pair of limbs. Free dorsal scutes. Nine pairs of feet (always?). Some segments with sensitive hairs. Last segment the smallest.

Family 1. Pauropidae.

Body slender. Dorsal scutes smooth. Limbs long and projecting from the lateral margins of the body. Colour pale.

The Structure of the Myriapoda.

Having now given a short view of the classification of the Class, I will proceed to give a general account of their structure, the variations in which have led to the divisions into the various Orders and Families. Their structure shows resemblances to several widely different classes of animals. One cannot help being impressed with their likeness to the Worms, at the same time they have affinities with the Crustaceans, and still more with the Insects. In the latter class the likeness of the Thysanuridae to Scolopendrella and Pauropus have induced a celebrated Italian anatomist, Professor Grassi, to claim the former as the ancestors of the Myriapoda.

Myriapods have a body which is segmented, as it is termed; that is, composed of a number of more or less similar parts or segments joined together.

One of the most important characteristics which distinguish Myriapods from other Arthropoda is the fact that they possess on the posterior segments of the body true legs which are jointed and take part in locomotion. The head is in all cases quite distinct from the body, and may be regarded as a number of segments fused together into one mass. Their heads are always provided with a single pair of antennae and mouth appendages, consisting of an upper lip, a pair of mandibles or jaws, and one to two pairs of maxillae. The mandibles resemble those of Insects, and are strongly toothed. In the Chilognatha a pair of maxillae are fused so as to form a single oval appendage. In the Chilopoda they each consist of a single blade bearing a short palp or feeler. The mouth parts may have the forms known as chewing, biting, or suctorial (Polyzonium) mouth appendages.

With the exception of the terminal segment, and in many cases the first or the seventh, each segment bears one or two pairs of limbs. These may be very long, as in Scutigera, or very short, as in Polyxenus. They may be attached close to one another near the ventral middle line of the body, or may have their bases far apart from each other, as in the Chilopoda. The exoskeleton or external armour is composed of chitin (Chilopoda) or of chitin with calcareous salts deposited in it (Chilognatha).

Their internal structure has a great likeness to that of Insects.

The general position of the internal organs may be seen from Fig. 28, which shows a Lithobius dissected so as to exhibit the digestive and nervous systems.

The digestive canal, which is a straight tube, extends throughout the whole length of the body, and terminates in the last segment of the body. It may be divided into the following parts:—

1. A narrow oesophagus, beginning with the mouth or buccal cavity, and receiving the contents of two or more salivary glands (d).

2. A wide mesenteron or mid-gut (n) extending throughout almost the whole length of the body.

3. A rectum which at its junction with the mid-gut receives the contents of two or four Malpighian tubes (g, h) which function as kidneys. Their function was for a long time unknown, but the discovery of crystals of uric acid in them placed the matter beyond doubt.

The heart has the form of a long pulsating dorsal vessel which extends through the whole length of the animal. It is divided into a number of chambers, which are attached to the dorsal wall of the body, and are furnished with muscles of a wing-like shape, which are known as the alary muscles, and which govern its pulsations. The chambers are furnished with valves and arteries for the exit of the blood, and slits known as ostia for the return of the blood to the heart. The blood enters the chambers of the heart from the body cavity through the ostia, and passes out through the arteries to circulate through the organs of the body and to return by the ostia.

Fig. 28.—Lithobius dissected. (After Vogt and Yung.) a, antennae. b, poison claws. c, brain. d, salivary glands. e, legs. f, nerve cord. g, Malpighian tube. h, Malpighian tube. i, vesicula seminalis. j, accessory gland. k, accessory gland. l, testis. m, thigh gland. n, digestive tube.

The two figures below (Figs. 29 and 30) show the position of the arteries and the ostia in a single segment of the body. The heart is too small and delicate to be seen with the naked eye; it therefore requires the aid of the microscope. A freshly-killed animal was therefore taken and prepared in the manner known to all microscopists, and extremely thin slices or sections cut horizontally from its back. One of these sections cut the whole length of the heart in one segment, which was accordingly drawn under the microscope (Fig. 29), and shows a longitudinal horizontal section through the whole length of the heart in a single segment, with the two ostia at each end of the segment and the two arteries in the middle.

The arteries, when they leave the body, pass into masses of fatty tissue on either side of the heart, and the other figure (Fig. 30) is intended to show the artery leaving the heart and penetrating into the fatty tissue. The figure is taken from the same section as the former one, but is much more highly magnified, so as to show more detail. The delicate coats of the heart are shown, the artery being covered with a clothing of large cells.

Fig. 29.—Heart of Julus terrestris showing ostia (ost) and arteries (Art) magnified.

Fig. 30.—Heart of Julus terrestris showing structure of artery (Art.) and external coat of heart (ext.c), also fat body (Fb), highly magnified. Ht, The cavity of the heart. The circular muscle fibres which surrounds the heart are shown just below the external coat (ext.c). ogl, Oil globules of the fat body.

Myriapods breathe by means of tracheae, with the exception of the Scutigeridae, which have an elementary form of lung which resembles that of spiders, and will be mentioned further on. These tracheae, as in Insects, are tubes lined with chitin, which is arranged in spiral bands. The tracheae open to the exterior by openings called stigmata, through which they receive the external air, which passes into the main tracheal tubes and into their ramifications, and thus effects the aeration of the blood.

The nervous system of the Myriapods consists, as in Insects, of a brain, which may be more or less developed, a circumoesophageal ring embracing the oesophagus, and a ventral chain of ganglia, and in some cases (Newport) of a system of visceral nerves. With the nervous system we may mention the sense organs, the eyes, which are present in most cases, though wanting, as has been already stated, in many groups. They are usually present as clusters of ocelli or eye spots closely packed together, or (in Scutigera) as peculiarly formed facetted eyes. The sensory hairs on the antennae must be reckoned as sense organs, as also the tufts of sense hairs on the head of Polyxenus. Scutigera has also a peculiar sense organ beneath the head, consisting of a sac opening on the under side of the head full of slender hairs, each of which is connected at its base with a nerve fibre. Except the eyes, the Myriapod sense organs have usually the form of hairs or groups of hairs connected with nerve fibres, which communicate with the central nervous system.

Fig. 31.—Under side of the head of Scutigera coleoptrata, with sense organ. eo, Opening of sense organ to the exterior; o, sense organ shown through the chitin; m, mouth; oc, eye; mxl, maxilla; f, furrow in the chitin. (Heathcote, Sense organ in Scutigera coleoptrata.)

Fig. 32.—Highly magnified section through head of Polyxenus lagurus, showing sense organ. ext.cut, external cuticle; t, tube surrounding base of sense hair; gang.c, ganglion cell. (Heathcote, Anatomy of Polyxenus lagurus.)

These two sense organs are shown in Figs. 31 and 32. Fig. 31 shows the under side of the head of Scutigera (Fig. 17), with the position of the sense organ and its opening. Fig. 32 is part of a section through the head of Polyxenus with two of the sense hairs. Each spine or sense hair fits into a cup in the chitin of the head; and the lower or internal part, which is divided from the upper or external part by a rim, is joined to a ganglionic nerve cell (gang.c.).

The Myriapods are of separate sexes, and the generative organs in both cases usually have the form of a long unpaired tube, which in the male is connected with accessory glands, and in the female is usually provided with double receptacula seminis. The generative openings usually lie near the base of the second pair of legs (Chilognatha), or at the posterior end of the body (Chilopoda). In the Chilognatha there is usually in the male an external copulatory organ at the base of the seventh pair of legs, remote from the genital opening.

The preceding account of the anatomy of the Myriapods has shown us the general characteristics of the whole group. I shall now take each of the five Orders into which the class is divided in the classification adopted in this account, and endeavour to explain the differences in anatomy which have led to the establishment of the Order. The first Order with which we have to do is that of the Chilognatha, which includes a large number of Myriapods; no less than eight families, some of them including a great number of forms.

Order I. Chilognatha.

The Chilognatha differ from other Orders in the shape of the body. This is in almost all cases, cylindrical or sub-cylindrical, instead of being more or less flattened as in the other Orders.

The body, as in all other Myriapods, is composed of segments, but in the Chilognatha these segments are composed, in almost all cases, of a complete ring of the substance of which the exoskeleton (as the shell of the animal is called) is composed. This substance is in the case of the Chilognatha chitin (a kind of horny substance, resembling, for instance, the outer case of a beetle's wing), containing a quantity of chalk salts and colouring matter; the substance thus formed is hard and tough. In other Orders the chitin of the exoskeleton is without chalky matter and is much more flexible. The length of the body, as may be seen from the classification, may be either very long, as in Julus, or very short, as in Glomeris.

The next anatomical character distinctive of the Order is the form of the appendages. First, the antennae. These are, as a general rule, much shorter than in the Chilopods, never reaching the length of half the body. They are, as a rule, club-shaped, the terminal half being thicker than the half adjoining the body.

The next appendages to be mentioned are the mouth parts. These differ in form from those of the other Orders, and their differences are connected very largely with the fact that the Chilognatha live on vegetable substances. Their mouth parts are adapted for chewing, except in the case of the Polyzoniidae, the eighth family of the Order, in which, according to Brandt, the mouth parts are adapted for sucking, and are prolonged into a kind of proboscis. The mouth parts of the Chilognatha consist of—

(1) An upper lip. A transversely-placed plate, which is fused with the rest of the head.

(2) A pair of powerful mandibles or jaws adapted for mastication, and moved by powerful muscles. f and g in Fig. 33 shows these mandibles, while the rest of the figure constitutes the broad plate (No. 3).

(3) A broad plate covering the under part of the head and partially enclosing the mouth. This structure, which, as we shall afterwards see, is formed by the fusion of two appendages which are distinct in the animal when just hatched, has been called the deutomalae, the jaws receiving the name of protomalae.

Fig. 33.—Mouth parts of Chilognatha. (From C. L. Koch, System der Myriapoden.) f and g, The mandibles. The parts marked a, b, c, d, e are firmly united and constitute the broad plate No. 3. They have received the following names—a, b, Internal stipes; c, external stipes; d, malellae; e, hypostoma.

After the mouth parts we come to the legs. We first notice the fact that the bases of the legs in each pair are closely approached to one another. They are so set into the body that the basal joints, or, as they are called, the coxal joints, nearly touch. This is the case in almost all Chilognatha, except in the Polyxenidae, and it is a fact connected with some important features in the internal anatomy. Then we have the peculiarity in the Chilognatha which has formed the basis of most classifications which have placed these animals in a group by themselves. This is the possession in most segments of two pairs of legs. This characteristic has caused the group to be called by some naturalists Diplopoda. As a general rule, the first four segments have only three pairs of legs between them, one of them being without a pair of legs. This legless or apodal segment is usually the third. From the fifth segment to the end of the body all the segments have two pairs of legs each. The legs are shorter than those of the Chilopods, and are all nearly equal in size. This is not the case in the other Orders. The legs are commonly wanting in the seventh segment of the male, and are replaced by a copulatory organ. This peculiarity is related to the different position of the generative openings in the Chilognatha. Another anatomical feature peculiar to the Chilognatha is the possession of the stink glands—the glandulae odoriferae before mentioned. This, however, is a character which does not hold for all the Chilognatha, since the Polyxenidae have none of these glands. All the other families, however, possess them, and they are present in none of the other Orders.

As regards the internal anatomy of the Chilognatha, the digestive canal differs mainly in the glands which supply it with secretions. It receives the saliva from two long tubular salivary glands, which open at the base of the four-lobed plate which has been mentioned as the third of the mouth appendages. The secretion of these glands is used, as has already been said, in the process of preparing the nest for the eggs. We cannot fail to be reminded of a similar function of salivary glands in those swallows, which prepare the nests of which bird's-nest soup is made with the secretion of the salivary glands. Another feature in the form of the digestive tube is that in many cases, if not in all, it is marked with constrictions which correspond with the segments of the body.

The heart in the Chilognatha is not such a highly developed organ as in the other Orders. The muscles which have already been mentioned as the alary muscles (or wing-shaped muscles) are not so highly developed, and consist for the most part of a few muscular fibres. The muscular walls of the heart, which consist of three layers, have the muscles less strongly developed, and are in general adapted for a less energetic circulation.

The tracheae, which open into the stigmata, as has already been said, branch into tufts of fine tubes, but the ramifications of these tufts never join (or anastomose, as it is called), and consequently we never get, as in the other Orders, long tracheal trunks running along the body.

The nervous system, in addition to the existence of the visceral nerve system described by Newport, shows a marked peculiarity in the form of the ventral ganglionic chain. As has already been said, the nerve system consists of a brain or mass of ganglia fused together and connected with the ventral nervous cord by a collar of nervous substance surrounding the oesophagus, and generally known as the circumoesophageal collar. The ventral nerve cord is a stout cord of nervous substance passing along the whole length of the animal, and situated below (or ventral to) the digestive tube and the generative system. This cord is enlarged at certain points, and these enlargements or swellings are called ganglia, while from the ganglia pass off nerves which supply the different organs of the body. In the Chilognatha the cord has a compressed appearance as if the ganglia were pressed into one another in such a way that it is very hard to distinguish any ganglia at all. If we use the microscope and examine sections cut transversely through the cord, we see that it is not a simple cord. Even if we examine the nerve cord with a simple lens, we see that a furrow runs longitudinally down it, and the use of the compound microscope shows us that this furrow represents a division into two cords in such a way that the single stout cord as it appeared to the naked eye is in reality two cords running side by side, and so compressed together that the substance is partly fused together. The ganglia too are double, being swellings of the two cords and not a single enlargement on a single cord. As we shall see in the other Orders, this arrangement constitutes a characteristic distinction.

The generative organs consist of a long tubular ovary or testis lying along almost the whole length of the body and placed between the digestive organ and the nervous system. Near its exit from the body the long tube divides into two short tubes, the oviducts in the female or the vasa deferentia in the male. These ducts open in the third segment of the body, unlike those of Myriapods belonging to other Orders. The accessory glands present in most other Myriapods are not present in the Chilognatha.

The general arrangement of the organs of the Chilognatha may be seen from Fig. 34, which represents a transverse section through the body of Polyxenus (Fig. 18). A comparison of these two figures (Figs. 34 and 18) will show the position of the organs mentioned in this account. The heart is shown with the suspensory and alary muscles attached.

Fig. 34.—Transverse section through Polyxenus lagurus: g.n.c, f.n.c, ganglionic and fibrous parts of nerve cord; Rec.sen, receptaculum seminis; ori.dct, oviduct; Spmzoa, spermatoza. (From Heathcote, Anatomy of Polyxenus lagurus.)

Order II. Chilopoda.

The shape of the body differs from that of the Order which has been just described (Chilognatha), inasmuch as it is not cylindrical but flattened, the back, however, being more arched than the ventral surface. In this respect, however, it cannot be said to differ from the other Orders which we have yet to describe.

The segments are not formed by a single ring of the exoskeleton, which in this Order is formed of chitin, and is tough and flexible rather than hard and strong; but of two or three plates which form a covering to the segment. The back is covered by a large plate known as the tergum, the sides by two plates known as pleura, and the ventral part by a plate called the sternum. The pleura and sternum are, however, in most cases fused together or indistinguishable. In this, as in most of the anatomical peculiarities, there is a much greater difference between the two Orders Chilopoda and Chilognatha than between the Chilopoda and the other three Orders which have still to be described.

The Chilopoda have only one pair of appendages to each segment of the body instead of two pairs like the Chilognatha.

The antennae of the Chilopoda are as a rule very long, and are always longer than in the Chilognatha which we have just described. They differ from those of the Schizotarsia (the third Order, which will be the next to be described) in having the basal joints nearer together; in other words, they are differently placed on the head. They differ from those of the Pauropoda (the fifth Order) in being straight and not branched. As a rule the antennae of the Chilopoda taper towards the extremity.

Fig. 35.—Mouth parts of Lithobius (Latzel). A, Head of Lithobius seen from the under surface after removal of poison claws; a, second maxilla; b, c, the two shafts of the first maxilla. B, One of the mandibles. C, The two poison claws.

The mouth parts are more numerous than in the Order we have just described (the Chilognatha). They consist of—

1. An upper lip. This is a transverse plate as just described in the case of the Chilognatha, but it is not always fused with the rest of the head. It is also usually composed of three pieces, two lateral and a middle piece.

2. A pair of jaws or mandibles, which are not of so simple a form as those of the Chilognatha, but rather resemble those of some of the Crustacea.

3 and 4. Two pairs of appendages called maxillae resembling feet, but used to aid the act of eating instead of locomotion. They are very different in different Chilopods, but are mostly slender and weak and usually provided with feelers (or palps) growing out of the main stem.

5. The next pair of appendages are the first pair of the legs of the body, which are also metamorphosed to serve a function different from the ambulatory function of the other limbs. These are the poison claws, and the possession of these forms another distinction between the Order we are now discussing and that of the Chilognatha. At the same time the third Order, that of the Schizotarsia, has poison claws, so that this feature does not separate the Chilopoda from all the other Orders. These poison claws are large curved claws connected with poison glands, the secretion of which flows through a canal which opens near the point.

The legs are longer than those of the Chilognatha, but not so long as those in the next Order to be described (the Schizotarsia). Their number is very various, from 15 pairs in Lithobius to 173 in the Geophilidae. Latzel notes a curious point in the number of the legs in this Order, namely, the number of pairs of legs is always an uneven one. There are always one pair to each segment. The last pair of legs is always longer than the other pairs, and this is a peculiarity of the Order.

The digestive tube resembles that of the other Orders, but the salivary glands are not long and tubular but short (Fig. 28, d). It is, moreover, not marked with constrictions corresponding with the segments of the body.

The tracheal system or the system of respiration may be said to be more highly developed in this Order than in any other. The tracheal branches anastomose with one another (that is, the branches join), and in some cases form long tracheal stems running along the body almost for its whole length. The number of the tracheal openings or stigmata varies and does not correspond with the number of segments.

The nervous system differs considerably from that in the Order Chilognatha; it resembles that in the Schizotarsia, and differs again from that in the other two Orders, Symphyla and Pauropoda. The brain shows some differences from other Orders chiefly in the development of the different lobes which are connected with the sense organs, the eyes and antennae, for instance; but the most marked difference is in the ventral ganglionic cord. First, the ganglionic swellings are much more clearly marked than in the Chilognatha. Secondly, the first three ganglia differ from the others in being nearer to one another and forming a single mass when seen by the naked eye, though when examined by the aid of a microscope we can see all the different parts are there. Thirdly, the division into two cords mentioned in the Chilognatha is carried to a much greater extent. The ganglia in each segment can be seen plainly to be double, and the cords connecting the ganglia are two in number. We can plainly see that the ventral nervous system of the Chilopoda consists of two cords lying parallel to one another, and each having a ganglionic enlargement in every segment. Whether a visceral nervous system is present in the group is doubtful.

The eighth family of the Chilognatha, the Polyxenidae, show an approach to the Chilopod nervous system.

The generative system differs chiefly in the opening of the genital apparatus at the end of the body instead of in the third segment; though this difference only separates the Order from the Chilognatha and not from the other Orders. They also have two pairs of large accessory glands (as they are called) connected with the genital openings.

Order III. Schizotarsia.

The third Order of Myriapods, the Schizotarsia, show a much greater resemblance to the Chilopoda than to the first Order, the Chilognatha. There are, however, important differences to distinguish them from all the other Orders.

The shape of the body is short, thick, and very compact. The composition of the individual segments resembles that found in Chilopoda rather than that of Chilognatha.

The antennae are very long, longer than in any of the Chilopods, and are composed of a great number of very small joints. The mouth parts show a greater length and slenderness than do those of the other Orders mentioned as yet. They consist of—

1. An upper lip partly free, but fused at the sides with the rest of the head. The upper lip is in three parts, as in the Chilopoda, but with the middle part very small and the lateral pieces large.

2. A pair of jaws or mandibles. These are provided not only with teeth, as in the other Myriapods, but also with a sort of comb of stiff bristles.

3 and 4. Two pairs of maxillae or foot jaws distinguished by their length and slenderness.

5. The poison claws long, slender, and not sharply curved. The bases of the poison claws hardly fused together and short.

The respiratory system in the Schizotarsia differs from that in all other Myriapods in the fact before mentioned, that they breathe by means of lungs and not by tracheae. There are, as before mentioned, eight dorsal scales in these animals; each dorsal scale except the last bears one of the peculiar organs which I have called lungs. At the hinder end of the scale there is a slit which leads into an air sac, from which a number of short tubes project into the blood in the space round the heart and serve to aerate it before it enters the heart. The heart, therefore, sends aerated blood to the organs, while in the tracheal-breathing Myriapods the blood is aerated in the organs themselves by means of tracheae.

The poison claws are followed by segments bearing fifteen pairs of true ambulatory legs. These are covered by eight large dorsal plates, increasing in size from before to the middle of the body, the middle plate being the largest, and then diminishing in size.

The nervous system resembles that of the Chilopoda, but there is a special pair of nerves which supply the sense organ, which has been mentioned as peculiar to the Order. The ventral nerve cord shows a very clear division into two, the ganglia of the two cords being almost entirely separate. The first few ganglia are fused, as has been mentioned in the Chilopoda.

The digestive tube resembles that of the Chilopoda. The legs are very long and slender, and the joints are beset with bristles. Both sexes have small hook-like appendages at the sides of the genital openings.

The eyes have already been mentioned as being more highly developed than in other classes, in correspondence with the more active habits of the animal. The generative organs open at the hind end of the body, as in Chilopoda.

The heart is highly developed, quite as much so as the Chilopod heart, the alary muscles being strong and broad, and the arteries being quite as perfect as those in any Myriapod. The muscular coats which govern the pulsations by their contractions are powerful and well developed.

Order IV. Symphyla.

We next come to one of the last two Orders which have been recently added to the Myriapoda. These little animals have a great resemblance to the Thysanura among the Insects, and especially to Campodea among the Thysanura. It will be well, therefore, to begin our account with a few of the reasons which have induced naturalists to include them among the Myriapods rather than among the Thysanura.

1. Campodea has three pairs of mouth appendages, while Scolopendrella has only two.

2. Scolopendrella has broad plates covering the back, not only on the anterior (thoracic) segments, but on the whole body.

3. The terminal appendages of Scolopendrella differ from those in Campodea.

4. Scolopendrella has a sense organ which is absent in Campodea.

5. Campodea breathes by means of three stigmata in the anterior part of the body. The stigmata of Scolopendrella are hard to see, and are not in the same position.

6. Scolopendrella has twelve pairs of legs, and Campodea, like all Insects, has only three.

I will now go on to an account of their anatomy. The body is small and slender, and is covered with a delicate shell or exoskeleton of chitin, which is so thin as to be almost transparent.

The antennae are long, and are composed of many joints of equal size.

The mouth parts consist of—

1. An upper lip.

2. A pair of mandibles.

3. A pair of maxillae.

The segments are not all of equal size. Some are larger than others. The larger and smaller segments are arranged alternately, and the smaller do not bear legs. As before stated, there are twelve leg-bearing segments.

At the end of the body there are two hook-like appendages which are pierced by a canal, through which is poured the secretion of a pair of glands. Near the sides of these appendages are a pair of sense organs, consisting of long hairs connected with nerves.

The digestive canal is a long straight tube passing through the length of the body. In the middle it is much enlarged, so as to form a stomach with a glandular coat. Posterior to the stomach the digestive tube receives the contents of two Malpighian tubes which act as kidneys.

The tracheal system consists of a single pair of stigmata on the under surface of the head, and the tracheae connected with them.

Order V. Pauropoda.

The Pauropoda, which form the fifth Order of Myriapods, are as yet very imperfectly known. Pauropus was discovered by Sir John Lubbock, and its discovery was announced by him in 1866. He found this little Centipede in his kitchen garden among some Thysanura, and at first considered it as a larval form, but continued observation showed that it was a mature creature. He described it as a small, white, bustling, intelligent little creature about 1⁄25 inch in length.

The antennae are very curious and highly characteristic of the Order. They resemble those of Crustacea rather than those of Myriapoda. Each antenna is composed in the following manner. First there is a shaft of four joints. From the fourth joint of this shaft spring two branches; one of these two branches is narrower than the other, and ends in a long thin bristle composed of a great number of joints. The other and broader branch bears two such bristles, and between them a small pear-shaped or globular body, the function of which is unknown.

The mouth parts consist of two minute pairs of appendages, the anterior pair toothed and the posterior pointed. The body is rather narrower in front; the segment behind the head has one pair of legs, the second, third, fourth, and fifth behind the head two each. The posterior legs are the longest; the genital organs open at the base of the second pair of legs, between these and the third pair. The manner of breathing is as yet unknown, tracheae not having been discovered.

Pauropus at first looks most like a Chilopod, but differs from that Order—

1. In the form of the antennae.

2. In the absence of poison claws and in the form of the mouth parts.

3. The opening of the generative organs being in the front part of the body.

It differs from Chilognatha in the following respects:—

1. The legs are not of equal length, the posterior legs being the longest, as in Chilopods.

2. The mouth parts differ from those of Chilognaths almost as much as from those of Chilopods.

3. The form of the antennae.

Only a few Pauropoda have been discovered as yet.

Embryology.

The preceding account of the anatomy of Myriapods would be incomplete without some reference to the wonderful manner in which the different organs of the body are built up; the whole of the complex organism proceeding by a gradual and regulated process of development from a simple cell called the ovum derived from the female body, and united with a cell from the male body (called the spermatozoon). I hope to be able to give my readers some idea of the interest which the pursuit of the difficult study of embryology adds to anatomy, by offering us a key to the interpretation of the relations between our knowledge of the forms at present living on the earth and those which, we learn from Palaeontology, have inhabited our planet in past ages.

Fig. 36.—Young ovum of Julus terrestris: nucl, nucleolus; nu, nucleus; R, first appearance of yolk; F, follicle cells.

Like all living creatures with which we are acquainted, the starting-point of Myriapod life is the ovum, as it is called. This ovum is a cell resembling the cells of which the body of all living animals are built up, and which may be compared to the bricks of which a building is composed. This cell or ovum is a small sphere of living transparent substance called protoplasm, and it is nucleated—that is, it contains a small spot of denser protoplasm called the nucleus, and within that a still smaller spot of still more dense protoplasm called the nucleolus. In the process of impregnation the ovum unites with the male cell, and the cell so formed is called the impregnated ovum. This ovum has the property of dividing into two cells, each resembling the parent cell from which it is derived; each of these cells has, like the parent cell, the same property of dividing into two more, and so on. Thus from this continual process of division or reproduction of every living cell, the materials are provided for the building up of the body.

The regularity of the process of the division of the ovum, or, as it is called, segmentation of the ovum, is interfered with by the presence of food yolk. The cells formed by the process of cell division just described need nourishment, and this nourishment is supplied to them by the food yolk formed in the body of the ovum before the process of segmentation begins. It is easy to understand that this yolk, which is not alive like the cells, cannot divide like them, and therefore the segmentation of the ovum in Myriapods is irregular, as it is called.

Fig. 37.—Later stage: nu, nucleolus; c.p, nucleus; y.sp, yolk spherules; ch, shell.

I will now go back a little and describe what happens to the ovum before the process of segmentation is complete. It increases in size and forms the supply of food yolk which is to provide the nutriment of the ovum. Then after impregnation the egg-shell is formed round it, and it becomes what we know as the egg. This egg is not a perfect sphere, but is oval (in most Myriapods) in shape. The egg is laid, and the process of segmentation begins shortly after it is laid, as has already been described.

When it has been laid for about 36 hours, if we take an egg and, after proper preparation, cut it into thin slices known to microscopists by the name of sections, and examine it by means of the microscope, we shall see that segmentation has resulted in this. Just beneath the egg-shell there is a thin layer of cells, one cell thick, which completely surrounds the egg. Inside this coat of cells is the food yolk, with a few cells scattered about in it at rare intervals, something like the raisins in a plum-pudding.

With the next process the formation of the young Myriapod may be said to begin. A strip along the length of the oval-shaped egg is thickened, and this thick mass of cells represents the future ventral surface of the animal. The rest of the thin layer of cells already mentioned just below the shell will form the shell or exoskeleton of the future animal. The thick strip of cells at the ventral surface has by this time split into layers, so that, resorting to our microscope again, a section through the short axis of the oval-shaped egg—a transverse section—will show us—

1. The egg-shell.

2. A layer of cells completely surrounding the egg, thin everywhere but on the ventral surface. This layer is known to embryologists as the epiblast. The thick part of the epiblast on the ventral surface gives rise to the nervous system.

3 and 4. Two layers of cells connected in the middle, along the line of the thick strip, but separate elsewhere, and not extending round the whole of the inside. These layers constitute what is known as the mesoblast, and give rise to the muscles and most of the internal organs.

5. The scattered cells in the yolk. They are known as the hypoblast and give rise to the digestive canal.

After this point is reached the formation of the organs begins. The segments are formed in order from before backwards. First the head, then the next segment, and so on. When the number of segments with which the animal will be hatched are formed, another process begins, and the tail end of the animal, which can already be distinguished, is bent towards the head. This is a process that takes place in many animals besides Myriapods, and is called the formation of the ventral flexure. Shortly after this the animal bursts the shell and comes into the outer world. The various processes may be understood by reference to the Figs. 36, 37, 38, 39, which are successive stages in the development of a Chilognath. Figs. 37, 38, are thin slices through the shorter diameter of the egg, which, as before mentioned, is an oval in shape. Fig. 39 is a section through the longer diameter of an egg in a more advanced stage of development, in fact just about to burst the shell. The body of the future animal is marked by constrictions, the future segments. Some of the organs are already formed, as the brain and the digestive tube, the openings of which will form the mouth (st) and the anus (pr).

Fig. 38.—Transverse section through next stage: mk, keel-like mass of cells from which the mesoblast is produced; ec, epiblast. (From Heathcote, Post. Emb. Dev. of Julus terrestris; Phil. Trans. vol. 179, 1888, B.)

Fig. 39.—Longitudinal section through later stage: Segs. 2, 3, etc., segments; Ceph. Seg, head; mes, mesoblast; en, hypoblast; st, future mouth; pr, future anus; mesen, gut; mem.ex, as in Fig. 41. (From Heathcote, Post. Emb. Dev. of Julus terrestris.)

Myriapods are hatched at different stages of development. The Chilognatha have only three appendages, which are so little developed that they are only small shapeless stumps, while the Chilopoda have the full number of legs in some cases; in others only a small number of legs, but yet more than the three pairs of legs of the Chilognatha, and fully developed instead of stump-like. The eyes are usually developed late in the life of the young animal. The bursting of the egg-shell is assisted in some Myriapods by a special kind of spike on the back part of the head.

The Fig. 40 shows a young Chilognath which has just burst the shell and come into the outer world. It is still surrounded with a membrane which has been formed by its skin or epiblast within the egg. One eye-spot has been formed.

Fig. 40.—Young Julus terrestris just hatched.

Fig. 41 shows a longitudinal section through the young Chilognath shown in Fig. 40, and the next (Fig. 42) a transverse section through the same. In comparing the two Figs. 41 and 42 it must be remembered that they are sections in different planes through the animal shown in Fig. 40, and therefore they only show a small portion, a thin slice, of the organs.

Fig. 41.—Longitudinal section through late stage: Sup.oe.gl, First appearance of brain; st, mouth; pr, anus; mesen, gut; n, nerve cord; n.gang, nerve ganglion; mem.ex, membrane surrounding the animal; v.f, ventral flexure; mes, mesoblast cells. (Heathcote, Post. Emb. Dev. of Julus terrestris.)

The first appearance of the mouth appendages has been already mentioned, and these are shown in Fig. 43, where the small stumps that later on change to jaws are shown. The figure shows the head of a young Chilognath seen from the lower side, and the second pair of stumps fuse together later on and produce the broad plate already mentioned as the characteristic mouth appendage of the Order.

Fig. 42.—G, gut; Malp.T, Malpighian tube; N.C, nerve cord; Tr.I, deep invagination by which the tracheae are formed; y.s, yolk spherules still present; L, first appearance of legs; S.S, part of mesoblast. (Heathcote, Post. Emb. Dev. of Julus terrestris.)

Fig. 43.—Under surface of the head of a young Julus terrestris: pro.m, rudimentary jaws; Deut.m, rudimentary mouth plate; an, antennae.

After the animal is hatched it has still, in the case of most Myriapods (those which are not hatched with all the segments complete), to undergo a further development, and in particular the eyes are still unformed. The process of development of the eye has only been followed out as yet in the Chilognatha, and in only one form, Julus, and is so curious that a short account may be of interest here. The development of the eye begins (in Julus) on the fourth day after hatching, and continues until the animal is full grown. A single ocellus or eye-spot appears first, and the rest are added one by one until the full number are reached.

The first appearances connected with the formation of the eye take place in the cellular layer just beneath the chitinous exoskeleton. This layer, called the hypodermis, plays an important part in the organisation of the animal. It forms the inner layer of what we may call the skin of the animal, and the cells of which it is composed secrete the chitin of which the shell or exoskeleton of the animal is composed, and which is moulted every year.

The first process in the formation of the eye-spot is the thickening of the hypodermis beneath the chitin, just in the place where the eye will come. At the same time the cells of this thickened mass of hypodermis secrete a quantity of pigment of a dark red brown colour. Next the cells of the thick mass of hypodermis begin to separate from one another in such a way that a vesicle is formed. This vesicle is hollow inside, and the thick walls are formed from the cells of the thickened hypodermic mass. This can be seen from Fig. 44, which represents a section through an ocellus when it is partly formed. From this vesicle the eye is formed.

The wall of the vesicle nearest the exoskeleton gives rise to the lens of the eye, while the other walls of the vesicle form the retinal parts of the eye. The cells from the brain grow out and form the optic nerve connecting the retina with the brain. The whole eye spot is covered internally by a thin membrane, formed not from the hypodermis but by cells from the inside of the body (mesoblast cells).

Fig. 44.—Section through eye when first forming: Hyp, hypodermis; Ln, lens; F.W.V, front wall of optic vesicle; b.w.v, back wall of vesicle; cap, capsule.

In the Chilognatha, the first Order of Myriapods, the young animal leaves the egg with three pairs of appendages; the first have already the form of antennae, the second will form the jaws, but have not yet taken their proper form, while the third pair will fuse together and alter their shape so as to form the curious plate that has already been mentioned as forming the second pair of mouth appendages. Behind the mouth appendages will come the first three pairs of legs. The whole young animal on leaving the egg is enveloped in two membranes. These membranes are secreted by the outside layer of cells in the same way that the shell or exoskeleton of the animal will be eventually formed, and represent the first two moults of the animal, which continues to moult its shell every year throughout life.

Of the Chilopoda, the second Order of Myriapods, all the families leave the egg-shell with the full number of legs, with the exception of the Lithobiidae, which have seven pairs of legs including the poison-claws. The Schizotarsia, the third Order, also have seven pairs of legs when hatched.

The legs make their appearance not one by one but in batches (in Julus terrestris in batches of five). The addition of legs and segments to the body takes place, not at the end of the body, but between the end segment and the penultimate.

This is a short sketch of the gradual development of the Myriapoda from the ovum to the fully-grown animal. It is, I am aware, a short and insufficient account of all the beautiful processes by which the different organs take their rise, but space is insufficient here, and too much detail would be out of place in a work of this nature, which only aims at giving an outline sketch of the group, which shall be intelligible to the general reader who has not made a special study of such matters. Before leaving the subject, however, I must mention a few of the points of interest which are to be learned from the examination of the course of development which has been sketched here. One of the greatest puzzles in the natural history of the Order Chilognatha has always been the double segments, as they are called; that is, in fact, the possession of two pairs of legs to each segment, which is, as we have already said, a distinguishing characteristic of the Order. As we have seen, the Chilognatha at an early stage of existence do not possess this characteristic, which is only peculiar to the adult and half-grown forms. Now what does this mean? Does each double segment in the full-grown Millepede represent two segments which have become fused together, or is each double segment, so called, a real segment resembling the segments present in the other Orders (for instance, Chilopoda), which has grown an extra pair of legs? Both these views have been advocated by distinguished naturalists. Neither of them is, in my opinion, quite right when viewed in the light cast on the subject by recent investigations into the life history of the Chilognatha.

A close examination into the minutiae of the growth of the different organs has shown us that the double characters of the double segments are more deeply seated than was imagined. The circulatory system, the nerve cord, and the first traces of segmentation in the mesoblast all show this double character, and the only single part about the segment is the broad plate covering the segment. Now in some of the most ancient of the fossil Myriapods this broad plate shows traces of a division, as if it were in reality two plates fused together. We have also to consider that the life history of the Chilognatha allows us to believe that the peculiar cylindrical shape of the body shown in the greatest degree in the Julidae is attained by the unequal development of the dorsal and ventral surfaces of the body; the ventral surface being compressed together till it is extremely narrow, and the dorsal surface, as it were, growing round it till the originally dorsal surface forms almost a complete ring round the body. Taking all this into consideration, we are justified, in my opinion, in concluding that each double segment in the Chilognatha is not two segments fused together, nor a single segment bearing two pairs of legs, but is two complete segments perfect in all particulars, but united by a large dorsal plate which was originally two plates which have been fused together, and which in most Chilognatha surrounds almost the whole of two segments in the form of a ring.

Again in the Chilopoda we see that a great distinctive feature that separates them from the Chilognatha is the character of the ventral nerve cord, the cord being double and not single, a character connected with the fact that the bases of the legs are widely separated from one another, and not closely approached to each other, as in the Chilognatha. As we before said, a more minute anatomical examination showed us that this difference was not so great as appeared at first sight, the cord showing traces of a duplication. Well, are these traces superficial, or do they represent a state of affairs more or less similar to that in the Chilopoda? Embryology helps us to answer this question also. In the early stages of the Chilognatha we find that the nerve cord has exactly the form of that in Chilopoda, showing us that the appearances in the anatomy had led us to a right conclusion, and giving us a valuable confirmation of our views. These two examples will serve to show the kind of interest which attaches to embryology.

Palaeontology.

We have seen that embryology enables us to look at the structure of the Myriapods from a new standpoint, and to correct and supplement the knowledge gained from an examination of the adult animal. In the same way a study of the forms of Myriapods which have become extinct on the globe, and have been preserved to us in a fossil form, gives a further opportunity of considering the relations of one form to another, and again of the relations of our group to other groups of animals now existing on the earth. Myriapod fossils have been found in strata of great antiquity. The oldest of such fossils must have been among the first land animals. The figure below shows a fossil Myriapod found in America, belonging to the Order of the Protosyngnatha which are only found in the Palaeozoic strata. It is a good example of the manner in which Myriapods were protected by bundles of bristles in the same way as the Polyxenus of the present time.

The oldest fossil Myriapods which have been discovered at the present time are two species which have been found in the Old Red Sandstone in Scotland. To realise the antiquity of these Myriapods, it will be worth while recalling the typical fossils found in the Old Red Sandstone, so as to see what the contemporaries of these ancient Myriapods were like. Among the plants there were Algae, Ferns, and Conifers, belonging to the lower divisions of the plant tribe. Among the animals there were Sponges, Corals, Starfish, Worms, Shell-fish, and Fishes, but none of the more highly organised of the animal or vegetable tribe had appeared on the earth. The Myriapods of the Old Red Sandstone, as has been before said, differ considerably from those of the present day, and as we proceed towards the species found in the more recent strata we find them more and more like the ones at present living, till we get to the Polyxenus and other species found in amber, which are hardly to be distinguished from living forms.

The next oldest fossil Myriapods are found in the coal measures, when both the animal and vegetable kingdoms were represented by more numerous and more specialised forms. The fossil fauna of this period is characterised by the number of gigantic Amphibia, many remains of which have been found. The great forests and the abundant vegetation of this time must have been favourable to the existence of our class, and accordingly we find no less than 32 species of fossil Myriapods. Of these most have been found in America, some in Great Britain, and some in Germany. One well-preserved fossil of Xylobius sigillariae was found by Dr. Dawson in America in the stump of a tree in the remains of a fossil forest. The eyes, head, and legs were plainly seen under the microscope. All these fossils belong to the earliest or Palaeozoic period.

Fig. 45.—Palaeocampa anthrax. (After Meek and Worth.) From Mazon Creek, Illinois.

The figure below (Fig. 46) shows a fossil also from the coal formations of Illinois, America, belonging to the family of the Euphoberiidae mentioned further on. It shows a nearer approach to the Julidae of the present time. The limbs, however, were of very curious shape, and may possibly have been adapted to locomotion in water as well as on land, and the small supposed branchiae on the ventral surface shown in Fig. 46, B, may possibly have been an arrangement to render respiration in the water possible.

In the secondary period the Myriapods were scantily represented, or, at any rate, geologists have failed to find their fossils. The class is represented by a single specimen found in the chalk in Greenland. This fossil, which has been included in the Julidae under the name of Julopsis cretacea, may perhaps belong to the Archipolypoda.

Passing on to the Tertiary or Recent period, we find the Myriapods again numerous, and more nearly resembling those living at the present time. They belong mostly to the Chilognatha and Chilopoda. They have been found in the fresh-water gypsum of Provence in France, the brown coal of Germany, and the green river formations of America. Several have been found in amber.

Fig. 46.—Acantherpestes major. (After Meek and Worth.) Mazon Creek, America. A, The whole animal; B, branchiae on the ventral surface.

Fossil Myriapods have been divided into four Orders, two of which coincide with the Orders of living Myriapods; the differences between the fossils and the living Myriapods having been held insufficient to warrant the establishment of a new Order. These two Orders are the Chilopoda and the Diplopoda or Chilognatha (Diplopoda is another name used by some writers for the group which we have hitherto called Chilognatha). The other two Orders have sufficient differences from living forms to render it necessary to include them in separate Orders.

The fossil Myriapods, then, are arranged as follows:—

The following table will show the species that have been discovered in the different strata:—

Devonian, or Old Red Sandstone		2 species of Archipolypoda
Carboniferous		01 species Protosyngnatha 31 species Archipolypoda
Permian (Rothliegendes of Germany), 4 specimens belonging to the Julidae or Archipolypoda.
Cretaceous		01 species		Archipolypoda or Chilognatha
Oligocene		17 species		Chilopoda
		23 species		Diplopoda (Chilognatha)
Miocene,		01 species		Diplopoda (Chilognatha)

I will now give a short account of the different Orders, and the fossil forms which are included in them.

Order I. Protosyngnatha.

This Order is represented by a single fossil (Fig. 45), discovered in the coal at Mazon Creek, Illinois, America, by Meek and Worth. It differs greatly from any of those in existence at the present day. The body is cylindrical, and composed of ten segments. The cephalic appendages (that is, the antennae and mouth parts) are inserted into a single unsegmented cephalic mass (the head). Each segment behind the head bears a single dorsal and ventral plate of equal breadth and length. The limbs are placed in these plates with a wide space between the base of each leg and that of the opposite one of the pair. Along the back, bundles of bristles are arranged in longitudinal rows.

Order II. Chilopoda.

The fossil forms of this Order resemble those of the Chilopoda of the present day. The oldest of them are found in amber. The following families have been found:—

Lithobiidae. Several species have been found in amber.

Scolopendridae. One species in amber, several species in later Tertiary formations.

Geophilidae. Three species in amber.

Two species resembling the Schizotarsia of the present day have been found in amber.

Order III. Archipolypoda.

The most numerous of the fossil families. With a few exceptions, all the Palaeozoic (that is, the oldest) Myriapods belong to this Order. The Carboniferous Archipolypoda seem to be much more numerous in the coal of America than in that of England. They resemble for the most part the Myriapods of the present day, except that all the segments without exception bear legs.

The families are three in number.

Family 1. Archidesmidae.

Resemble the Polydesmidae of the present day. Two species have been found by Page in the Old Red Sandstone of Forfarshire. He named them Kampecaris. One found by Peach in the same formation is called Archidesmus.

Family 2. Euphoberiidae.

They show some resemblance to the Julidae of the present day, but the dorsal scutes, or plates of the back, are more or less perfectly divided into two divisions corresponding with the pairs of legs. The following are the principal fossils of this family:—

Acantherpestes. Found by Meek and Worth in the coal at Mazon Creek in America (Fig. 46).

Euphoberia. About 12 species found at the same place as the last named.

Amylispes. Found by Scudder, Mazon Creek, America.

Eileticus. Scudder, Mazon Creek, America.

Family 3. Archijulidae.

The dorsal plates nearly consolidated, but the division still apparent. Fossil forms are—

Trichijulus. Scudder, Mazon Creek, America.

Xylobius. Dawson. Found in the coal in Nova Scotia. Two species found at Mazon Creek, America.

Order IV. Chilognatha.

Families corresponding to those of the present day. The oldest specimens come from the chalk in Greenland; most of the others from amber.

Family 1. Glomeridae. One form, G. denticulata, has been found in amber.

Family 2. Polydesmidae. Two species in amber.

Family 3. Lysiopetalidae. A number of species, amongst which are 6 Craspedosoma, mostly from amber.

Family 4. Julidae. A number of species of this family have been found, some in amber, some in other Tertiary strata. Amongst the latter a probable example of Julus terrestris, living at the present time.

Family 5. Polyxenidae. Five species have been found in amber.

Now that we have considered the structure of the Myriapods and the groups into which they are subdivided or classified, we may proceed to consider what position they hold in the household of nature. That they present certain features of similarity to other classes has been already mentioned, and that this is the fact cannot be doubted when we look back at the way in which they have been classified in the works of early writers. For example, Lamarck, the great French naturalist, classifies them with spiders in his well-known work, La Philosophie Zoologique, under the name of Arachnides antennistes. Cuvier, the comparative anatomist, unites them with the Insects, making them the first Order, while the Thysanura is the second. We have already seen that one Order of Myriapods, the Symphyla, bears a great resemblance to the Thysanura. The English naturalist Leach was the first to establish Myriapods as a class, and his arrangement has been followed by all naturalists after his time. But while their peculiarities of structure and form are sufficiently marked to separate them as a class, it cannot be denied that the older naturalists were right to recognise that they have many essential characteristics in common with other classes of animals. And recent investigations have emphasised this fact. For instance, let us consider the recent discoveries of the Orders of Symphyla and Pauropoda, Orders which, while bearing so many of the characters of Myriapods that naturalists have agreed to place them in that class, yet resemble in many important points the Insect Order of Thysanura. This seems to justify Cuvier in claiming the close relationship for them that he did.

Recent investigations have also brought out more prominently the resemblances to the Worms. Of late, considerable attention has been directed to Peripatus (see pp. [1-26]), and the resemblances to the Myriapods in its anatomy and development are such that Latzel has actually included it in the Myriapods as an Order, Malacopoda. Now Peripatus also shows resemblances to the annelid Worms, and thus affords us a connexion to the Worm type hardly less striking than that to the Insect. This resemblance to the Worms, which Myriapods certainly bear, was noticed by the ancient writers, and as they had for the most part only external appearances to consider, they pushed this idea to extremes in actually including some of the marine Worms (Annelida) among the Centipedes. Pliny talks of a marine Scolopendra as a very poisonous animal, and there is little doubt that he meant one of the marine worms. An old German naturalist, Gesner, in a very curious book published in 1669 gives an account of an annelid sea-worm which he calls Scolopendra marina, and which is in all probability the sea Scolopendra which Pliny mentions. From Gesner's account it seems to have been used as a medicine (externally only). "The use of this animal in medicine. The animal soaked in oil makes the hair fall off. So do its ashes mixed in oil." It was also pounded up with honey.

This idea of Centipedes living in water survived among later naturalists. Charles Owen, the author before quoted, mentions them as amphibious in 1742. "The Scolopendra is a little venomous worm and amphibious. When it wounds any, there follows a blueness about the affected part and an itch all over the body like that caused by nettles. Its weapons of mischief are much the same with those of the spider, only larger; its bite is very tormenting, and produces not only pruriginous pain in the flesh, but very often distraction of mind. These little creatures make but a mean figure in the ranks of animals, yet have been terrible in their exploits, particularly in driving people out of their country. Thus the people of Rhytium, a city of Crete, were constrained to leave their quarters for them (Aelian, lib. xv. cap. 26)."

Myriapods have been considered to bear resemblances to the Crustacea, and this to a certain extent is true, though only to a certain extent, the resemblances being confined to the more general characteristics that they share with other groups of animals.

Of late years attempts have been made to speculate about the origin of the Myriapods—that is, to endeavour to obtain by means of investigation of their anatomy, embryology, and palaeontological history, some idea of the history of the group. Such attempts at research into the phylogeny, as it is called, of a group must be more or less speculative until our knowledge is much greater than it is at present. But such inquiries have their value, and the schemes of descent and phylogenetic trees, at any rate, indicate a real relation to different groups, even if they do not provide us with a real and actual history of the animals.

There have been two main theories about the descent of the Myriapoda. One of these derives them directly from the Insecta through the forms known as the Thysanura, which resemble in such a degree the Myriapod Orders of Symphyla and Pauropoda. The other theory holds that the Myriapods, as well as the Insecta, have been derived from some ancestor bearing a resemblance to Peripatus. In other words, one theory claims that the relationship of Myriapoda to Insecta is that of father and son; the other that the relationship between the two is that of brother to brother. The arguments by which these theories are respectively supported consist for the most part of an analysis of the different characters of the anatomy and embryology and the determination of the most primitive among them. For example, the supporters of the theory that the Thysanura are the most nearly allied to the Myriapod ancestor lay great weight on the fact that some Myriapods are born with three pairs of legs only, and they compare this stage in the life history of the Myriapoda to the metamorphosis and larval stage of Insects. For the supporters of this view the Orders of Symphyla and Pauropoda are the most primitive of the Myriapods. On the other hand, the followers of the other theory do not allow that the characters in which the Myriapods are like Insects are primitive ones, but they lay more stress on the characters found in the early development, such as the character of the process of the formation of the body segments, the mesoblastic segmentation, and the origin of the various organs of the body.

It may be easily understood that such differences in the estimation of the primitive characters of the embryology of a group may arise. Embryology has been compared by one of the greatest of modern embryologists to "an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." What wonder is it that different people examining such a record should come to different conclusions as to the more doubtful and difficult portions of it. It is this very difficulty which makes the principal interest in the study, and although our knowledge of the language in which this manuscript is written is as yet imperfect, still we hope that constant study may teach us more and more, and enable us to read the great book of nature with more and more ease and certainty.

If any of my readers should wish for a more full account of the natural history of this group I must refer them to the following works, which I have used in compiling the above account. In the first of these there is an excellent bibliography of the subject:—

Latzel, Die Myriapoden der Oesterreichisch-Ungarischen Monarchie, Wien, 1880.

Zittel, Handbuch der Palaeontologie, 1 Abth, II. Bd., Leipzig, 1881-1885.

Korschelt and Heider, Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere, Jena 1891.

INSECTA

BY

DAVID SHARP, M.A., M.B., F.R.S.

CHAPTER III

CHARACTERISTIC FEATURES OF INSECT LIFE–SOCIAL INSECTS–DEFINITION OF THE CLASS INSECTA–COMPOSITION OF INSECT SKELETON–NUMBER OF SEGMENTS–NATURE OF SCLERITES–HEAD–APPENDAGES OF THE MOUTH–EYES–THORAX–ENTOTHORAX–LEGS–WINGS–ABDOMEN OR HIND BODY–SPIRACLES–SYSTEMATIC ORIENTATION.

Insects form by far the larger part of the land animals of the world; they outnumber in species all the other terrestrial animals together, while compared with the Vertebrates their numbers are simply enormous. Yet they attract but little attention from the ordinary observer, this being probably primarily due to the small size of the individual Insect, which leads the unreflecting to treat the creature as of little importance. "It can be crushed in a moment" is perhaps the unformulated idea that underlies the almost complete neglect of knowledge concerning Insects that prevails even in the educated classes of society. The largest Insects scarcely exceed in bulk a mouse or a wren, while the smallest are almost or quite imperceptible to the naked eye, and yet the larger part of the animal matter existing on the lands of the globe is in all probability locked up in the forms of Insects. Taken as a whole they are the most successful of all the forms of terrestrial animals.

In the waters of the globe the predominance of Insect life disappears. In the smaller collections of fresh water many Insects find a home during a portion of their lives, and some few contrive to pass their whole existence in such places; but of the larger bodies of fresh water they invade merely the fringes, and they make only the feeblest attempt at existence in the ocean; the genus Halobates containing, so far as we know, the sole Insects that are capable of using the ocean as a medium of existence at a distance from the shore.

It will probably be asked, how has it come about that creatures so insignificant in size and strength have nevertheless been so successful in what we call the struggle for existence? And it is possible that the answer will be found in the peculiar relations that exist in Insects between the great functions of circulation and respiration; these being of such a nature that the nutrition of the organs of the body can be carried on very rapidly and very efficiently so long as a certain bulk is not exceeded.

Rapidity of growth is carried to an almost incredible extent in some Insects, and the powers of multiplication—which may be considered as equivalent to the growth of the species—even surpass the rapidity of the increase of the individual; while, as if to augment the favourable results attainable by the more usual routine of the physiological processes, "metamorphosis" has been adopted, as a consequence of which growth and development can be isolated from one another, thus allowing the former to go on unchecked or uncomplicated by the latter. A very simple calculation will show how favourable some of the chief features of Insect life are. Let it be supposed that growth of the individual takes time in proportion to the bulk attained, and let A be an animal that weighs one ounce, B a creature that weighs ten ounces, each having the power of producing 100 young when full grown; a simple calculation shows that after the lapse of a time necessary for the production of one generation of the larger creature the produce of the smaller animal will enormously outweigh that of its bulkier rival. Probably it was some consideration of this sort that led Linnaeus to make his somewhat paradoxical statement to the effect that three flies consume the carcase of a horse as quickly as a lion.[^[16]]

Astonishing as may be the rapidity of the physiological processes of Insects, the results attained by them are, it must be admitted, scarcely less admirable: the structures of the Insect's body exhibit a perfection that, from a mechanical point of view, is unsurpassed, while the external beauty of some of the creatures makes them fit associates of the most delicate flowers or no mean rivals of the most gorgeous of the feathered world. The words of Linnaeus, "Natura in minimis maxime miranda," are not a mere rhetorical effort, but the expression of a simple truth. Saint Augustine, too, though speaking from a point of view somewhat remote from that of the great Swedish naturalist, expressed an idea that leads to a similar conclusion when he said, "Creavit in coelum angelos, in terram vermiculos; nec major in illis nec minor in istis."

The formation of organised societies by some kinds of Insects is a phenomenon of great interest, for there are very few animals except man and Insects that display this method of existence. Particulars as to some of these societies will be given when we treat of the Termitidae, and of the Hymenoptera Aculeata; but we will take this opportunity of directing attention to some points of general interest in connexion with this subject. In Insect societies we find that not only do great numbers of separate individuals live together and adopt different modes of industrial action in accordance with the position they occupy in the association, but also that such individuals are profoundly modified in the structures of their body and in their physiological processes in such ways as to specially fit them for the parts they have to play. We may also see these societies in what may be considered different stages of evolution; the phenomena we are alluding to being in some species much less marked than they are in others, and these more primitive kinds of societies being composed of a smaller number of individuals, which are also much less different from one another. We, moreover, meet with complex societies exhibiting some remarkably similar features among Insects that are very different systematically. The true ants and the white ants belong to groups that are in structure and in the mode of growth of the individual essentially dissimilar, though their social lives are in several important respects analogous.

It should be remarked that the phenomena connected with the social life of Insects are still only very imperfectly known; many highly important points being quite obscure, and our ideas being too much based on fragments gathered from the lives of different species. The honey bee is the only social Insect of whose economy we have anything approaching to a wide knowledge, and even in the case of this Insect our information is neither so complete nor so precise as is desirable.

The various branches of knowledge connected with Insects are called collectively Entomology. Although entomology is only a department of the great science of zoology, yet it is in practice a very distinct one; owing to its vast extent few of those who work at other branches of zoology also occupy themselves with entomology, while entomologists usually confine themselves to work in the vast field thus abandoned to them.

Before passing to the consideration of the natural history and structure of the members of the various Orders of Insects we will give a verbal diagrammatic sketch, if we may use such an expression, with a view to explaining the various terms that are ordinarily used. We shall make it as brief as possible, taking in succession (1) the external structure, (2) internal structure, (3) development of the individual, (4) classification.

In the course of this introductory sketch we shall find it necessary to mention the names of some of the Orders of Insects that will only be explained or defined in subsequent pages. We may therefore here state that the term "Orthoptera" includes grasshoppers, locusts, earwigs, cockroaches; "Neuroptera" comprises dragon-flies, May-flies, lacewings, stone-flies and caddis-flies; to the "Hymenoptera" belong bees, wasps, ants, sawflies, and a host of little creatures scarcely noticed by the ordinary observer: "Coleoptera" are beetles; "Lepidoptera," butterflies and moths; "Diptera," house-flies, blue-bottles, daddy-longlegs, and such; "Hemiptera" or "Rhynchota" are bugs, greenfly, etc.

Class Insecta: or Insecta Hexapoda.

Definition.—Insects are small animals, having the body divided into three regions placed in longitudinal succession—head, thorax, and abdomen: they take in air by means of tracheae, a system of tubes distributed throughout the body, and opening externally by means of orifices placed at the sides of the body. They have six legs, and a pair of antennae; these latter are placed on the head, while the legs are attached to the thorax, or second of the three great body divisions; the abdomen has no true legs, but not infrequently has terminal appendages and, on the under surface, protuberances which serve as feet. Very frequently there are two pairs of wings, sometimes only one pair, in other cases none: the wings are always placed on the thorax. Insects are transversely segmented—that is to say, the body has the form of a succession of rings; but this condition is in many cases obscure; the number of these rings rarely, if ever, exceeds thirteen in addition to the head and to a terminal piece that sometimes exists. Insects usually change much in appearance in the course of their growth, the annulose or ringed condition being most evident in the early part of the individual's life. The legs are usually elongate and apparently jointed, but in the immature condition may be altogether absent, or very short; in the latter case the jointing is obscure. The number of jointed legs is always six.

External Structure.

The series of rings of which the external crust or skeleton of Insects is composed exhibits great modifications, not only in the various kinds of Insects but even in the different parts of the same individual, and at successive periods of its development; so that in the majority of mature Insects the separate rings are readily distinguished only in the hind body or abdomen. The total number of the visible rings, segments, somites, or arthromeres, as they are variously called by different writers, is frequently thirteen in addition to the head. This latter part is considered to be itself composed of the elements of several rings, but morphologists are not yet agreed as to their number, some thinking this is three while others place it as high as seven; three or four being, perhaps, the figures at present most in favour, though Viallanes, who has recently discussed[^[17]] the subject, considers six, the number suggested by Huxley, as the most probable. Cholodkovsky is of a similar opinion. However this may be, the three rings behind the head constitute the thorax, which is always largely developed, though, like the head, its segmentation is usually very much obscured by unequal development of different parts, or by consolidation of some of them, or by both of these conditions. The third great division of the body, the abdomen, is also usually much modified by one or more of the terminal segments being changed in form, or even entirely withdrawn into the interior of the body. The existence of ten segments in the hind body can, however, be very frequently actually demonstrated, so that it is correct to speak of ten as the normal number.

Fig. 47—Diagram of exterior of insect: the two vertical dotted lines indicate the divisions between H, head; T, thorax; and A, abdomen: a, antenna; b, labrum; c, mandible; d, maxillary palpus; e, labial palpus; f, facetted eye; g, pronotum; h, mesonotum; i, metanotum; k, wings; l₁ to l₁₀, abdominal segments; m, the internal membranous portions uniting the apparently separated segments; n, cerci; o, stigma; p, abdominal pleuron bearing small stigmata; q₁, q₂, q₃, pro-, meso-, meta-sterna; r₁, mesothoracic episternum; s₁, epimeron, these two forming the mesopleuron; r₂, s₂, metathoracic episternum and epimeron; t, coxa; v, trochanter; w, femur; x, tibia; y, tarsus; z, gula.

It is no reproach to morphologists that they have not yet agreed as to the number of segments that may be taken as typical for an Insect, for all the branches of evidence bearing on the point are still imperfect. It may be well, therefore, to state the most extreme views that appear to be at all admissible. Hagen[^[18]] has recently stated the opinion that each thoracic segment consists really of three segments—an anterior or wing-bearer, a middle or leg-bearer, and a posterior or stigma-bearer. There seems to be no reason for treating the stigma as being at all of the nature of an appendage, and the theory of a triple origin for these segments may be dismissed. There are, however, several facts that indicate a duplicity in these somites, among which we may specially mention the remarkable constancy of two pleural pieces on each side of each thoracic segment. The hypothesis of these rings being each the representative of two segments cannot therefore be at present considered entirely untenable, and in that case the maximum and minimum numbers that can be suggested appear to be twenty-four and eleven, distributed as follows:—

Although it is not probable that ultimately so great a difference as these figures indicate will be found to prevail, it is certainly at present premature to say that all Insects are made up of the same number of primary segments.

A brief account of the structure of the integument will be found in the chapter dealing with the post-embryonic development.

The three great regions of the Insect body are functionally as well as anatomically distinct. The head bears the most important of the sense organs, viz. the antennae and ocular organs; it includes the greater of the nerve-centres, and carries the mouth as well as the appendages, the trophi, connected therewith. The thorax is chiefly devoted to the organs of locomotion, bearing externally the wings and legs, and including considerable masses of muscles, as well as the nerve centres by which they are innervated; through the thorax there pass, however, in the longitudinal direction, those structures by which the unity of the organisation is completed, viz. the alimentary canal, the dorsal vessel or "heart" for distributing the nutritive fluid, and also the nerve cords. The abdomen includes the greater part of the organs for carrying on the life of the individual and of the species; it also frequently bears externally, at or near its termination, appendages that are doubtless usually organs of sense of a tactile nature.

In the lower forms of Insect life there is little or no actual internal triple division of the body; but in the higher forms such separation becomes wonderfully complete, so that the head may communicate with the thorax only by a narrow isthmus, and the thorax with the abdomen only by a very slender link. This arrangement is carried to its greatest extreme in the Hymenoptera Aculeata. It may be looked on as possibly a means for separating the nutrition of the parts included in the three great body divisions.

Along each side of the body extends a series of orifices for the admission of air, the stigmata or spiracles; there are none of these on the head, but on each side of most of the other segments there is one of these spiracles. This, however, is a rule subject to many exceptions, and it is doubtful whether there is ever a spiracle on the last abdominal segment. Even in the young stage of the Insect the number of these stigmata is variable; while in the perfect Insect the positions of some of the stigmata may be much modified correlatively with the unequal development or consolidation of parts, especially of the thorax when it is highly modified for bearing the wings.

The segments of the Insect are not separate parts connected with one another by joints and ligaments; the condition of the Insect crust is in fact that of a continuous long sac, in which there are slight constrictions giving rise to the segments, the interior of the sac being always traversed from end to end by a tube, or rather by the invaginated ends of the sac itself which connect with an included second sac, the stomach. The more prominent or exposed parts of the external sac are more or less hard, while the constricted parts remain delicate, and thus the continuous bag comes to consist of a series of more or less hard rings connected by more delicate membranes. This condition is readily seen in distended larvae, and is shown by our figure 48 which is taken from the same specimen, whose portrait, drawn during life, will be given when we come to the Coleoptera, family Cleridae. The nature of the concealed connexions between the apparently separate segments of Insects is shown at m, Fig. 47, p. [88].

Fig. 48—Tillus elongatus, fully distended larva.

As the number of segments in the adult Insect corresponds—except in the head—with the number of divisions that appear very early in the embryo, we conclude that the segmentation of the adult is, even in Insects which change their form very greatly during growth, due to the condition that existed in the embryo; but it must not be forgotten that important secondary changes occur in the somites during the growth and development of the individual. Hence in some cases there appear to be more than the usual number of segments, e.g. Cardiophorus larva, and in others the number of somites is diminished by amalgamation, or by the extreme reduction in size of some of the parts.

Besides the division of the body into consecutive segments, another feature is usually conspicuous; the upper part, in many segments, being differentiated from the lower and the two being connected together by intervening parts in somewhat the same sort of way as the segments themselves are connected. Such a differentiation is never visible on the head, but may frequently be seen in the thorax, and almost always in the abdomen. A dorsal and a ventral aspect are thus separated, while the connecting bond on either side forms a pleuron. By this differentiation a second form of symmetry is introduced, for whereas there is but one upper and one lower aspect, and the two do not correspond, there are two lateral and similar areas. This bilateral symmetry is conspicuous in nearly all the external parts of the body, and extends to most of the internal organs. The pleura, or lateral regions of the sac, frequently remain membranous when the dorsal and ventral aspects are hard. The dorsal parts of the Insect's rings are also called by writers terga, or nota, and the ventral parts sterna.

The appendages of the body are:—(1) a pair of antennae; (2) the trophi, constituted by three pairs of mouth-parts; (3) three pairs of legs; (4) the wings[^[19]]; (5) abdominal appendages of various kinds, but usually jointed. Before considering these in detail we shall do well to make ourselves more fully acquainted with the elementary details of the structure of the trunk.

In the adult Insect the integument or crust of the body is more or less hard or shell-like, sometimes, indeed, very hard, and on examination it will be seen that besides the divisions into segments and into dorsal, ventral, and pleural regions, there are lines indicating the existence of other divisions, and it will be found that by dissection along these lines distinct pieces can be readily separated. Each hard piece that can be so separated is called a sclerite, and the individual sclerites of a segment have received names from entomotomists. The sclerites are not really quite separate pieces, though we are in the habit of speaking of them as if such were the case. If an Insect be distended by pressure from the interior, many of the sclerites can be forced apart, and it is then seen that they are connected by delicate membrane. The structure is thus made up of hard parts meeting one another along certain lines of union—sutures—so that the original membranous continuity may be quite concealed. In many Insects, or in parts of them, the sclerites do not come into apposition by sutures, and are thus, as it were, islands of hard matter surrounded by membrane. A brief consideration of some of the more important sclerites is all that is necessary for our present purpose: we will begin with the head.

Fig. 49.—Capsule of head of beetle, Harpalus caliginosus: A, upper; B, under surface: a, clypeus; b, epicranium; c, protocranium; d, gula; e, facetted eye; f, occipital foramen; g, submentum; h, cavity for insertion of antenna.

The head is most variable in size and form; as a part of its surface is occupied by the eyes and as these organs differ in shape, extent, and position to a surprising degree, it is not a matter for astonishment that it is almost impossible to agree as to terms for the areas of the head. Of the sclerites of the head itself there are only three that are sufficiently constant and definite to be worthy of description here. These are the clypeus, the epicranium, and the gula. The clypeus is situate on the upper surface of the head-capsule, in front; it bears the labrum which may be briefly described as a sort of flap forming an upper lip. The labrum is usually possessed of some amount of mobility. The clypeus itself is excessively variable in size and form, and sometimes cannot be delimited owing to the obliteration of the suture of connexion with the more posterior part of the head; it is rarely or never a paired piece. Occasionally there is a more or less distinct piece interposed between the clypeus and the labrum, and which is the source of considerable difficulty, as it may be taken for the clypeus. Some authors call the clypeus the epistome, but it is better to use this latter term for the purpose of indicating the part that is immediately behind the labrum, whether that part be the clypeus, or some other sclerite; the term is very convenient in those cases where the structure cannot be, or has not been, satisfactorily determined morphologically.

In Figure 50 the parts usually visible on the anterior aspect of the head and its appendages are shown so far as these latter can be seen when the mouth is closed; in the case of the Insect here represented the bases of the mandibles are clearly seen (g), while their apical portions are entirely covered by the labrum, just below the lower margin of which the tips of the maxillae are seen, looking as if they were the continuations of the mandibles.

The labrum is a somewhat perplexing piece, morphologists being not yet agreed as to its nature; it is usually placed quite on the front of the head, and varies extremely in form; it is nearly always a single or unpaired piece; the French morphologist Chatin considers that it is really a paired structure.

Fig. 50.—Front view of head of field-cricket (Gryllus): a, epicranium; b, compound eye; c, antenna; d, post-: e, ante-clypeus; f, labrum; g, base of mandible; h, maxillary palpus; i, labial palpus; k, apex of maxilla.

The gula (Fig. 49, B d, and Fig. 47, z) is a piece existing in the middle longitudinally of the under-surface of the head; in front it bears the mentum or the submentum, and extends backwards to the great occipital foramen, but in some Insects the gula is in front very distant from the edge of the buccal cavity. The epicranium forms the larger part of the head, and is consequently most inconstant in size and shape; it usually occupies the larger part of the upper-surface, and is reflected to the under-surface to meet the gula. Sometimes a transverse line exists (Fig. 49, A) dividing the epicranium into two parts, the posterior of which has been called the protocranium; which, however, is not a good term. The epicranium bears the antennae; these organs do not come out between the epicranium and the clypeus, the foramen for their insertion being seated entirely in the epicranium (see Fig. 50). In some Insects there are traces of the epicranium being divided longitudinally along the middle line. When this part is much modified the antennae may appear to be inserted on the lateral portions of the head, or even on its under-side; this arises from extension of some part of the epicranium, as shown in Fig. 49, B, where h, the cavity of insertion of the antenna, appears to be situate on the under-surface of the epicranium, the appearance being due to an infolding of an angle of the part.

There is always a gap in the back of the head for the passage of the alimentary canal and other organs into the thorax; this opening is called the occipital foramen. Various terms, such as frons, vertex, occiput, temples, and cheeks, have been used for designating areas of the head. The only one of these which is of importance is the gena, and even this can only be defined as the anterior part of the lateral portion of the head-capsule. An extended study of the comparative anatomy of the head-capsule is still a desideratum in entomology. The appendages of the head that are engaged in the operations of feeding are frequently spoken of collectively as the trophi, a term which includes the labrum as well as the true buccal appendages.

The appendages forming the parts of the mouth are paired, and consist of the mandibles, the maxillae, and the labium, the pair in this latter part being combined to form a single body. The buccal appendages are frequently spoken of as gnathites. The gnathites are some, if not all, of them composed of apparently numerous parts, some of these being distinct sclerites, others membranous structures which may be either bare or pubescent—that is, covered with delicate short hair. In Insects the mouth functions in two quite different ways, by biting or by sucking. The Insects that bite are called Mandibulata, and those that suck Haustellata. In the mandibulate Insects the composition of the gnathites is readily comprehensible, so that in nearly the whole of the vast number of species of that type the corresponding parts can be recognised with something like certainty. This, however, is not the case with the sucking Insects; in them the parts of the mouth are very different indeed, so that in some cases morphologists are not agreed as to what parts really correspond with some of the structures of the Mandibulata. At present it will be sufficient for us to consider only the mandibulate mouth, leaving the various forms of sucking mouth to be discussed when we treat of the Orders of Haustellata in detail.

The upper or anterior pair of gnathites is the mandibles, (Fig. 50, g). There is no part of the body that varies more than does the mandible, even in the mandibulate Insects. It can scarcely be detected in some, while in others, as in the male stag-beetle, it may attain the length of the whole of the rest of the body; its form, too, varies as much as its size; most usually, however, the pair of mandibles are somewhat of the form of callipers, and are used for biting, cutting, holding, or crushing purposes. The mandibles are frequently armed with processes spoken of as teeth, but which must not be in any way confounded with the teeth of Vertebrates. The only Insects that possess an articulated tooth are the Passalidae, beetles armed with a rather large mandible bearing a single mobile tooth among others that are not so. Wood Mason and Chatin consider the mandibles to be, morphologically, jointed appendages, and the latter authority states that in the mandible of Embia he has been able to distinguish the same elements as exist in the maxillae. In aculeate Hymenoptera the mandibles are used to a considerable extent for industrial purposes.

Fig. 51.—Mandibles, maxillae, and labium of Locusta viridissima: A, mandibles; B, maxillae (lateral parts) and labium (middle parts) united: a, cardo; b, stipes; c, palpiger; d, max. palp.; e, lacinia; f, galea; g, submentum; h, mentum; i, palpiger; k, labial palpus; l, ligula; m, paraglossa (galea); n, lacinia; o, lingua.

The maxilla is a complex organ consisting of numerous pieces, viz. cardo, stipes, palpiger, galea, lacinia, palpus. The galea and lacinia are frequently called the lobes of the maxilla. The maxilla no doubt acts as a sense organ as well as a mechanical apparatus for holding; this latter function being subordinate to the other. In Fig. 68, p. [122], we have represented a complex maxillary sense-organ.

The labium or lower lip has as its basal portion the undivided mentum, and closes the mouth beneath or behind, according as the position of the head varies. In most Insects the labium appears very different from the maxilla, but in many cases several of the parts corresponding to those of the maxilla can be clearly traced in the labium.

Fig. 52.—Maxilla and lower lip of Coleoptera. A, Maxilla of Passalus: a, cardo; b, stipes; c, palpiger; d, palpus; e, inner or inferior lobe or lacinia; f, outer or superior lobe or galea: B, Labium of Harpalus caliginosus: a, mentum; b, hypoglottis; c, palpiger (support of the labial palp); d, palp; e, ligula; f, paraglossa.

The mentum is an undivided, frequently very hard, piece, continuous with either the submentum or the gula, and anterior to this are placed the other parts, viz. the labial palpi and their supports, the palpigers; beyond and between these exists a central piece (Fig. 52, B, e), about whose name some difference of opinion prevails, but which may be called the ligula (languette of French authors), and on each side of this is a paraglossa. In the Orthoptera the single median piece—the ligula of Coleopterists—is represented by two divided parts. In some Insects (many Coleoptera) there is interposed between the mentum and the palpigers a piece called the hypoglottis (Fig. 52, B, b). It is not so well ascertained as it should be, that the pieces of the lower lip bearing the same names in different Orders are in all cases really homologous, and comparison suggests that the hypoglottis of Coleoptera may possibly represent the piece corresponding to the mentum of Orthopterists, the so-called mentum of beetles being in that case the submentum of Orthopterists.

There is another part of the mouth to which we may call special attention, as it has recently attracted more attention than it formerly did; it is a membranous lobe in the interior of the mouth, very conspicuous in Orthoptera, and called the tongue, lingua, or hypopharynx; it reposes, in the interior of the mouth (Fig. 51, o), on the middle parts of the front of the labium; it is probably not entirely lost in Coleoptera, but enters into the composition of the complex middle part of the lip by amalgamation with the paraglossae. It has recently been proposed to treat this lingua as the morphological equivalent of the labium or of the maxillae, giving it the name of the endolabium, but the propriety of this course remains to be proved;[^[20]] the view is apparently suggested chiefly by the structure of the mouth of Hemimerus, a very rare and most peculiar Insect that has not as yet been sufficiently studied.

As the maxillae and labium are largely used by taxonomists in the systematic arrangement of the mandibulate Insects, we give a figure of them as seen in Coleoptera, where the parts, though closely amalgamated, can nevertheless be distinguished. This Fig. 52 should be compared with Fig. 51.

In speaking of the segments of the body we pointed out that they were not separate parts but constituted an uninterrupted whole, and it is well to remark here that this is also true of the gnathites. Although the mouth parts are spoken of as separate pieces, they really form only projections from the great body wall. Fig. 51, B, shows the intimate connexion that exists between the maxillae and labium; the continuity of the mandibles with the membrane of the buccal cavity is capable of very easy demonstration.

The head bears, besides the pieces we have considered, a pair of antennae. These organs, though varying excessively in form, are always present in the adult Insect, and exist even in the majority of young Insects. They are very mobile, highly sensitive organs, situate on or near the front part of the head. The antennae arise in the embryo from the procephalic lobes, the morphological import of which parts is one of the most difficult points connected with Insect embryology.

The eyes of Insects are of two sorts, simple and compound. The simple eyes, or ocelli, vary in number from one to as many as eighteen or twenty; when thus numerous they are situated in groups on each side of the head. In their most perfect form, as found in adult aculeate Hymenoptera, in Orthoptera and Diptera, ocelli are usually two or three in number, and present the appearance of small, perfectly transparent lenses inserted in the integument. In their simplest form they are said to consist of some masses of pigment in connexion with a nerve.

Fig. 53.—Two ommatidia from the eye of Colymbetes fuscus, × 160. (After Exner.) a, Cornea; b, crystalline cone; c, rhabdom; d, fenestrate membrane with nerve structures below it; e, iris-pigment; f, retina-pigment.

The compound, or facetted, eyes are the most remarkable of all the structures of the Insect, and in the higher and more active forms, such as the Dragon-flies and hovering Diptera, attain a complexity and delicacy of organisation that elicit the highest admiration from every one who studies them. They are totally different in structure and very distinct in function from the eyes of Vertebrata, and are seated on very large special lobes of the brain (see Fig. 65), which indeed are so large and so complex in structure that Insects may be described as possessing special ocular brains brought into relation with the lights, shades, and movements of the external world by a remarkably complex optical apparatus. This instrumental part of the eye is called the dioptric part in contradistinction from the percipient portion, and consists of an outer corneal lens (a, Fig. 53), whose exposed surface forms one of the facets of the eye; under the lens is placed the crystalline cone (b), this latter being borne on a rod-like object (c), called the rhabdom. There are two layers of pigment, the outer (e), called the iris-pigment, the inner (f), the retinal-pigment; underneath, or rather we should say more central than, the rhabdoms is the fenestrate membrane (d), beyond which there is an extremely complex mass of nerve-fibres; nerves also penetrate the fenestrate membrane, and their distal extremities are connected with the delicate sheaths by one of which each rhabdom is surrounded, the combination of sheath and nerves forming a retinula. Each set of the parts above the fenestrate membrane constitutes an ommatidium, and there may be many of these ommatidia in an eye; indeed, it is said that the eye of a small beetle, Mordella, contains as many as 25,000 ommatidia. As a rule the larvae of Insects with a complete metamorphosis bear only simple eyes. In the young of Dragon-flies, as well as of some other Insects having a less perfect metamorphosis, the compound eyes exist in the early stages, but they have then an obscure appearance, and are probably functionally imperfect.

In the interior of the head there exists a horny framework called the tentorium, whose chief office apparently is to protect the brain. It is different in kind according to the species. The head shows a remarkable and unique relation to the following segments. It is the rule in Insect structure that the back of a segment overlaps the front part of the one following it; in other words, each segment receives within it the front of the one behind it. Though this is one of the most constant features of Insect anatomy, it is departed from in the case of the head, which may be either received into, or overlapped by, the segment following it, but never itself overlaps the latter. There is perhaps but a single Insect (Hypocephalus, an anomalous beetle) in which the relation between the head and thorax can be considered to be at all similar to that which exists between each of the other segments of the body and that following it; and even in Hypocephalus it is only the posterior angles of the head that overlap the thorax. Although the head usually appears to be very closely connected with the thorax, and is very frequently in repose received to a considerable extent within the latter, it nevertheless enjoys great freedom of motion; this is obtained by means of a large membrane, capable of much corrugation, and in which there are seated some sclerites, so arranged as to fold together and occupy little space when the head is retracted, but which help to prop and support it when extended for feeding or other purposes. These pieces are called the cervical sclerites or plates. They are very largely developed in Hymenoptera, in many Coleoptera, and in Blattidæ, and have not yet received from anatomists a sufficient amount of attention. Huxley suggested that they may be portions of head segments.

Fig. 54.—Extended head and front of thorax of a beetle, Euchroma: a, back of head; b, front of pronotum; c, chitinous retractile band; d, cervical sclerites.

Thorax.

The thorax, being composed of the three consecutive rings behind the head, falls naturally into three divisions—pro-, meso-, and metathorax. These three segments differ greatly in their relative proportions in different Insects, and in different stages of the same Insect's life. In their more highly developed conditions each of the three divisions is of complex structure, and the sclerites of which it is externally made up are sufficiently constant in their numbers and relative positions to permit of their identification in a vast number of cases; hence the sclerites have received names, and their nomenclature is of practical importance, because some, if not all, of these parts are made use of in the classification of Insects. Each division of the thorax has an upper region, called synonymically dorsum, notum, or tergum; an inferior or ventral region, called sternum; and on each side a lateral region, the pleuron. These regions of each of the three thoracic divisions are further distinguished by joining to their name an indication of the segment spoken of, in the form of the prefixes pro-, meso-, and meta-; thus the pronotum, prosternum, and propleura make up the prothorax. The thoracic regions are each made up of sclerites whose nomenclature is due to Audouin.[^[21]] He considered that every thoracic ring is composed of the pieces shown in Fig. 55, viz. (1) the sternum (B', a), an unpaired ventral piece; (2) the notum (A), composed of four pieces placed in consecutive longitudinal order (A'), and named praescutum (a), scutum (b), scutellum (c), and post-scutellum (d); (3) lateral pieces, of which he distinguished on each side an episternum (B', c), epimeron (e), and parapteron (d), these together forming the pleuron. We give Audouin's Figure, but we cannot enter on a full discussion of his views as to the thorax; they have become widely known, though the constancy of the parts is not so great as he supposed it would prove to be. Sometimes it is impossible to find all the elements he thought should be present in a thoracic ring, while in other cases too many sclerites exist. As a rule the notum of the meso- and metathoraces is in greater part composed of two pieces, the scutum and the scutellum; while in the pronotum only one dorsal piece can be satisfactorily distinguished, though a study of the development may show that really two are frequently, if not usually, present. On the other hand, one, or more, of the notal sclerites in some cases shows evidence of longitudinal division along the middle. The sternum or ventral piece, though varying greatly in form, is the most constant element of a thoracic segment, but it has sometimes the appearance of consisting of two parts, an anterior and a posterior. The pleuron nearly always consists quite evidently of two parts, the episternum, the more anterior and inferior, and the epimeron.[^[22]] The relations between these two parts vary much; in some cases the episternum is conspicuously the more anterior, while in others the epimeron is placed much above it, and may extend nearly as far forwards as it. It may be said, as a rule, that when the sternum extends farther backwards than the notum, the epimeron is above the episternum, as in many Coleoptera; but if the sternum be anterior to the notum, then the episternum is superior to the epimeron, as in dragon-flies. We would here again reiterate the fact that these "pieces" are really not separate parts, but are more or less indurated portions of a continuous integument, which is frequently entirely occupied by them; hence a portion of a sclerite that in one species is hard, may in an allied form be wholly or partly membranous, and in such case its delimitation may be very evident on some of its sides, and quite obscure on another.

Fig. 55.—Mesothorax of Dytiscus, after Audouin. A, notum; A', pieces of the notum separated: a, praescutum; b, scutum; c, scutellum; d, post-scutellum: B, the sternum and pleura united; B', their parts separated: a, sternum; c, episternum; d, parapteron; e, epimeron.

The parapteron of Audouin does not appear to be really a distinct portion of the pleuron; in the case of Dytiscus it is apparently merely a thickening of an edge. Audouin supposed this part to be specially connected with the wing-articulation, and the term has been subsequently used by other writers in connexion with several little pieces that exist in the pleural region of winged Insects.

The prothorax is even more subject to variation in its development than the other divisions of the thorax are. In the Hymenoptera the prosternum is disconnected from the pronotum and is capable, together with the first pair of legs, of movement independent of its corresponding dorsal part, the pronotum, which in this Order is always more or less completely united with the meso-thorax; in the Diptera the rule is that the three thoracic segments are closely consolidated into one mass. In the majority of Insects the prothorax is comparatively free, that is to say, it is not so closely united with the other two thoracic segments as they are with one another. The three thoracic rings are seen in a comparatively uniform state of development in a great number of larvae; also in the adult stages of some Aptera, and among winged insects in some Neuroptera such as the Embiidae, Termitidae, and Perlidae. In Lepidoptera the pronotum bears a pair of erectile processes called patagia; though frequently of moderately large size, they escape observation, being covered with scales and usually closely adpressed to the sides of the pronotum.

The two great divisions of the body—the mesothorax and the metathorax—are usually very intimately combined in winged Insects, and even when the prothorax is free, as in Coleoptera, these posterior two thoracic rings are very greatly amalgamated. In the higher forms of the Order just mentioned the mesosternum and mesopleuron become changed in direction, and form as it were a diaphragm closing the front of the metasternum. The meso- and meta-thorax frequently each bear a pair of wings.

We have described briefly and figured (Fig. 55) the sclerites of the mesothorax, and those of the metathorax correspond fairly well with them. In addition to the sclerites usually described as constituting these two thoracic divisions, there are some small pieces at the bases of the wings. Jurine discriminated and named no less than seven of these at the base of the anterior wing of a Hymenopteron. One of them becomes of considerable size and importance in the Order just mentioned, and seems to be articulated so as to exert pressure on the base of the costa of the wing. This structure attains its maximum of development in a genus (? nondescript) of Scoliidae, as shown in Fig. 56. The best name for this sclerite seems to be that proposed by Kirby and Spence, tegula. Some writers call it paraptère, hypoptère, or squamule, and others have termed it patagium; this latter name is, however, inadmissible, as it is applied to a process of the prothorax we have already alluded to.

Fig. 56.—Head and thorax of wasp from Bogota: t, tegula; b, base of wing.

To complete our account of the structure of the thorax it is necessary to mention certain hard parts projecting into its interior, but of which there is usually little or no trace externally. A large process in many Insects projects upwards from the sternum in a forked manner. It was called by Audouin the entothorax; some modern authors prefer the term apophysis. Longitudinal partitions of very large size, descending from the dorsum into the interior, also exist; these are called phragmas, and are of great importance in some Insects with perfect flight, such as Hymenoptera, Lepidoptera, and Diptera. There is no phragma in connection with the pronotum, but behind this part there may be three. A phragma has the appearance of being a fold of the dorsum; it serves as an attachment for muscles, and may probably be of service in other ways. More insignificant projections into the interior are the little pieces called apodemes (Fig. 57, e); these are placed at the sides of the thorax near the wings. The apophyses are no doubt useful in preserving the delicate vital organs from shocks, or from derangement by the muscular movements and the changes of position of the body.

Fig. 57.—Transverse section of skeleton of metathorax of Goliathus druryi, seen from behind: a, metanotum; b, metasternum; c, phragma; d, entothorax (apophysis or furca); e, apodeme; f, tendon of articulation. (After Kolbe.)

The appendages of the thorax are (a) inferior, the legs; (b) superior, the wings. The legs are always six in number, and are usually present even in larvae, though there exist many apodal larvae, especially in Diptera. The three pairs of legs form one of the most constant of the characters of Insects. They are jointed appendages and consist of foot, otherwise tarsus; tibia, femur, trochanter, and coxa; another piece, called trochantin more or less distinctly separated from the coxa, exists in many Insects. The legs are prolongations of the body sac, and are in closer relation with the epimera and with the episterna than with other parts of the crust, though they have a close relation with the sternum. If we look at the body and leg of a neuropterous Insect (Fig. 58) we see that the basal part of the leg—the coxa—is apparently a continuation of one of the two pleural pieces or of both; in the latter case one of the prolonged pieces forms the coxa proper, and the tip of the other forms a supporting piece, which may possibly be the homologue of the trochantin of some Insects. In some Orthoptera, especially in Blattidae, and in Termitidae, there is a transverse chitinised fold interposed between the sternum and the coxa, and this has the appearance of being the same piece as the trochantin of the anterior legs of Coleoptera.

Fig. 58.—Hind leg of Panorpa: a, episternum; a′, epimeron; b, coxa; b′, coxal fold of epimeron; c, trochanter; d, femur; e, tibia; f, tarsus.

Beyond the coxa comes the trochanter; this in many Hymenoptera is a double piece, though in other Insects it is single; usually it is the most insignificant part of the leg. The femur is, on the whole, the least variable part of the leg; the tibia, which follows it, being frequently highly modified for industrial or other purposes. The joint between the femur and the tibia is usually bent, and is therefore the most conspicuous one in the leg; it is called the knee. The other joints have not corresponding names, though that between the tibia and the tarsus is of great importance. The spines at the tip of the tibia, projecting beyond it, are called spurs, or calcares. The tarsus or foot is extremely variable; it is very rarely absent, but may consist of only one piece—joint, as it is frequently called[^[23]]—or of any larger number up to five, which may be considered the characteristic number in the higher Insect forms. The terminal joint of the tarsus bears normally a pair of claws; between the claws there is frequently a lobe or process, according to circumstances very varied in different Insects, called empodium, arolium, palmula, plantula, pseudonychium, or pulvillus. This latter name should only be used in those cases in which the sole of the foot is covered with a dense pubescence. The form of the individual tarsal joints and the armature or vestiture of the lower surface are highly variable. The most remarkable tarsus is that found on the front foot of the male Dytiscus.

It has been suggested that the claws and the terminal appendage of the tarsus ought to be counted as forming a distinct joint; hence some authors state that the higher Insects have six joints to the feet. These parts, however, are never counted as separate joints by systematic entomologists, and it has recently been stated that they are not such originally.

The parts of the foot at the extremity of the last tarsal joint proper are of great importance to the creature, and vary greatly in different Insects. The most constant part of this apparatus is a pair of claws, or a single claw. Between the two claws there may exist the additional apparatus referred to above. This in some Insects—notably in the Diptera—reaches a very complex development. We figure these structures in Pelopaeus spinolae, a fossorial Hymenopteron, remarking that our figures exhibit the apparatus in a state of retraction (Fig. 59). According to the nomenclature of Dahl and Ockler[^[24]] the plate (b) on the dorsal aspect is the pressure plate (Druck-Platte), and acts as an agent of pressure on the sole of the pad (C, e); c and d on the underside are considered to be extension-agents; c, extension-plate; d, extension-sole (Streck-Platte, Streck-Sohle). These agents are assisted in acting on the pad by means of an elastic bow placed in the interior of the latter. The pad (e) is a very remarkable structure, capable of much extension and retraction; when extended it is seen that the pressure plate is bent twice at a right angle so as to form a step, the distal part of which runs along the upper face of the basal part of the pad; the apical portion of this latter consists of two large lobes, which in repose, as shown in our Figure (f), fall back on the pad, something in the fashion of the retracted claws of the cat, and conceal the pressure-plate.

The mode in which Insects are able to walk on smooth perpendicular surfaces has been much discussed, and it appears highly probable that the method by which this is accomplished is the exudation of moisture from the foot; there is still, however, much to be ascertained before the process can be satisfactorily comprehended. The theory to the effect that the method is the pressure of the atmosphere acting on the foot when the sole is in perfect apposition with the object walked on, or when a slight vacuum is created between the two, has apparently less to support it.

Fig. 59.—Foot of Pelopaeus, a fossorial wasp: A, tarsus entire; B, terminal joint, upper side; C, under side. a, claw; b, base of pressure-plate; c, extension-plate; d, extension-sole; e, pad; f, lobe of pad retracted.

The legs of the young Insect are usually more simple than those of the adult, and in caterpillars they are short appendages, and only imperfectly jointed. If a young larva, with feet, of a beetle, such as Crioceris asparagi be examined, it may be seen that the leg is formed by protuberance of the integument, which becomes divided into parts by simple creases; an observation suggesting that the more highly developed jointed leg is formed in a similar manner. This appears to be really the case, for the actual continuity of the limb at the chief joint—the knee—can be demonstrated in many Insects by splitting the outer integument longitudinally and then pulling the pieces a little apart; while in other cases even this is not necessary, the knee along its inner face being membranous to a considerable extent, and the membrane continuous from femur to tibia.

Turning to the wings, we remark that there may be one or two pairs of these appendages. When there is but one pair it is nearly always mesothoracic, when there are two pairs one is invariably mesothoracic, the other metathoracic. The situation of the wing is always at the edge of the notum, but the attachment varies in other respects. It may be limited to a small spot, and this is usually the case with the anterior wing; or the attachment may extend for a considerable distance along the edge of the notum, a condition which frequently occurs, especially in the case of the posterior wings. The actual connexion of the wings with the thorax takes place by means of strong horny lines in them which come into very close relation with the little pieces in the thorax which we have already described, and which were styled by Audouin articulatory epidemes. There is extreme variety in the size, form, texture, and clothing of the wings, but there is so much resemblance in general characters amongst the members of each one of the Orders, that it is usually possible for an expert, seeing only a wing, to say with certainty what Order of Insects its possessor belonged to. We shall allude to these characters in treating of the Orders of Insects.

Each wing consists of two layers, an upper and a lower, and between them there may be tracheae and other structures, especially obvious when the wings are newly developed. It has been shown by Hagen that the two layers can be separated when the wings are recently formed, and it is then seen that each layer is traversed by lines of harder matter, the nervures. These ribs are frequently called wing-veins, or nerves, but as they have no relation to the anatomical structures bearing those names, it is better to make use of the term nervures. The strength, number, form and inter-relations of these nervures vary exceedingly; they are thus most important aids in the classification of Insects. Hence various efforts have been made to establish a system of nomenclature that shall be uniform throughout the different Orders, but at present success has not attended these efforts, and it is probable that no real homology exists between the nervures of the different Orders of Insects. We shall not therefore discuss the question here. We may, however, mention that German savants have recently distinguished two forms of nervures which they consider essentially distinct, viz. convex and concave. These, to some extent, alternate with one another, but a fork given off by a convex one is not considered to be a concave one. The terms convex and concave are not happily chosen; they do not refer to the shape of the nervures, but appear to have been suggested by the fact that the surface of the wing being somewhat undulating the convex veins more usually run along the ridges, the concave veins along the depressions. The convex are the more important of the two, being the stronger, and more closely connected with the articulation of the wing.

The wings, broadly speaking, may be said to be three-margined: the margin that is anterior when the wings are extended is called the costa, and the edge that is then most distant from the body is the outer margin, while the limit that lies along the body when the wings are closed is the inner margin.

The only great Order of Insects provided with a single pair of wings is the Diptera, and in these the metathorax possesses, instead of wings, a pair of little capitate bodies called halteres or poisers. In the abnormal Strepsiptera, where a large pair of wings is placed on the metathorax, there are on the mesothorax some small appendages that are considered to represent the anterior wings. In the great Order Coleoptera, or beetles, the anterior wings are replaced by a pair of horny sheaths that close together over the back of the Insect, concealing the hind-wings, so that the beetle looks like a wingless Insect: in other four-winged Insects it is usually the front wings that are most useful in flight, but the elytra, as these parts are called in Coleoptera, take no active part in flight, and it has been recently suggested by Hoffbauer[^[25]] that they are not the homologues of the front wings, but of the tegulae (see Fig. 56), of other Insects. In the Orthoptera the front wings also differ in consistence from the other pair over which they lie in repose, and are called tegmina. There are many Insects in which the wings exist in a more or less rudimentary or vestigial condition, though they are never used for purposes of flight.

The abdomen, or hind body, is the least modified part of the body, though some of the numerous rings of which it is composed may be extremely altered from the usual simple form. Such change takes place at its two extremities, but usually to a much greater extent at the distal extremity than at the base. This latter part is attached to the thorax, and it is a curious fact that in many Insects the base of the abdomen is so closely connected with the thorax that it has all the appearance of being a portion of this latter division of the body; indeed it is sometimes difficult to trace the real division between the two parts. In such cases a further differentiation may occur, and the part of the abdomen that on its anterior aspect is intimately attached to the thorax may on its posterior aspect be very slightly connected with the rest of the abdomen. Under such circumstances it is difficult at first sight to recognise the real state of the case. When a segment is thus transferred from the abdomen to the metathorax, the part is called a median segment. The most remarkable median segment exists in those Hymenoptera which have a stalked abdomen, but a similar though less perfect condition exists in many Insects. When such a union occurs, it is usually most complete on the dorsal surface, and the first ventral plate may almost totally disappear: such an alteration may involve a certain amount of change in the sclerites of the next segment, so that the morphological determination of the parts at the back of the thorax and front of the abdomen is by no means a simple matter. A highly modified hind-body exists in the higher ants, Myrmicidae. In Fig. 60 we contrast the simple abdomen of Japyx with the highly modified state of the same part in an ant.

Fig. 60.—Simple abdomen of Japyx (A) contrasted with the highly modified one of an ant, Cryptocerus (B). The segments are numbered from before backwards.

Unlike the head and thorax, the abdomen is so loosely knitted together that it can undergo much expansion and contraction. This is facilitated by an imbricated arrangement of the plates, and by their being connected by means of membranes admitting of much movement (Fig. 47, m, p. [88]). In order to understand the structure of the abdomen it should be studied in its most distended state; it is then seen that there is a dorsal and a ventral hard plate to each ring, and there is also usually a stigma; there may be foldings or plications near the line of junction of the dorsal and ventral plates, but these margins are not really distinct pieces. The pleura, in fact, remain membranous in the abdominal region, contrasting strongly with the condition of these parts in the thorax. The proportions of the plates vary greatly; sometimes the ventral are very large in proportion to the dorsal, as is usually the case in Coleoptera, while in the Orthoptera the reverse condition prevails.

Cerci or other appendages frequently exist at the extremity of the abdomen (Fig. 47, n, p. [88]); the former are sometimes like antennae, while in other cases they may be short compressed processes consisting of very few joints. The females of many Insects possess saws or piercing instruments concealed within the apical part of the abdomen; in other cases an elongate exserted organ, called ovipositor, used for placing the eggs in suitable positions, is present. Such organs consist, it is thought, either of modified appendages, called gonapophyses, or of dorsal, ventral, or pleural plates. The males frequently bear within the extremity of the body a more or less complicated apparatus called the genital armour. The term gonapophysis is at present a vague one, including stings, some ovipositors, portions of male copulatory apparatus, or other structures, of which the origin is more or less obscure.

The caterpillar, or larva, of the Lepidoptera and some other Insects, bears a greater number of legs than the three pairs we have mentioned as being the normal number in Insects, but the posterior feet are in this case very different from the anterior, and are called false legs or prolegs. These prolegs, which are placed on the hind body, bear a series of hooks in Lepidopterous larvae, but the analogous structures of Sawfly larvae are destitute of such hooks.

Placed along the sides of the body, usually quite visible in the larva, but more or less concealed in the perfect Insect, are little apertures for the admittance of air to the respiratory system. They are called spiracles or stigmata. There is extreme variety in their structure and size; the largest and most remarkable are found on the prothorax of Coleoptera, especially in the groups Copridae and Cerambycidae.

The exact position of the stigmata varies greatly, as does also their number. In the Order Aptera there may be none, while the maximum number of eleven pairs is said by Grassi[^[26]] to be attained in Japyx solifugus: in no other Insect have more than ten pairs been recorded, and this number is comparatively rare. Both position and number frequently differ in the early and later stages of the same Insect. The structure of the stigmata is quite as inconstant as the other points we have mentioned are.

Fig. 61.—Membranous space between pro- and meso-thoraces of a beetle Euchroma, showing stigma (st); a, hind margin of pronotum; b, front leg; c, front margin of mesonotum; d, base of elytra; e, mesosternum.

The admission of air to the tracheal system and its confinement there, as well as the exclusion of foreign bodies, have to be provided for. The control of the air within the system is, according to Landois[^[27]] and Krancher,[^[28]] usually accomplished by means of an occluding apparatus placed on the tracheal trunk a little inside of the stigma, and in such case this latter orifice serves chiefly as a means for preventing the intrusion of foreign bodies. The occluding apparatus consists of muscular and mechanical parts, which differ much in their details in different Insects. Lowne supposes that the air is maintained in the tracheal system in a compressed condition, and if this be so, this apparatus must be of great importance in the Insect economy. Miall and Denny[^[29]] state that in the anterior stigmata of the cockroach the valves act as the occluding agents, muscles being attached directly to the inner face of the valves, and in some other Insects the spiracular valves appear to act partially by muscular agency, but there are many stigmata having valves destitute of muscles. According to Lowne[^[30]] there exist valves in the blowfly at the entrance to the trachea proper, and he gives the following as the arrangement of parts for the admission of air:—there is a spiracle leading into a chamber, the atrium, which is limited inwardly by the occluding apparatus; and beyond this there is a second chamber, the vestibule, separated from the tracheae proper by a valvular arrangement. He considers that the vestibule acts as a pump to force the air into the tracheae.

Fig. 62.—Diagrammatic Insect to explain terms of position. A, apex; B, base: 1, tibia; 2, last abdominal segment; 3, ideal centre.

Systematic Orientation.

Terms relating to position are unfortunately used by writers on entomology in various, even in opposite senses. Great confusion exists as to the application of such words as base, apex, transverse, longitudinal. We can best explain the way in which the relative positions and directions of parts should be described by reference to Figure 62. The spot 3 represents an imaginary centre, situated between the thorax and abdomen, to which all the parts of the body are supposed to be related. The Insect should always be described as if it were in the position shown in the Figure, and the terms used should not vary as the position is changed. The creature is placed with ventral surface beneath, and with the appendages extended, like the Insect itself, in a horizontal plane. In the Figure the legs are, for clearness, made to radiate, but in the proper position the anterior pair should be approximate in front, and the middle and hind pairs directed backwards under the body. The legs are not to be treated as if they were hanging from the body, though that is the position they frequently actually assume. The right and left sides, and the upper and lower faces (these latter are frequently also spoken of as sides), are still to retain the same nomenclature even when the position of the specimen is reversed. The base of an organ is that margin that is nearest to the ideal centre, the apex that which is most distant. Thus in Fig. 62, where 1 indicates the front tibia, the apex (A) is broader than the base (B); in the antennae the apex is the front part, while in the cerci the apex is the posterior part; in the last abdominal segment (2) the base (B) is in front of the apex (A). The terms longitudinal and transverse should always be used with reference to the two chief axes of the body-surface; longitudinal referring to the axis extending from before backwards, and transverse to that going across, i.e. from side to side.

CHAPTER IV

ARRANGEMENT OF INTERNAL ORGANS–MUSCLES–NERVOUS SYSTEM–GANGLIONIC CHAIN–BRAIN–SENSE-ORGANS–ALIMENTARY CANAL–MALPIGHIAN TUBES–RESPIRATION–TRACHEAL SYSTEM–FUNCTION OF RESPIRATION–BLOOD OR BLOOD-CHYLE–DORSAL VESSEL OR HEART–FAT-BODY–OVARIES–TESTES–PARTHENOGENESIS–GLANDS.

The internal anatomy of Insects may be conveniently dealt with under the following heads:—(1) Muscular system; (2) nervous system; (3) alimentary system (under which may be included secretion and excretion, about which in Insects very little is known); (4) respiratory organs; (5) circulatory system; (6) fat-body; (7) reproductive system.

Fig. 63.—Diagram of arrangement of some of the internal organs of an Insect: a, mouth; b, mandible; c, pharynx; d, oesophagus; e, salivary glands (usually extending further backwards); f, eye; g, supra-oesophageal ganglion; h, sub-oesophageal ganglion; i, tentorium; j, aorta; k₁, k₂, k₃, entothorax; l₁-l₈, ventral nervous chain; m, crop; n, proventriculus; o, stomach; p, Malpighian tubes; q, small intestine; r, large intestine; s, heart; t, pericardial septum; u, ovary composed of four egg-tubes; v, oviduct; w, spermatheca (or an accessory gland); x, retractile ovipositor; y, cercus; z, labrum.

Many of the anatomical structures have positions in the body that are fairly constant throughout the class. Parts of the respiratory and muscular systems and the fat-body occur in most of the districts of the body. The heart is placed just below the dorsal surface; the alimentary canal extends along the middle from the head to the end of the body. The chief parts of the nervous system are below the alimentary canal, except that the brain is placed above the beginning of the canal in the head. The reproductive system extends in the abdomen obliquely from above downwards, commencing anteriorly at the upper part and terminating posteriorly at the lower part of the body cavity.

In Fig. 63 we show the arrangement of some of the chief organs of the body, with the exception of the muscular and respiratory systems, and the fat-body. It is scarcely necessary to point out that the figure is merely diagrammatic, and does not show the shapes and sizes of the organs as they will be found in any one Insect.

Muscles.

The muscular system of Insects is very extensive, Lyonnet[^[31]] having found, it is said, nearly 4000 muscles in the caterpillar of the goat-moth; a large part of this number are segmental repetitions, nevertheless the muscular system is really complex, as may be seen by referring to the study of the flight of dragon-flies by von Lendenfeld.[^[32]]

The minute structure of the muscles does not differ essentially from what obtains in Vertebrate animals. The muscles are aggregations of minute fibrils which are transversely striated, though in variable degree. Those in the thorax are yellow or pale brown, but in other parts the colour is more nearly white. The muscles of flight are described as being penetrated by numerous tracheae, while those found elsewhere are merely surrounded by these aerating tubules.

The force brought into play by the contractions of Insect muscles is very great, and has been repeatedly stated to be much superior to that of Vertebrate animals; very little reliance can, however, be placed on the assumptions and calculations that are supposed to prove this, and it is not supported by Camerano's recent researches.[^[33]]

Some of the tendons to which the muscles are attached are very elaborate structures, and are as hard as the chitinous skeleton, so as to be like small bones in their nature. A very elaborate tendon of this kind is connected with the prothoracic trochantin in Coleoptera, and may be readily examined in Hydrophilus. It has been suggested that the entothorax is tendinous in its origin, but other morphologists treat it, with more reason, as an elaborate fold inwards of the integument.

Fig. 64.—Cephalic and ventral chain of ganglia: A, larva of Chironomus; B, imago of Hippobosca. (After Brandt.)

Nervous System.

Insects are provided with a very complex nervous system, which may be treated as consisting of three divisions:—(1) The cephalic system; (2) the ventral, or ganglionic chain; (3) an accessory sympathetic system, or systems. All these divisions are intimately connected. We will consider first the most extensive, viz. the ventral chain. This consists of a series of small masses of nervous matter called ganglia which extend in the longitudinal direction of the body along the median line of the lower aspect, and are connected by longitudinal commissures, each ganglion being joined to that following it by two threads of nervous matter. Each of the ganglia of the ventral chain really consists of two ganglia placed side by side and connected by commissures as well as cellular matter. In larvae some of the ganglia may be contiguous, so that the commissures do not exist. From the ganglia motor nerves proceed to the various parts of the body for the purpose of stimulating and co-ordinating the contractions of the muscles. The number of the ganglia in the ventral chain differs greatly in different Insects, and even in the different stages of metamorphosis of the same species, but never exceeds thirteen. As this number is that of the segments of the body, it has been considered that each segment had primitively a single ganglion. Thirteen ganglia for the ventral chain can, however, be only demonstrated in the embryonic state; in the later stages of life eleven appears to be the largest number that can be distinguished, and so many as this are found but rarely, and then chiefly in the larval stage. The diminution in number takes place by the amalgamation or coalescence of some of the ganglia, and hence those Insects in which the ganglia are few are said to have a highly concentrated nervous system. The modes in which these ganglia combine are very various; the most usual is perhaps that of the combination of the three terminal ganglia into one body. As a rule it may be said that concentration is the concomitant of a more forward position of the ganglia. As a result of this it is found that in some cases, as in Lamellicorn beetles, there are no ganglia situate in the abdomen. In the perfect state of the higher Diptera, the thoracic and abdominal ganglia are so completely concentrated in the thorax as to form a sort of thoracic brain. In Fig. 64 we represent a very diffuse and a very concentrated ganglionic chain; A being that of the larva of Chironomus, B that of the imago of Hippobosca. In both these sketches the cephalic ganglia as well as those of the ventral chain are shown.

Turning next to the cephalic masses, we find these in the perfect Insect to be nearly always two in number: a very large and complex one placed above the oesophagus, and therefore called the supra-oesophageal ganglion; and a smaller one, the sub- or infra-oesophageal, placed below the oesophagus. The latter ganglion is in many Insects so closely approximated to the supra-oesophageal ganglion that it appears to be a part thereof, and is sometimes spoken of as the lower brain. In other Insects these two ganglia are more remote, and the infra-oesophageal one then appears part of the ventral chain. In the embryo it is said that the mode of development of the supra-oesophageal ganglion lends support to the idea that it may be the equivalent of three ganglia; there being at one time three lobes, which afterwards coalesce, on each side of the mouth. This is in accordance with the view formulated by Viallanes[^[34]] to the effect that this great nerve-centre, or brain, as it is frequently called, consists essentially of three parts, viz. a Proto-, a Deuto-, and a Trito-cerebron. It is, however, only proper to say that though the brain and the ventral chain of ganglia may appear to be one system, and in the early embryonic condition to be actually continuous, these points cannot be considered to be fully established. Dr. L. Will has informed us[^[35]] that in Aphididae the brain has a separate origin, and is only subsequently united with the ganglionic chain. Some authorities say that in the early condition the sub-oesophageal ganglion is formed from two, and the supra-oesophageal from the same number of ganglia; the division in that case being 2 and 2, not 3 and 1, as Viallanes' views would suggest. The inquiries that are necessary to establish such points involve very complex and delicate investigations, so that it is not a matter of surprise that it cannot yet be said whether each of these views may be in certain cases correct. The supra- and sub-oesophageal ganglia are always intimately connected by a commissure on each side of the oesophagus; when very closely approximated they look like one mass through which passes the oesophagus (Fig. 66, A). The large supra-oesophageal ganglion supplies the great nerves of the cephalic sense-organs, while the smaller sub-oesophageal centre gives off the nerves to the parts of the mouth. From the lower and anterior part of the supra-oesophageal ganglion a nervous filament extends as a ring round the anterior part of the oesophagus, and supplies a nerve to the upper lip.[^[36]] This structure is not very well known, and has been chiefly studied by Liénard,[^[37]] who considers that it will prove to be present in all Insects.

Whether the two cephalic ganglia be considered as really part of a single great ganglionic chain, or the reverse, they are at any rate always intimately connected with the ventral ganglia. We have already stated that the two cephalic masses are themselves closely approximated in many Insects, and may add that in some Hemiptera the first thoracic ganglion of the ventral chain is amalgamated into one body with the sub-oesophageal ganglion, and further that there are a few Insects in which this latter centre is wanting. If the cephalic ganglia and ventral chain be looked on as part of one system, this may be considered as composed originally of seventeen ganglia, which number has been demonstrated in some embryos.

The anatomy of the supra-oesophageal ganglion is very complex; it has been recently investigated by Viallanes[^[38]] in the wasp (Vespa) and in a grasshopper (Caloptenus italicus). The development and complication of its inner structure and of some of its outer parts appear to be proportional with the state of advancement of the instinct or intelligence of the Insect, and Viallanes found the brain of the grasshopper to be of a more simple nature than that of the wasp.

Fig. 65.—Brain of Worker Ant of Formica rufa. (After Leydig, highly magnified.) Explanation in text.

Brandt, to whom is due a large part of our knowledge of the anatomy of the nervous system in Insects, says that the supra-oesophageal ganglion varies greatly in size in various Insects, its mass being to a great extent proportional with the development of the compound eyes; hence the absolute size is not a criterion for the amount of intelligence, and we must rather look to the complication of the structure and to the development of certain parts for an index of this nature. The drone in the honey-bee has, correlatively with the superior development of its eyes, a larger brain than the worker, but the size of the hemispheres, and the development of the gyri cerebrales is superior in the latter. In other words, the mass of those great lobes of the brain that are directly connected with the faceted eyes must not be taken into account in a consideration of the relation of the size and development of the brain to the intelligence of the individual. The weight of the brain in Insects is said by Lowne to vary from 1⁄150 to 1⁄2500 of the weight of the body.

Figure 65 gives a view of one side of the supra-oesophageal ganglion of the worker of an ant,—Formica rufa,—and is taken from Leydig, who gives the following elucidation of it: A, primary lobe, a, homogeneous granular inner substance, b, cellular envelope; B, stalked bodies (gyri cerebrales), a, b, as before; C, presumed olfactory lobes, c, inner substance, d, ganglionic masses; D, ocular lobes, e, f, g, h, various layers of the same; E, origin of lateral commissures; F, median commissure in interior of brain; G, lower brain (sub-oesophageal ganglion); H, ocelli; J, faceted eye.

Fig. 66.—Stomato-gastric nerves of Cockroach: A, with brain in situ, after Koestler; B, with the brain removed, after Miall and Denny: s.g, supra-oesophageal ganglion; o, optic nerve; a, antennary nerve; f.g, frontal ganglion; oe, oesophagus; c, connective; p.g, paired ganglia; v.g, crop or ventricular ganglion; r, recurrent nerve.

Besides the brain and the great chain of ganglia there exists an accessory system, or systems, sometimes called the sympathetic, vagus, or visceral system. Although complex, these parts are delicate and difficult of dissection, and are consequently not so well known as is the ganglionic chain. There is a connecting or median nerve cord, communicating with the longitudinal commissures of each segment, and itself dilating into ganglia at intervals; this is sometimes called the unpaired system. There is another group of nerves having paired ganglia, starting from a small ganglion in the forehead, then connecting with the brain, and afterwards extending along the oesophagus to the crop and proventriculus (Fig. 66). This is usually called the stomatogastric system. The oesophageal ring we have already spoken of.

By means of these accessory nervous systems all the organs of the body are brought into more or less direct relation with the brain and the ganglionic chain.

Our knowledge of these subsidiary nervous systems is by no means extensive, and their nomenclature is very unsettled; little is actually known as to their functions.

Organs of Sense.

Insects have most delicate powers of perception, indeed they are perhaps superior in this respect to the other classes of animals. Their senses, though probably on the whole analogous to those of the Vertebrata, are certainly far from corresponding therewith, and their sense organs seem to be even more different from those of what we call the higher animals than the functions themselves are. We have already briefly sketched the structure of the optical organs, which are invariably situate on the head. This is not the case with the ears, which certainly exist in one Order,—the Orthoptera,—and are placed either on the front legs below the knee, or at the base of the abdomen. Notwithstanding their strange situation, the structures alluded to are undoubtedly auditory, and somewhat approximate in nature to the ear of Vertebrates, being placed in proximity to the inner face of a tense membrane; we shall refer to them when considering the Orthoptera. Sir John Lubbock considers—no doubt with reason—that some ants have auditory organs in the tibia. Many Insects possess rod-like or bristle-like structures in various parts of the body, called chordotonal organs; they are considered by Graber[^[39]] and others to have auditory functions, though they are not to be compared with the definite ears of the Orthoptera.

The other senses and sense organs of Insects are even less known, and have given rise to much perplexity; for though many structures have been detected that may with more or less probability be looked on as sense organs, it is difficult to assign a particular function to any of them, except it be to the sensory hairs. These are seated on various parts of the body. The chitinous covering, being a dead, hard substance, has no nerves distributed in it, but it is pierced with orifices, and in some of these there is implanted a hair which at its base is in connexion with a nerve; such a structure may possibly be sensitive not only to contact with solid bodies, but even to various kinds of vibration. We give a figure (Fig. 67) of some of these hairs on the caudal appendage of a cricket, after Vom Rath. The small hairs on the outer surface of the chitin in this figure have no sensory function, but each of the others probably has; and these latter, being each accompanied by a different structure, must, though so closely approximated, be supposed to have a different function; but in what way those that have no direct connexion with a nerve may act it is difficult to guess.

The antennae of Insects are the seats of a great variety of sense organs, many of which are modifications of the hair, pit and nerve structure we have described above, but others cannot be brought within this category. Amongst these we may mention the pits covered with membrane (figured by various writers), perforations of the chitin without any hair, and membranous bodies either concealed in cavities or partially protruding therefrom.

Fig. 67.—Longitudinal section of portion of caudal appendage of Acheta domestica (after Vom Rath): ch, chitin; hyp, hypodermis; n, nerve; h¹, integumental hairs, not sensitive; h², ordinary hair; h³, sensory hair; h⁴, bladder-like hair; sz, sense-cell.

Fig. 68.—Longitudinal section of apex of palpus of Pieris brassicae: sch, scales; ch, chitin; hyp, hypodermis; n, nerve; sz, sense cells; sh, sense hairs. (After Vom Rath.)

Various parts of the mouth are also the seats of sense organs of different kinds, some of them of a compound character; in such cases there may be a considerable number of hairs seated on branches of a common nerve as figured by Vom Rath[^[40]] on the apex of the maxillary palp of Locusta viridissima, or a compound organ such as we represent in Fig. 68 may be located in the interior of the apical portion of the palp.

The functions of the various structures that have been detected are, as already remarked, very difficult to discover. Vom Rath thinks the cones he describes on the antennae and palpi are organs of smell, while he assigns to those on the maxillae, lower lip, epipharynx, and hypopharynx the rôle of taste organs, but admits he cannot draw any absolute line of distinction between the two forms. The opinions of Kraepelin, Hauser, and Will, as well as those of various earlier writers, are considered in Sir John Lubbock's book on this subject.[^[41]]

Alimentary and Nutritive System.

The alimentary canal occupies the median longitudinal axis of the body, being situated below the dorsal vessel, and above the ventral nervous chain; it extends from the mouth to the opposite extremity of the body. It varies greatly in the different kinds of Insects, but in all its forms it is recognised as consisting essentially of three divisions: anterior, middle, and posterior. The first and last of these divisions are considered to be of quite different morphological nature from the middle part, or true stomach, and to be, as it were, invaginations of the extremities of a closed bag; it is ascertained that in the embryo these invaginations have really blind extremities (see Fig. 82, p. [151]), and only subsequently become connected with the middle part of the canal. There are even some larvae of Insects in which the posterior portion of the canal is not opened till near the close of the larval life; this is the case with many Hymenoptera, and it is probable, though not as frequently stated certain, that the occlusion marks the point of junction of the proctodaeum with the stomach. The anterior and posterior parts of the canal are formed by the ectoderm of the embryo, and in embryological and morphological language are called respectively the stomodaeum and proctodaeum; the true stomach is formed from the endoderm, and the muscular layer of the whole canal from the mesoderm.

Fig. 69.—Digestive system of Xyphidria camelus (after Dufour): a, head capsule; b, salivary glands; c, oesophagus; d, crop; e, proventriculus; f, chyle, or true stomach; g, small intestine; h, large intestine; i, Malpighian tubes; k, termination of body.

The alimentary canal is more complex anatomically than it is morphologically, and various parts are distinguished, viz. the canal and its appendicula; the former consisting of oesophagus, crop, gizzard, true stomach, and an intestine divided into two or more parts. It should be remarked that though it is probable that the morphological distinctions correspond to a great extent with the anatomical lines of demarcation, yet this has not been sufficiently ascertained: the origin of the proctodaeum in Musca is indeed a point of special difficulty, and one on which there is considerable diversity of opinion. In some Hemiptera the division of the canal into three parts is very obscure, so that it would be more correct, as Dufour says, to define it as consisting in these Insects of two main divisions—one anterior to, the other posterior to, the insertion of the Malpighian tubes.

It should be borne in mind that the alimentary canal is very different in different Insects, so that the brief general description we must confine ourselves to will not be found to apply satisfactorily to any one Insect. The oesophagus is the part behind the mouth, and is usually narrow, as it has to pass through the most important nervous centres; extremely variable in length, it dilates behind to form the crop. It may, too, have a dilatation immediately behind the mouth, and in such case a pharynx is considered to exist. The crop is broader than the oesophagus, and must be looked on as a mere dilatation of the latter, as no line of demarcation can be pointed out between the two, and the crop may be totally absent.

In some of the sucking Insects there is a lateral diverticulum, having a stalk of greater or less length, called the sucking-stomach; it is by no means certain that the function this name implies is correctly assigned to the organ.

The gizzard or proventriculus (French, gésier; German, Kaumagen) is a small body interposed in some Insects between the true stomach and the crop or oesophagus. It is frequently remarkable for the development of its chitinous lining into strong toothed or ridged processes that look as if they were well adapted for the comminution of food. The function of the proventriculus in some Insects is obscure; its structure is used by systematists in the classification of ants. The extremity of the proventriculus not infrequently projects into the cavity of the stomach.

The true stomach, or chylific ventricle (Magen or Mitteldarm of the Germans), is present in all the post-embryonic stages of the Insect's life, existing even in the imagines of those who live only for a few hours, and do not use the stomach for any alimentary purpose. It is so variable in shape and capacity that no general description of it can be given. Sometimes it is very elongate, so that it is coiled and like an intestine in shape; it very frequently bears diverticula or pouches, which are placed on the anterior part, and vary greatly in size, sometimes they are only two in number, while in other cases they are so numerous that a portion of the outside of the stomach looks as if it were covered with villi. A division of the stomach into two parts is in some cases very marked, and the posterior portion may, in certain cases, be mistaken for the intestine; but the position of the Malpighian tubes serves as a mark for the distinction of the two structures, the tubes being inserted just at the junction of the stomach with the intestine.

The intestine is very variable in length: the anterior part is the smaller, and is frequently spoken of as the colon; at the extremity of the body the gut becomes much larger, so as to form a rectum. There is occasionally a diverticulum or "caecum" connected with the rectum, and in some Insects stink-glands. In some Hemiptera there is no small intestine, the Malpighian tubes being inserted at the junction of the stomach with the rectum. The total length of the alimentary canal is extremely variable; it is necessarily at least as long as the distance between the mouth and anal orifice, but sometimes it is five or six times as long as this, and some of its parts then form coils in the abdominal cavity.

The alimentary canal has two coats of muscles: a longitudinal and a transverse or annular. Both coexist in most of its parts. Internal to these coats there exists in the anterior and posterior parts of the canal a chitinous layer, which in the stomach is replaced by a remarkable epithelium, the cells of which are renewed, new ones growing while the old are still in activity. We figure a portion of this structure after Miall and Denny, and may remark that Oudemans[^[42]] has verified the correctness of their representation. The layers below represent the longitudinal and transverse muscles.

Fig. 70.—Epithelium of stomach of Cockroach (after Miall and Denny): the lower parts indicate the transverse and longitudinal muscular layers.

In addition to the various diverticula we have mentioned, there are two important sets of organs connected with the alimentary canal, viz. the salivary glands and the Malpighian tubes.

The salivary glands are present in many Insects, but are absent in others. They are situate in the anterior portion of the body, and are very variable in their development, being sometimes very extensive, in other cases inconspicuous. They consist either of simple tubes lined with cells, or of branched tubes, or of tubes dilated laterally into little acini or groups of bags, the arrangement then somewhat resembling that of a bunch of grapes. There are sometimes large sacs or reservoirs connected with the efferent tubes proceeding from the secreting portions of the glands. The salivary glands ultimately discharge into the mouth, so that the fluid secreted by them has to be swallowed in the same manner as the food, not improbably along with it. The silk so copiously produced by some larvae comes from very long tubes similar in form and situation to the simple tubes of the salivary glands.

The Malpighian tubules are present in most Insects, though they are considered on good authority to be absent in many Collembola and in some Thysanura. They are placed near the posterior part of the body, usually opening into the alimentary canal just at the junction of the stomach and the intestine, at a spot called the pylorus. They vary excessively in length and in number,[^[43]] being sometimes only two, while in other cases there may be a hundred or even more of them. In some cases they are budded off from the hind-gut of the embryo when this is still very small; in other cases they appear later; frequently their number is greater in the adult than it is in the young. In Gryllotalpa there is one tube or duct with a considerable number of finer tubes at the end of it. There is no muscular layer in the Malpighian tubes, they being lined with cells which leave a free canal in the centre. The tubes are now thought, on considerable evidence, to be organs for the excretion of uric acid or urates, but it is not known how they are emptied. Marchal has stated[^[44]] that he has seen the Malpighian tubes, on extraction from the body, undergo worm-like movements; he suggests that their contents may be expelled by similar movements when they are in the body.

The functions of the different portions of the alimentary canal, and the extent to which the ingested food is acted on by their mechanical structures or their products is very obscure, and different opinions prevail on important points. It would appear that the saliva exercises a preparatory action on the food, and that the absorption of the nutritive matter into the body cavity takes place chiefly from the true stomach, while the Malpighian tubes perform an excretory function. Beyond these elementary, though but vaguely ascertained facts, little is known, though Plateau's[^[45]] and Jousset's researches on the digestion of Insects throw some light on the subject.

Respiratory Organs.

The respiration of Insects is carried on by means of a system of vessels for the conveyance of air to all parts of the body; this system is most remarkably developed and elaborate, and contrasts strongly with the mechanism for the circulation of the blood, which is as much reduced as the air system is highly developed, as well as with the arrangement that exists in the Vertebrates. There are in Insects no lungs, but air is carried to every part of the body directly by means of tracheae. These tracheae connect with the spiracles—the orifices at the sides of the body we have already mentioned when describing the external structures—and the air thus finds its way into the most remote recesses of the Insect's body. The tracheae are all intimately connected. Large tubes connect the spiracles longitudinally, others pass from side to side of the body, and a set of tracheae for the lower part of the body is connected with another set on the upper surface by means of several descending tubes. From these main channels smaller branches extend in all directions, forking and giving off twigs, so that all the organs inside the body can be supplied with air in the most liberal manner. On opening a freshly deceased Insect the abundance of the tracheae is one of the peculiarities that most attracts the attention; and as these tubes have a peculiar white glistening appearance, they are recognised without difficulty. In Insects of active flight, possibly in some that are more passive, though never in larvae, there are air-sacs, of more than one kind, connected with the tracheae, and these are sufficiently capacious to have a considerable effect in diminishing the specific gravity of the Insect. The most usual situation for these sacs is the basal portion of the abdominal cavity, on the great lateral tracheal conduits. In speaking of the external structure we have remarked that the stigmata, or spiracles, by which the air is admitted are very various in their size and in the manner in which they open and close. Some spiracles have no power of opening; while others are provided with a muscular and valvular apparatus for the purpose of opening and closing effectually.

The structure of the tracheae is remarkable: they are elastic and consist of an outer cellular, and an inner chitinous layer; this latter is strengthened by a peculiar spiral fibre, which gives to the tubes, when examined with the microscope, a transversely, closely striated appearance. Packard considers[^[46]] that in some tracheae this fibre is not really spiral, but consists of a large number of closely placed rings. Such a condition has not, however, been recorded by any other observer. The spiral fibre is absent in the fine capillary twigs of the tracheal system, as well as from the expanded sacs. The mode of termination of the capillary branches is not clear. Some have supposed that the finest twigs anastomose with others; on the other hand it has been said that they terminate by penetrating cells, or that they simply come to an end with either open or closed extremities. Wistinghausen[^[47]] states that in the silk-glands the tracheal twigs anastomose, and he is of opinion that the fine terminal portions contain fluid. However this may be, it is certain that all the organs are abundantly supplied with a capillary tracheal network, or arboreal ramification, and that in some cases the tubes enter the substance of tissues. Near their terminations they are said to be 1⁄30 to 1⁄60 millimetre in diameter.

Fig. 71.—Portion of the abdominal part of tracheal system of a Locust (Oedipoda): a, spiracular orifices; b, tracheal tubes; c, vesicular dilatations; d, tracheal twigs or capillaries. (After Dufour.)

We must repeat that such a system as we have just sketched forms a striking contrast to the imperfect blood-vascular system, and that Insects differ profoundly in these respects from Vertebrate animals. In the latter the blood-vessels penetrate to all the tissues and form capillaries, while the aerating apparatus is confined to one part of the body; in Insects the blood-circulating system is very limited, and air is carried directly by complex vessels to all parts; thus the tracheal system is universally recognised as one of the most remarkable of the characters of Insects. Many Insects have a very active respiratory system, as is shown by the rapidity with which they are affected by agents like chloroform; but the exact manner in which the breathing is carried on is unknown. In living Insects rapid movements of contraction and expansion of parts of the body, chiefly the abdomen, may be observed, and these body contractions are sometimes accompanied by opening and shutting the spiracular orifices: it has been inferred that these phenomena are respiratory. Although such movements are not always present, it is possible that when they occur they may force the air onwards to the tissues, though this is by no means certain. It is clear that the tracheal system is the usual means of supplying the organisation with oxygen, but it appears to be improbable that it can also act as the agent for removing the carbonaceous products of tissue-changes. It has been thought possible that carbonic acid might reach the spiracles from the remote capillaries by a process of diffusion,[^[48]] but it should be recollected that as some Insects have no tracheal system, there must exist some other mode of eliminating carbonic acid, and it is possible that this mode may continue to operate as an important agent of purification, even when the tracheal system is, as a bearer of air to the tissues, highly developed. Eisig[^[49]] has suggested that the formation of chitin is an act of excretion; if so this is capable of relieving the system of carbonic acid to some extent. Others have maintained that transpiration takes place through the delicate portions of the integument. Lubbock[^[50]] has shown that Melolontha larvae breathe "partly by means of their skin." The mode in which the carbon of tissue-change, and the nitrogen of inspiration are removed, is still obscure; but it appears probable that the views expressed by Réaumur, Lyonnet, and Lowne[^[51]] as to inspiration and expiration may prove to be nearer the truth than those which are more widely current. In connexion with this it should be recollected that the outer integument consists of chitin, and is cast and renewed several times during the life of the individual. Now as chitin consists largely of carbon and nitrogen, it is evident that the moulting must itself serve as a carbonaceous and nitrogenous excretion. If, as is suggested by Bataillon's researches,[^[52]] the condition accompanying metamorphosis be that of asphyxia, it is probable that the secretion of the new coat of chitin may figure as an act of excretion of considerable importance. If there be any truth in this suggestion it may prove the means of enabling us to comprehend some points in the development of Insects that have hitherto proved very perplexing.

Peyrou has shown[^[53]] that the atmosphere extracted from the bodies of Insects (Melolontha) is much less rich in oxygen than the surrounding atmosphere is, and at ordinary temperatures always contains a much larger proportion of carbonic acid: he finds, too, that as in the leaves with which he makes a comparison, the proportion of oxygen augments as the protoplasmic activity diminishes. Were such an observation carried out so as to distinguish between the air in the tracheal system and the gas in other parts of the body the result would be still more interesting.

We know very little as to the animal heat produced by insects, but it is clear from various observations[^[54]] that the amount evolved in repose is very small. In different conditions of activity the temperature of the insect may rise to be several degrees above that of the surrounding medium, but there seems to be at present no information as to the physiological mode of its production, and as to the channel by which the products—whether carbonic acid or other matters—may be disposed of.

In the order Aptera (Thysanura and Collembola) the tracheal system is highly peculiar. In some Collembola it apparently does not exist, and in this case we may presume with greater certainty that transpiration of gases occurs through the integument: in other members of this Order tracheae are present in a more or less imperfect state of development, but the tracheae of different segments do not communicate with one another, thus forming a remarkable contrast to the amalgamated tracheal system of the other Orders of Insects, where, even when the tracheal system is much reduced in extent (as in Coccidae), it is nevertheless completely unified. Gryllotalpa is, however, said by Dohrn[^[55]] to be exceptional in this respect; the tracheae connected with each spiracle remaining unconnected.

Water Insects have usually peculiarities in their respiratory systems, though these are not so great as might à priori have been anticipated. Some breathe by coming to the surface and taking in a supply of air in various manners, but some apparently obtain from the water itself the air necessary for their physiological processes. Aquatic Insects are frequently provided with gills, which may be either wing-like expansions of the integument containing some tracheae (Ephemeridae larvae), or bunches of tubes, or single tubes (Trichoptera larvae). Such Insects may either possess stigmata in addition to the gills, or be destitute of them. In other cases air is obtained by taking water into the posterior part of the alimentary canal (many dragon-flies), which part is then provided with special tracheae. Some water-larvae appear to possess neither stigmata nor gills (certain Perlidae and Diptera), and it is supposed that these obtain air through the integument; in such Insects tracheal twigs may frequently be seen on the interior of the skin. In the imago state it is the rule that Water Insects breathe by means of stigmata, and that they carry about with them a supply of air sufficient for a longer or shorter period. A great many Insects that live in water in their earlier stages and breathe there by peculiar means, in their perfect imago state live in the air and breathe in the usual manner. There are, in both terrestrial and aquatic Insects, a few cases of exsertile sacs without tracheae, but filled with blood (Pelobius larva, Machilis, etc.); and such organs are supposed to be of a respiratory nature, though there does not appear to be any positive evidence to that effect.

Blood and Blood-Circulation.

Owing to the great complexity of the tracheal system, and to its general diffusion in the body, the blood and its circulation are very different in Insects from what they are in Vertebrates, so that it is scarcely conducive to the progress of physiological knowledge to call two fluids with such different functions by one name. The blood of Insects varies according to the species, and in all probability even in conformity with the stage of the life of the individual. Its primary office is that of feeding the tissues it bathes, and it cannot be considered as having any aerating function. It is frequently crowded with fatty substances. Graber says: "The richness of Insect blood in unsaponified or unelaborated fat shows in the plainest manner that it is more properly a mixture of blood and chyle; or indeed we might say with greater accuracy, leaving out of consideration certain matters to be eliminated from it, that it is a refined or distilled chyle." Connected in the most intimate manner with the blood there is a large quantity of material called vaguely the fat-body; the blood and its adjuncts of this kind being called by Wielowiejski[^[56]] the blood-tissue. We shall return to the consideration of this tissue after sketching the apparatus for distributing the refined chyle, or blood as we must, using the ordinary term, call it.

There is in Insects no complete system of blood-vessels, though there is a pulsating vessel to ensure distribution of the nutritive fluid. This dorsal vessel, or heart as it is frequently called, may be distinguished and its pulsations watched, in transparent Insects when alive. It is situate at the upper part of the body, extending from the posterior extremity, or near it, to the head or thorax, and is an elongate tube, consisting as it were of a number of united chambers; it is closed behind, except in some larvae, but is open in front, and has several orifices at the sides; these orifices, or ostia, are frequently absent from the front part of the tube, which portion is also narrower, being called the aorta—by no means a suitable term. Near the lateral orifices there are delicate folds, which act to some extent as valves, facilitating, in conjunction with the mode of contraction of the vessel, a forward movement of the blood. The composition of the tube, or series of chambers, is that of a muscular layer, with internal and external membranous coverings, the intima and adventitia. Olga Poletajewa states[^[57]] that in Bombus the dorsal vessel consists of five chambers placed in longitudinal succession, and not very intimately connected, and that there is but little valvular structure. In Cimbex she finds a similar arrangement, but there are ten chambers, and no aorta.

The dorsal vessel is connected with the roof of the body by some short muscles, and is usually much surrounded by fat-body into which tracheae penetrate; by these various means it is kept in position, though only loosely attached; beneath it there is a delicate, incomplete or fenestrate, membrane, delimiting a sort of space called the pericardial chamber or sinus; connected with this membrane are some very delicate muscles, the alary muscles, extending inwards from the body wall (b, Fig. 72): the curtain formed by these muscles and the fenestrate membrane is called the pericardial diaphragm or septum. The alary muscles are not directly connected with the heart.

Fig. 72.—Dorsal vessel (c), and alary muscles (b), of Gryllotalpa (after Graber); a, aorta. N.B.—The ventral aspect is here dorsal, and nearly the whole of the body is removed to show these parts.

Fig. 73.—Diagram of transverse section of pericardial sinus of Oedipoda coerulescens. (After Graber, Arch. Mikr. Anat. ix.) H, heart; s, septum; m, muscles—the upper suspensory, the lower alary.

It has been thought by some that delicate vessels exist beyond the aorta through which the fluid is distributed in definite channels, but this does not appear to be really the case, although the fluid may frequently be seen to move in definite lines at some distance from the heart.

There is still much uncertainty as to some of the details of the action of the heart, and more especially as to the influence of the alary muscles. The effect of the contraction of these must be to increase the area of the pericardial chamber by rendering its floor or septum less arched, as shown in our diagram (Fig. 73), representing a transverse section through the pericardial chamber, H being the dorsal vessel with m its suspensory muscles, and s its septum, with m the alary muscles. The contraction of these latter would draw the septum into the position of the dotted line, thus increasing the area of the sinus above; but as this floor or septum is a fenestrated structure, its contraction allows fluid to pass through it to the chamber above; thus this arrangement may be looked on as a means of keeping up a supply of fluid to the dorsal vessel, the perforated septum, when it contracts, exerting pressure on the tissues below; these are saturated with fluid, which passes through the apertures to the enlarged pericardial chamber.

Some misconception has prevailed, too, as to the function of the pericardial chamber. This space frequently contains a large quantity of fat-body—pericardial tissue—together with tracheae, and this has given rise to the idea that it might be lung-like in function; but, as Miall and Denny[^[58]] have pointed out, this is erroneous; the tissues in Insects have their own ample supplies of air. It has also been supposed that the alary muscles cause the contraction of the heart, but this is not directly the case, for they are not attached to it, and it pulsates after they have been severed. It has been suggested that the contractions of this vessel are regulated by small ganglia placed on, or in, its substance. However this may be, these contractions vary enormously according to the condition of the Insect; they may be as many, it is said, as 100 or more in a minute, or they may be very slow and feeble, if not altogether absent, without the death of the Insect ensuing.

The expulsion of the blood from the front of the dorsal vessel seems to be due to the rhythm of the contraction of the vessel as well as to its mechanical structure. Bataillon says,[^[59]] confirming an observation of Réaumur, that at the period when the silkworm is about to change to the chrysalis condition, the circulation undergoes periodical changes, the fluid moving during some intervals of about ten minutes' duration in a reversed direction, while at other times the blood is expelled in front and backwards simultaneously, owing apparently to a rhythmical change in the mode of contraction of the dorsal vessel.

As the dorsal vessel consists of a number of distinct chambers, it has been suggested that there is normally one of these for each segment of the body; and it appears that the total number is sometimes thirteen, which is frequently that of the segments of the body without the head. The number of chambers differs, however, greatly, as we have previously stated, and cannot be considered to support the idea of an original segmental arrangement of the chambers. The dorsal vessel, though in the adult a single organ, arises in the embryo from two lateral, widely separated parts which only in a subsequent stage of the embryonic development coalesce in the median line.

Fat-Body.

In discussing the tracheae we remarked on the importance of their function and on their abundant presence in the body. Equally conspicuous, and perhaps scarcely less important in function, is the fat-body, which on opening some Insects, especially such as are in the larval stage, at once attracts attention. It consists of masses of various size and indefinite form distributed throughout the body, loosely connected together, and more or less surrounding and concealing the different organs. The colour varies according to the species of Insect. This fat-body is much connected with fine tracheal twigs, so that an organisation extending throughout the body is thus formed. It may be looked on as a store of nutritious matter which may be added to or drawn on with great rapidity; and it is no doubt on this that many of the internal parasites, so common in the earlier stages of Insects' lives, subsist before attacking the more permanent tissues of their hosts. There is some reason to suppose that the fat-body may have some potency in determining the hunger of the Insect, for some parasitised larvae eat incessantly.

The matter extracted from the food taken into the stomach of the Insect, after undergoing some elaboration—on which point very little is known—finds its way into the body-cavity of the creature, and as it is not confined in any special vessels the fat-body has as unlimited a supply of the nutritive fluid as the other organs: if nutriment be present in much greater quantity than is required for the purposes of immediate activity, metamorphosis or reproduction, it is no doubt taken up by the fat-body which thus maintains, as it were, an independent feeble life, subject to the demands of the higher parts of the organisation. It undoubtedly is very important in metamorphosis, indeed it is possible that one of the advantages of the larval state may be found in the fact that it facilitates, by means of the fat-body, the storage in the organisation of large quantities of material in a comparatively short period of time.

A considerable quantity of fat tissue is found in the pericardial sinus, where it is frequently of somewhat peculiar form, and is spoken of as pericardial cells, or pericardial tissue. Some large cells, frequently of pale yellow colour, and containing no fat, are called oenocytes by Wielowiejski. They are connected with the general fat-body, but are not entirely mingled with it; several kinds have been already distinguished, and they are probably generally present. The phagocytes, or leucocytes, the cells that institute the process of histolysis in the metamorphosis of Muscidae, are a form of blood cell; though these cells are amoeboid some writers derive them from the fat-body. The cells in the blood have no doubt generally an intimate relation with the fat-body, but very little accurate information has been obtained as to these important physiological points, though Graber has inaugurated their study.[^[60]]

Organs of Sex.

The continuation of the species is effected in Insects by means of two sexes, each endowed with special reproductive organs. It has been stated that there are three sexes in some Insects—male, female, and neuter; but this is not correct, as the so-called neuters are truly sexed individuals,—generally females,—though, as a rule, they are not occupied with the direct physiological processes for continuing the species.

The offspring is usually produced in the shape of eggs, which are formed in ovaries. These organs consist of egg-tubes, a cluster of which is placed on each side of the body, and is suspended, according to Leydig[^[61]] and others, to the tissue connected with the heart by means of the thread-like terminations of the tubes.

Fig. 74.—Sex organs of female of Scolia interrupta (after Dufour); a, egg-tubes; b, oviducts; c, poison glands; d, duct of accessory gland (or spermatheca); e, external terminal parts of body.

The number of egg-tubes varies greatly in different Insects; there may be only one to each ovary (Campodea), but usually the number is greater, and in the queen-bee it is increased to about 180. In the Queens of the Termitidae, or white ants, the ovaries take on an extraordinary development; they fill the whole of the greatly distended hind-body. Three thousand egg-tubes, each containing many hundred eggs, may be found in a Queen Termite, so that, as has been said by Hagen,[^[62]] an offspring of millions in number is probable. There is considerable variety in the arrangements for the growth of the eggs in the egg-tubes. Speaking concisely, the tubes may be considered to be centres of attraction for nutritive material, of which they frequently contain considerable stores. Next to the terminal thread, of which we have already spoken, there is a greater or smaller enlargement of the tube, called the terminal chamber; and there may also be nutriment chambers, in addition to the dilatations which form the egg-chambers proper. Korschelt[^[63]] distinguishes three principal forms of egg-tubes, viz. (1) there are no special nutriment chambers, a condition shown in Figure 74; (2) nutriment chambers alternate with the egg-chambers, as shown in our Figure of an egg-tube of Dytiscus marginalis; (3) the terminal chamber takes on an unusual development, acting as a large nutriment chamber, there being no other special nutriment chambers. This condition is found in Rhizotrogus solstitialis. The arrangements as to successive or simultaneous production of the eggs in the tubes seem to differ in different Insects. In some forms, such as the white ants, the process of egg-formation (oogenesis) attains a rapidity that is almost incredible, and is continued, it is said, for periods of many months. There is no point in which Insects differ more than in that of the number of eggs produced by one female. The egg-tubes are connected with a duct for the conveyance of the eggs to the exterior, and the arrangements of the tubes with regard to the oviduct also vary much. An interesting condition is found in Machilis (see Fig. 94, p. [188]), where the seven egg-tubes are not arranged in a bunch, but open at a distance from one another into the elongated duct. The two oviducts usually unite into one chamber, called the azygos portion or the uterus, near their termination. There are a few Insects (Ephemeridae) in which the two oviducts do not unite, but have a pair of orifices at the extremity of the body. Hatchett-Jackson has recently shown[^[64]] that in Vanessa io of the Order Lepidoptera, the paired larval oviducts are solid, and are fixed ventrally so as to represent an Ephemeridean stage; that the azygos system of ducts and appended structures develop separately from the original oviducts, and that they pass through stages represented in other Orders of Insects to the stage peculiar to the Lepidoptera. Machilis, according to Oudemans, is a complete connecting link between the Insects with single and those with paired orifices.

There are in different Insects more than one kind of diverticula and accessory glands in connexion with the oviducts or uterus; a receptaculum seminis, also called spermatheca, is common. In the Lepidoptera there is added a remarkable structure, the bursa copulatrix, which is a pouch connected by a tubular isthmus with the common portion of the oviduct, but having at the same time a separate external orifice, so that there are two sexual orifices, the opening of the bursa copulatrix being the lower or more anterior. The organ called by Dufour in his various contributions glande sébifique, is now considered to be, in some cases at any rate, a spermatheca. The special functions of the accessory glands are still very obscure.

Fig. 75.—Egg-tube of Dytiscus marginalis; e.c, egg-chamber; n.c, nutriment chamber; t.c, terminal chamber; t.t, terminal thread. (After Korschelt.)

The ovaries of the female are replaced in the male by a pair of testes, organs exhibiting much variety of form. The structure may consist of an extremely long and fine convoluted tube, packed into a small space and covered with a capsule; or there may be several shorter tubes. As another extreme may be mentioned the existence of a number of small follicles opening into a common tube, several of these small bodies forming together a testis. As a rule each testis has its own capsule, but cases occur—very frequently in the Lepidoptera—in which the two testes are enclosed in a common capsule; so that there then appears to be only one testis. The secretion of each testis is conveyed outwards by means of a slender tube, the vas deferens, and there are always two such tubes, even when the two testes are placed in one capsule. The vasa deferentia differ greatly in their length in different Insects, and are in some cases many times the length of the body; they open into a common duct, the ductus ejaculatorius. Usually at some part of the vas deferens there exists a reservoir in the form of a sac or dilatation, called the vesicula seminalis. There are in the male, as well as in the female, frequently diverticula, or glands, in connexion with the sexual passages; these sometimes exhibit very remarkable forms, as in the common cockroach, but their functions are quite obscure. There is, as we have already remarked, extreme variety in the details of the structure of the internal reproductive apparatus in the male, and there are a few cases in which the vasa deferentia do not unite behind, but terminate in a pair of separate orifices. The genus Machilis is as remarkable in the form of the sexual glands and ducts of the male as we have already mentioned it to be in the corresponding parts of the female.

Fig. 76.—Tenthredo cincta. a, a, testes; b, b, vasa deferentia; c, c, vesiculæ seminales; d, extremity of body with copulatory armature. (After Dufour.)

Although the internal sexual organs are only fully developed in the imago or terminal stage of the individual life, yet in reality their rudiments appear very early, and may be detected from the embryo state onwards through the other preparatory stages.

The spermatozoa of a considerable number of Insects, especially of Coleoptera, have been examined by Ballowitz;[^[65]] they exhibit great variety; usually they are of extremely elongate form, thread-like, with curious sagittate or simply pointed heads, and are of a fibrillar structure, breaking up at various parts into finer threads.

External Sexual Organs.—The terminal segments of the body are usually very highly modified in connexion with the external sexual organs, and this modification occurs in such a great variety of forms as to render it impossible to give any general account thereof, or of the organs themselves. Some of these segments—or parts of the segments, for it may be dorsal plates or ventral plates, or both—may be withdrawn into the interior, and changed in shape, or may be doubled over, so that the true termination of the body may be concealed. The comparative anatomy of all these parts is especially complex in the males, and has been as yet but little elucidated, and as the various terms made use of by descriptive entomologists are of an unsatisfactory nature we may be excused from enumerating them. We may, however, mention that when a terminal chamber is found, with which both the alimentary canal and the sexual organs are connected, it is called a cloaca, as in other animals.

Parthenogenesis.

There are undoubted cases in Insects of the occurrence of parthenogenesis, that is, the production of young by a female without concurrence of a male. This phenomenon is usually limited to a small number of generations, as in the case of the Aphididae, or even to a single generation, as occurs in the alternation of generations of many Cynipidae, a parthenogenetic alternating with a sexual generation. There are, however, a few species of Insects of which no male is known (in Tenthredinidae, Cynipidae, Coccidae), and these must be looked on as perpetually parthenogenetic. It is a curious fact that the result of parthenogenesis in some species is the production of only one sex, which in some Insects is female, in others male; the phenomenon in the former case is called by Taschenberg[^[66]] Thelyotoky, in the latter case Arrhenotoky; Deuterotoky being applied to the cases in which two sexes are produced. In some forms of parthenogenesis the young are produced alive instead of in the form of eggs. A very rare kind of parthenogenesis, called paedogenesis, has been found to exist in two or three species of Diptera, young being produced by the immature Insect, either larva or pupa.

Glands.

Insects are provided with a variety of glands, some of which we have alluded to in describing the alimentary canal and the organs of sex; but in addition to these there are others in connexion with the outer integument; they may be either single cells, as described by Miall in Dicranota larva,[^[67]] or groups of cells, isolated in tubes, or pouches. The minute structure of Insect glands has been to some extent described by Leydig;[^[68]] they appear to be essentially of a simple nature, but their special functions are very problematic, it being difficult to obtain sufficient of their products for satisfactory examination.

CHAPTER V

DEVELOPMENT

EMBRYOLOGY–EGGS–MICROPYLES–FORMATION OF EMBRYO–VENTRAL PLATE–ECTODERM AND ENDODERM–SEGMENTATION–LATER STAGES–DIRECT OBSERVATION OF EMBRYO–METAMORPHOSIS–COMPLETE AND INCOMPLETE–INSTAR–HYPERMETAMORPHOSIS–METAMORPHOSIS OF INTERNAL ORGANS–INTEGUMENT–METAMORPHOSIS OF BLOWFLY–HISTOLYSIS–IMAGINAL DISCS–PHYSIOLOGY OF METAMORPHOSIS–ECDYSIS.

The processes for the maintenance of the life of the individual are in Insects of less proportional importance in comparison with those for the maintenance of the species than they are in Vertebrates. The generations of Insects are numerous, and the individuals produced in each generation are still more profuse. The individuals have as a rule only a short life; several successive generations may indeed make their appearances and disappear in the course of a single year.

Although eggs are laid by the great majority of Insects, a few species nevertheless increase their numbers by the production of living young, in a shape more or less closely similar to that of the parent. This is well known to take place in the Aphididae or green-fly Insects, whose rapid increase in numbers is such a plague to the farmer and gardener. These and some other cases are, however, exceptional, and only emphasise the fact that Insects are pre-eminently oviparous. Leydig, indeed, has found in the same Aphis, and even in the same ovary, an egg-tube producing eggs while a neighbouring tube is producing viviparous individuals.[^[69]] In the Diptera pupipara the young are produced one at a time, and are born in the pupal stage of their development, the earlier larval state being undergone in the body of the parent: thus a single large egg is laid, which is really a pupa.

The eggs are usually of rather large size in comparison with the parent, and are produced in numbers varying according to the species from a few—15 or even less in some fossorial Hymenoptera—to many thousands in the social Insects: somewhere between 50 and 100 may perhaps be taken as an average number for one female to produce. The whole number is frequently deposited with rapidity, and the parent then dies at once. Some of the migratory locusts are known to deposit batches of eggs after considerable intervals of time and change of locality. The social Insects present extraordinary anomalies as to the production of the eggs and the prolongation of the life of the female parent, who is in such cases called a queen.

The living matter contained in the egg of an Insect is protected by three external coats: (1) a delicate interior oolemm; (2) a stronger, usually shell-like, covering called the chorion; (3) a layer of material added to the exterior of the egg from glands, at or near the time when it is deposited, and of very various character, sometimes forming a coat on each egg and sometimes a common covering or capsule for a number of eggs. The egg-shell proper, or chorion, is frequently covered in whole or part with a complex minute sculpture, of a symmetrical character, and in some cases this is very highly developed, forming an ornamentation of much delicacy; hence some Insects' eggs are objects of admirable appearance, though the microscope is of course necessary to reveal their charms. One of the families of butterflies, the Lycaenidae, is remarkable for the complex forms displayed by the ornamentation of the chorion (see Fig. 78, B).

Fig. 77.—Upper or micropylar aspect of egg of Vanessa cardui. (After Scudder.)

The egg-shell at one pole of the egg is perforated by one or more minute orifices for the admission to the interior of the spermatozoon, and it is the rule that the shell hereabouts is symmetrically sculptured (see Fig. 77), even when it is unornamented elsewhere: the apertures in question are called micropyles. They are sometimes protected by a micropyle apparatus, consisting of raised processes, or porches: these are developed to an extraordinary extent in some eggs, especially in those of Hemiptera-Heteroptera (see Fig. 78, C). Some of these peculiar structures have been described and figured by Leuckart.[^[70]] The purpose they serve is quite obscure.

Fig. 78.—Eggs of Insects: A, blowfly (after Henking); B, butterfly, Thecla (after Scudder); C, Hemipteron (Reduviid).

Formation of Embryo.

The mature, but unfertilised, egg is filled with matter that should ultimately become the future individual, and in the process of attaining this end is the seat of a most remarkable series of changes, which in some Insects are passed through with extreme rapidity. The egg-contents consist of a comparatively structureless matrix of a protoplasmic nature and of yolk, both of which are distributed throughout the egg in an approximately even manner. The yolk, however, is by no means of a simple nature, but consists, even in a single egg, of two or three kinds of spherular or granular constituents; and these vary much in their appearance and arrangement in the early stages of the development of an egg, the yolk of the same egg being either of a homogeneously granular nature, or consisting of granules and larger masses, as well as of particles of fatty matter; these latter when seen through the microscope looking sometimes like shining, nearly colourless, globules.

Fig. 79.—Showing the two extruded polar bodies P₁, P₂ now nearly fused and reincluded, and the formation of the spindle by junction of the male and female pronuclei. (After Henking.)

The nature of the matrix—which term we may apply to both the protoplasm and yolk as distinguished from the minute formative portions of the egg—and the changes that take place in it have been to some extent studied, and Kowalewsky, Dohrn,[^[71]] Woodworth,[^[72]] and others have given some particulars about them. The early changes in the formative parts of the mature egg have been observed by Henking in several Insects, and particularly in Pyrrhocoris, his observations being of considerable interest. When the egg is in the ovary and before it is quite mature,—at the time, in fact, when it is receiving nutriment from ovarian cells,—it contains a germinal vesicle including a germinal spot, but when the egg is mature the germinal vesicle has disappeared, and there exists in its place at one portion of the periphery of the egg-contents a cluster of minute bodies called chromosomes by Henking, whom we shall follow in briefly describing their changes. The group divides into two, each of which is arranged in a rod or spindle-like manner, and may then be called a directive rod or spindle. The outer of these two groups travels quite to the periphery of the egg, and there with some adjacent matter is extruded quite outside the egg-contents (not outside the egg-coverings), being in its augmented form called a polar or directive body. While this is going on the second directive spindle itself divides into two groups, the outer of which is then extruded in the manner we have already described in the case of the first polar body, thus completing the extrusion of two directive bodies. The essential parts of the bodies that are successively formed during these processes are the aggregates, called chromosomes; the number of these chromosomes appears to be constant in each species; their movements and dispositions are of a very interesting character, the systems they form in the course of their development having polar and equatorial arrangements. These we cannot further allude to, but may mention that the extrusion of the directive bodies is only temporary, they being again included within the periphery of the egg by the growth and extension of adjacent parts which meet over and thus enclose the bodies.

The arrangements and movements we have briefly alluded to have been limited to the unfertilised condition of the egg (we should rather say, the fertilising element has taken no part in them), and have as their result the union of the chromosomes existing after the extrusion of the two polar bodies, into a small body called the female pronucleus or egg-nucleus (Eikern), while the position of the movements has been an extremely minute portion of the egg near to its outer surface or periphery. The introduction of a sperm, or male, element to the egg through the micropyle gives rise to the formation of another minute body placed more in the interior of the egg, and called the sperm-nucleus. The egg-nucleus, travelling more into the interior of the egg, meets the sperm-nucleus; the two amalgamate, forming a nucleus or body that goes through a series of changes resulting in its division into two daughter-bodies. These two again divide, and by repetitions of such division a large number of nuclei are formed which become arranged in a continuous manner so as to form an envelope enclosing a considerable part (if not quite the whole) of the egg-mass. This envelope is called the blastoderm, and together with its contents will form the embryo. We must merely allude to the fact that it has been considered that some of the nuclei forming the blastoderm arise directly from the egg-mass by a process of amalgamation, and if this prove to be correct it may be admitted that some portions of the embryo are not entirely the result of division or segmentation of combined germ and sperm-nuclei. Wheeler states[^[73]] that some of the nuclei formed by the first differentiation go to form the vitellophags scattered throughout the yolk. We should also remark that, according to Henking, the blastoderm when completed shows at one part a thickening, immediately under which (i.e. included in the area the blastoderm encloses) are the two polar bodies, which, as we have seen, were formed by the germinating body at an earlier stage of its activity. Fig. 79 represents a stage in the development of Pyrrhocoris, showing the interior of the egg after a body has been formed by the union of the sperm and egg-nuclei; this body is about to undergo division or segmentation, and the equatorial arrangement where this will take place is seen. The two polar bodies P₁, P₂, after having been excluded, are nearly reincluded in the egg.

The Ventral Plate.

The next important change after the formation of the blastoderm is the partial detachment of a part of its periphery to become placed in the interior of the other and larger portion. The way in which this takes place will be gathered from the accompanying diagrammatic figures taken from Graber: a thickened portion (a b) of the blastoderm becomes indrawn so as to leave a fold (c d) at each point of its withdrawal, and these folds afterwards grow and meet so as to enclose the thickened portion. The outer envelope, formed in part by the original blastoderm and in part by the new growth, is called the serosa (e f), the inner layer (g) of the conjoined new folds being termed the amnion: the part withdrawn to the interior and covered by the serosa and amnion is called the ventral plate, or germinal band (Keimstreif), and becomes developed into the future animal. The details of the withdrawal of the ventral plate to the interior are very different in the various Insects that have been investigated.

Fig. 80.—Stages of the enclosure of the ventral plate: A, a, b, ventral plate; B, c, d, folds of the blastoderm that form the commencement of the amnion and serosa; C, e, f, part of the serosa; g, amnion.

One of the earliest stages in the development is a differentiation of a portion of the ventral plate into layers from which the future parts of the organisation will be derived. This separation of endoderm from ectoderm takes place by a sort of invagination, analogous with that by which the ventral plate itself is formed. A longitudinal depression running along the middle of the ventral plate appears, and forms a groove or channel, which becomes obliterated as to its outer face by the meeting together of the two margins of the groove (except on the anterior part, which remains open). The more internal layer of the periphery of this closed canal is the origin of the endoderm and its derivatives. Subsequently the ventral plate and its derivatives grow so as to form the ventral part and the internal organs of the Insect, the dorsal part being completed much later by growths that differ much in different Insects; Graber, who has specially investigated this matter, informing us[^[74]] that an astonishing multifariousness is displayed. It would appear that the various modes of this development do not coincide with the divisions into Orders and Families adopted by any systematists.

We should observe that the terms ectoderm, mesoderm, and endoderm will probably be no longer applied to the layers of the embryo when embryologists shall have decided as to the nature of the derived layers, and shall have agreed as to names for them. According to the nomenclature of Graber[^[75]] the blastoderm differentiates into Ectoblast and Endoblast; this latter undergoing a further differentiation into Coeloblast and Myoblast. This talented embryologist gives the following table of the relations of the embryonic layers and their nomenclature, the first term of each group being the one he proposed to use:—

Nussbaum considers[^[76]] that "there are four layers in the cockroach-embryo, viz. (1) epiblast, from which the integument and nervous system are developed; (2) somatic layer of mesoblast, mainly converted into the muscles of the body-wall; (3) splanchnic layer of mesoblast, yielding the muscular coat of the alimentary canal; and (4) hypoblast, yielding the epithelium of the mesenteron."

Fig. 81.—Early stages of the segmentation of a beetle (Lina): A, segmentation not visible, 1 day; B, segmentation of head visible; C, segmentation still more advanced, 2¼ days; PC, procephalic lobes; g¹, g², g³, segments bearing appendages of the head; th, thorax; th¹, th², th³, segments of the thorax; a¹, a², anterior abdominal.

Turning our attention to the origin of the segmentation, that is so marked a feature of Insect structure, we find that evidence of division or arrangement of the body into segments appears very early, as shown in our Figure of some of the early stages of development of Lina (a beetle), Fig. 81. In A the segmentation of the ectoderm has not commenced, but the procephalic lobes (P C) are seen; in B the three head segments are distinct, while in C the thoracic segmentation has occurred, and that of the abdomen has commenced. Graber considers that in this species the abdomen consists of ten segmental lobes, and a terminal piece or telson. According to Graber[^[77]] this is not a primitive condition, but is preceded by a division into three or four parts, corresponding with the divisions that will afterwards be head, thorax, and abdomen. This primary segmentation, he says, takes place in the Hypoblast (Endoderm) layer of the ventral plate; this layer being, in an early stage of the development of a common grasshopper (Stenobothrus variabilis), divided into four sections, two of which go to form the head, while the others become thorax and abdomen respectively. In Lina the primary segmentation is, Graber says, into three instead of four parts. Graber's opinion on the primary segmentation does not appear to be generally accepted, and Wheeler, who has studied[^[78]] the embryology of another Orthopteron, considers it will prove to be incorrect. When the secondary segmentation occurs the anterior of the two cephalic divisions remains intact, while the second divides into the three parts that afterwards bear the mouth parts as appendages. The thoracic mass subsequently segments into three parts, and still later the hind part of the ventral plate undergoes a similar differentiation so as to form the abdominal segments; what the exact number of these may be is, however, by no means easy to decide, the division being but vague, especially posteriorly, and not occurring all at once, but progressing from before backwards.

The investigations that have been made in reference to the segmentation of the ventral plate do not at present justify us in asserting that all Insects are formed from the same number of embryonic segments. The matter is summarised by Lowne, to the effect that posterior to the procephalic lobes there are three head segments and three thoracic segments, and a number of abdominal segments, "rarely less than nine or more than eleven." It will be seen by referring to Figure 81 that the segmentation appears, not simultaneously, but progressively from the head backwards; this of course greatly increases the difficulty of determining by means of a section the real number of segments.

Fig. 82.—Embryo of a moth (Zygaena) at the fifth day (after Graber): am, amnion; s, serosa; p, procephalic lobes; st, stomodaeum; pr, proctodaeum; g¹, g², g³, the mouth parts or head appendages; th¹, th², th³, appendages of the thoracic segments; a¹-a¹⁰, abdominal segments; s.g, salivary gland.

The later stages in the development of Insects are already proved to be so various that it would be impossible to attempt to follow them in detail; but in Fig. 82 we represent a median section of the embryo of Zygaena filipendula at the fifth day. It shows well some of the more important of the general features of the development at a stage subsequent to those represented in Fig. 81, A, B, C. The very distinct stomodaeum (st) and proctodaeum (pr) are seen as inflexions of the external wall of the body; the segmentation and the development of the ventral parts of the embryo are well advanced, while the dorsal part of the embryo is still quite incomplete.

The method of investigation by which embryologists chiefly carry on their researches is that of dividing the egg after proper preparation, into a large number of thin sections, which are afterwards examined in detail, so as to allow the arrangement to be completely inferred and described. Valuable as this method is, it is nevertheless clear that it should, if possible, be supplemented by direct observation of the processes as they take place in the living egg: this method was formerly used, and by its aid we may still hope to obtain exact knowledge as to the arrangements and rearrangements of particles by which the structures develop. Such questions as whether the whole formative power in the egg is absolutely confined to one or two small centres to which the whole of the other egg contents are merely, as it were, passive accessories, or whether an egg is a combination in which some portion of the powers of rearrangement is possessed by other particles, as well as the chromosomes, in virtue of their own nature or of their position at an early period in the whole, can scarcely be settled without the aid of direct observation of the processes during life.

The importance of the yolk is recognised by most of the recent writers. Nussbaum states (loc. cit.) that "scattered yolk-cells associate themselves with the mesoblast cells, so that the constituents of the mesoblast have a twofold origin." Wheeler finds[^[79]] that amoeboid cells—he styles them vitellophags—traverse the yolk and assist in its rearrangement; he insists on the importance both as regards quantity and quality of the yolk.

The eggs of some insects are fairly transparent, and the process of development in them can, to a certain extent, be observed by simple inspection with the microscope; a method that was used by Weismann in his observations on the embryology of Chironomus. There is a moth (Limacodes testudo), that has no objection to depositing its eggs on glass microscope-slides. These eggs are about a millimetre long, somewhat more than half that width, are very flat, and the egg-shell or chorion is very thin and perfectly transparent. When first laid the contents of this egg appear nearly homogeneous and evenly distributed, a finely granular appearance being presented throughout; but in twenty-four hours a great change is found to have taken place. The whole superficial contents of the egg are at that time arranged in groups, having the appearance of separate rounded or oval masses, pressed together so as to destroy much of their globular symmetry. The egg contents are also divided into very distinct forms, a granular matter, and a large number of transparent globules, these latter being the fatty portion of the yolk; these are present everywhere, though in the centre there is a space where they are very scanty, and they also do not extend quite to the circumference. But the most remarkable change that has taken place is the appearance in the middle of the field of an area different from the rest in several particulars; it occupies about one-third of the width and one-third of the length; it has a whiter and more opaque appearance, and the fat globules in it are fewer in number and more indistinct. This area is afterwards seen to be occupied by the developing embryo, the outlines of which become gradually more distinct. Fig. 83 gives an idea of the appearance of the egg about the middle period of the development. In warm weather the larva emerges from this egg ten or eleven days after it has been deposited.

Fig. 83.—A, Egg of Limacodes testudo about the middle of the development of the embryo; B, micropyles and surrounding sculpture of chorion.

The period occupied by the development of the embryo is very different in the various kinds of Insects; the blowfly embryo is fully developed in less than twenty-four hours, while in some of the Orthoptera the embryonic stage may be prolonged through several months. According to Woodworth the blastoderm in Vanessa antiopa is complete in twenty-four hours after the deposition of the egg, and the involution of the ventral plate is accomplished within three days of deposition.

Metamorphosis.

The ontogeny, or life history of the individual, of Insects is peculiar, inasmuch as a very large part of the development takes place only late in life and after growth has been completed. Insects leave the egg in a certain form, and in that condition they continue—with, however, a greater or less amount of change according to kind—till growth is completed, when, in many cases, a very great change of form takes place. Post-embryonic development, or change of form of this kind, is called metamorphosis. It is not a phenomenon peculiar to Insects, but exists to a greater or less extent in other groups of the Metazoa; while simpler post-embryonic development occurs in nearly all, as in scarcely any complex animals are all the organs completely formed at the time the individual becomes possessed of a separate existence. In many animals other than Insects the post-embryonic development assumes most remarkable and complex forms, though there are perhaps none in which the phenomenon is very similar to the metamorphosis of Insects. The essential features of metamorphosis, as exhibited in the great class we are writing of, appear to be the separation in time of growth and development, and the limitation of the reproductive processes to a short period at the end of the individual life. The peculiar phenomena of the post-embryonic development of the white ants show that there exists some remarkable correlation between the condition of the reproductive organs and the development of the other parts of the organisation. If we take it that the post-embryonic physiological processes of any individual Insect are of three kinds,—growth, development, and reproduction,—then we may say that in the higher Insects these three processes are almost completely separated, and go on consecutively, the order being,—first, growth; second, development; third, reproduction. While, if we complete the view by including the processes comprised in the formation of the egg and the development therein, the series will be—(1) oogenesis, or egg-growth; (2) development (embryonic); (3) growth (post-embryonic); (4) development (post-embryonic); (5) reproduction.

The metamorphosis of Insects is one of the most interesting parts of entomology. It is, however, as yet very little known from a scientific point of view, although the simpler of its external characters have for many ages past attracted the attention and elicited the admiration of lovers of nature. It may seem incorrect to say that little is yet known scientifically of a phenomenon concerning which references almost innumerable are to be found in literature: nevertheless the observations that have been made as to metamorphosis, and the analysis that has been commenced of the facts are at present little more than sufficient to show us how vast and complex is the subject, and how great are the difficulties it presents.

There are three great fields of inquiry in regard to metamorphosis, viz. (1) the external form at the different stages; (2) the internal organs and their changes; (3) the physiological processes. Of these only the first has yet received any extensive attention, though it is the third that precedes or underlies the other two, and is the most important. We will say a few words about each of these departments of the inquiry. Taking first the external form—the instar. But before turning to this we must point out that in limiting the inquiry to the post-embryonic development, we are making one of those limitations that give rise to much misconception, though they are necessary for the acquisition of knowledge as to any complex set of phenomena. If we assume five well-marked stages as constituting the life of an Insect with extreme metamorphosis, viz. (1) the formation and growth of the egg; (2) the changes in the egg culminating in its hatching after fertilisation; (3) the period of growth; (4) the pupal changes; (5) the life of the perfect Insect; and if we limit our inquiry about development to the latter three, we are then shutting out of view a great preliminary question, viz. whether some Insects leave the egg in a different stage of development to others, and we are consequently exposing ourselves to the risk of forgetting that some of the distinctions we observe in the subsequent metamorphosis may be consequential on differences in the embryonic development.

Instar and Stadium.

Figs. 84 and 85 represent corresponding stages in the life of two different Insects, Fig. 84 showing a locust (Acridium), and Fig. 85 a white butterfly. In each A represents the newly-hatched individual; B, the insect just before its perfect state; C, the perfect or imago stage. On comparing the two sets of figures we see that the C stages correspond pretty well as regards the most important features (the position of the wings being unimportant), that the A stages are moderately different, while the B states are not to be recognised as equivalent conditions.

Fig. 84.—Locust (Acridium peregrinum): A, newly hatched; B, just antecedent to last ecdysis; C, perfect Insect.

Fig. 85.—Butterfly (Pieris): A, the newly hatched young, or larva magnified; B, pupa (natural size) just antecedent to last ecdysis; C, perfect Insect.

Every Insect after leaving the egg undergoes during the process of growth castings of the skin, each of which is called a moult or ecdysis. Taking for our present purpose five as the number of ecdyses undergone by both the locust and butterfly, we may express the differences in the successions of change we portray in Figs. 84 and 85 by saying that previous to the first ecdysis the two Insects are moderately dissimilar, that the locust undergoes a moderate change before reaching the fifth ecdysis, and undergoes another moderate change at this moult, thus reaching its perfect condition by a slight, rather gradual series of alterations of form. On the other hand, the butterfly undergoes but little modification, remaining much in the condition shown by A, Fig. 85, till the fourth, or penultimate, ecdysis, but then suffers a complete change of form and condition, which apparently is only inferior to another astonishing change that takes place at the fifth or final moult. The chief, though by no means the only, difference between the two series consists in the fact that the butterfly has interposed between the penultimate and the final ecdyses a completely quiescent helpless condition, in which it is deprived of external organs of sense, locomotion, and nutrition; while in the locust there is no loss of these organs, and such quiescent period as exists is confined to a short period just at the fifth ecdysis. The changes exhibited by the butterfly are called "complete metamorphosis," while this phenomenon in the locust is said to be "incomplete." The Insect with complete metamorphosis is in its early stage called a larva, and in the quiescent state a pupa. The adult state in both butterfly and locust is known as imago or perfect Insect.

The most conspicuous of the differences between Insects with complete and those with incomplete metamorphosis is, as we have remarked, the existence in the former of a pupa. The pupal state is by no means similar in all the Insects that possess it. The most anomalous conditions in regard to it occur in the Order Neuroptera. In some members of that Order—the Caddis-flies for instance—the pupa is at first quiescent, but becomes active before the last ecdysis; while in another division—the May-flies—the last ecdysis is not preceded by a formed pupa, nor is there even a distinct pupal period, but the penultimate ecdysis is accompanied by a change of form to the winged condition, the final ecdysis being merely a casting of the skin after the winged state has been assumed. In the Odonata or Dragon-flies there is no pupal stage, but the change of form occurring at the last ecdysis is very great. In those Insects where the interval between the last two moults is not accompanied by the creature's passing into a definite, quiescent pupa, the individual is frequently called then a nymph; but the term nymph has merely a distinctive meaning, and is not capable of accurate definition, owing to the variety of different conditions covered by the word. Eaton, in describing this term as it is used for Ephemeridae, says, "Nymphs are young which lead an active life, quitting the egg at a tolerably advanced stage of morphological development, and having the mouth-parts formed after the same main type of construction as those of the adult insect."[^[80]]

The intervals between the ecdyses are called stadia, the first stadium being the period between hatching and the first ecdysis. Unfortunately no term is in general use to express the form of the Insect at the various stadia; entomologists say, "the form assumed at the first moult," and so on. To avoid this circumlocution it may be well to adopt a term suggested by Fischer,[^[81]] and call the Insect as it appears at hatching the first instar, what it is as it emerges from the first ecdysis the second instar, and so on; in that case the pupa of a Lepidopteron that assumed that condition at the fifth ecdysis would be the sixth instar, and the butterfly itself would be the seventh instar.

Various terms are used to express the differences that exist in the metamorphoses of Insects, and as these terms refer chiefly to the changes in the outer form, we will here mention them. As already stated, the locust is, in our own language, said to have an incomplete metamorphosis, the butterfly a complete one. The term Holometabola has been proposed for Insects with complete metamorphosis, while the appellations Ametabola, Hemimetabola, Heterometabola, and Paurometabola have been invented for the various forms of incomplete, or rather less complex, metamorphosis. Some writers use the term Ametabola for Insects that are supposed to exhibit no change of external form after quitting the egg, the contrasted series of all other Insects being then called Metabola. Westwood and others use the word Homomorpha for Insects in which the condition on hatching more or less resembles that attained at the close of the development, and Heteromorpha for those in which the form on emergence from the egg differs much from what it ultimately becomes.

Hypermetamorphosis.

There are certain unusual changes to which the term hypermetamorphosis has been applied; these we can here only briefly allude to.

Insects that have complete metamorphoses, and are not supplied with food by their parents or guardians, are provided during their larval life with special modifications of extremely various kinds to fit them for the period of life during which they are obtaining food and growing. Thus caterpillars possess numerous adaptations to fit them for the period during which they live on leaves, while maggots have modifications enabling them to live amongst decomposing flesh. Some larvae are greatly modified in this adaptive way, and when the adaptations change greatly during the life of the larva, hypermetamorphosis is said to exist. As an instance we may mention some beetle larvae that are born with legs by whose aid they can cling to a bee, and so get carried to its nest, where they will in future live on the stores of food the bee provides for its own young. In order that they may be accommodated to their totally different second circumstances, they change their first form, losing their legs, and becoming almost bladder-like creatures, fitted for floating on the honey without being injured by it. Such an occurrence has been described by Fabre[^[82]] in the case of Sitaris humeralis, and his figures have been reproduced in Sir John Lubbock's book on the metamorphoses of Insects,[^[83]] as well as in other works, yet they are of so much interest that we give them again, especially as the subject is still only in its infancy; we at present see no sufficient reason for the later of these larval states. Little is, we believe, known as to the internal anatomy of the various instars in these curious cases.

Fig. 86.—Preparatory stages of Sitaris humeralis: 9, 10, 11, 12, first, second, third, and fourth larval instars; 13, pupa. (After Lubbock and Fabre.)

There are certain minute Hymenoptera that deposit their eggs inside the eggs of other Insects, where the beings hatched from the parasitic eggs subsequently undergo their development and growth, finding their sustenance in the yolk or embryo contained in the host-egg. It is evident that such a life is very anomalous as regards both food and the conditions for respiration, and we consequently find that these tiny egg-parasites go through a series of changes of form of a most remarkable character.[^[84]] It would appear that in these cases the embryonic and post-embryonic developments are not separated in the same way as they are in other Insects. We are not aware that any term has yet been proposed for this very curious kind of Insect development, which, as pointed out by Brauer,[^[85]] is doubtless of a different nature from the hypermetamorphosis of Sitaris.

Changes in Internal Organs.

In relation to the post-embryonic development of the internal organs of the body there is but little exact generalisation to be made, the anatomical condition of these organs at the time of emergence from the egg having been ascertained in but few Insects. We know that in Holometabolous Insects the internal anatomy differs profoundly in the larval and imaginal instars. As to Insects with more imperfect metamorphosis very little information exists, but it appears probable that in many no extensive distinctions exist between the newly-hatched and the adult forms, except in the condition of the reproductive organs. Differences of minor importance doubtless exist, but there is almost no information as to their extent, or as to the periods at which the changes occur; so that we do not know to what extent they may be concentrated at the final ecdysis. In Insects with perfect metamorphosis the structures of the internal organs are, as we have said, in many cases totally different in the larval and imaginal periods of the life; but these changes are far from being uniform in all Holometabola. The nervous system in some cases undergoes a great concentration of the ganglia, in others does not, and important distinctions exist in this respect even within the limits of a single Order, such as the Coleoptera. Some Insects take the same kind of food throughout their lives, but many others change totally in this respect, and their organs for the prehension and digestion of food undergo a corresponding change. Butterflies suck food in the form of liquid juices from flowers by means of a delicate and long proboscis, while the young butterfly—the caterpillar—disdains sweets, and consumes, by the assistance of powerful mandibles, a great bulk of leaves. Other Holometabola undergo no such total change of habits; the tiger-beetle, for instance, is as ferocious a consumer of the juices of Insects in its young stage as it is in the adult condition. Hence Brauer[^[86]] divides Insects, as regards this point, into three categories. The forms in which both the young and adult take food by suction he calls Menorhyncha; those in which both the imago and immature forms feed by mandibles he calls Menognatha; while his Metagnatha consists of those insects that take food by jaws when young, but by suction with tubular mouths when mature. Besides these main divisions there are some exceptional cases to which we need not here allude, our present object being to indicate that in the Metagnatha the digestive organs are of a very different nature in the young and in the adult states of existence.

The internal organs for the continuance of the species are known to be present in a rudimentary stage in the embryo, and it is a rule that they do not attain their full development until growth has been completed; to this rule there may possibly be an exception in the case of the Aptera. But little information of a comparative character exists as to the dorsal vessel and the changes it undergoes during metamorphosis. There is considerable difficulty in connexion with the examination of this structure, but it appears probable that it is one of the organs that changes the least during the process of metamorphosis.

The exact nature of the internal changes that occur during metamorphosis is almost a modern subject. It is of course a matter of great difficulty to observe and record changes that go on in the interior of such small creatures as Insects, and when the phenomena occur with great rapidity, as is frequently the case in Insect metamorphosis, the difficulty is much increased. Nevertheless the subject is of such great interest that it has been investigated with a skill and perseverance that call for the highest admiration. The greater part of the information obtained refers to a single Insect, the blowfly; and amongst those who have made important contributions to it we may mention Weismann,[^[87]] Viallanes,[^[88]] Ganin,[^[89]] and Van Rees,[^[90]] and it is at present under investigation by Lowne. A good deal, too, is becoming known about the processes in the case of the silkworm.

Integument and Ecdysis.

The integument consists of a cellular layer, usually called the hypodermis, situated on a basement membrane. The hypodermis, or layer of chitinogenous cells, excretes a matter which remains attached to the body, forming the hard outer layer of the skin. This layer consists of chitin and has no vitality, but its presence no doubt exerts a very important influence on the physiological processes of the Insect. The chitinous investment varies much in thickness and in other properties; in some Insects it is hard, even glassy, so as to be difficult to pierce with a pin, in others it is pliable, and in some very delicate. Chitin is a substance very difficult to investigate; according to the recent researches of Krawkow[^[91]] it may prove to be of somewhat variable chemical composition.

After a time the hypodermis excretes a fresh supply of chitin, and, possibly by the commencement of this process, the older chitinous investment becomes separated and is shed. The details have, however, not been ascertained, though their importance has been suggested by Hatchett Jackson.[^[92]] The newly exposed layer of integument is pallid, but afterwards becomes coloured in a manner varying according to the species, the process being possibly due to some secondary exudation permeating the freshly exposed chitin, or modifying some part of its exterior.

Lowne informs us that in the imago of the blowfly the great majority of the hypodermic cells themselves enter into the composition of the chitinous integument; and it is perhaps not a matter for surprise that the cells should die on the completion of their functional activity, and should form a part of the chitinous investment. Some writers say that the chitinous layer may be shown to be covered by a delicate extima or outer coat.

The number of ecdyses varies greatly in Insects, but has been definitely ascertained in only a few forms outside the Order Lepidoptera. In Campodea Grassi says there is a single fragmentary moult, and in many Hymenoptera the skin that is cast is extremely delicate, and the process perhaps only occurs twice or three times previous to the pupal stage. In most Insects, however, ecdysis is a much more important affair, and the whole of the chitinous integument is cast off entire, even the linings of the tracheae, and of the alimentary canal and its adjuncts being parted with. Sir John Lubbock observed twenty-three moults in a May-fly of the genus Cloëon,[^[93]] this being the maximum yet recorded, though Sommer states[^[94]] that in Macrotoma plumbea moulting goes on as long as life lasts, even after the Insect has attained its full size.

Some Insects get quit of a considerable quantity of matter by their ecdyses, while in others the amount is comparatively slight. It has been thought that the moulting is effected in order to permit of increase of size of the Insect, but there are facts which point to the conclusion that this is only a factor of secondary importance in the matter. One of these is that many Insects make their first ecdysis almost immediately after they leave the egg; this is the case with the young larva of the blowfly, which, according to Lowne, moults within two hours of its emergence from the egg. We have already referred to the important suggestion made by Eisig[^[95]] that, since chitin is a nitrogenous substance, the ecdyses may be a means of getting rid of waste nitrogenous matter; to which we have added that as chitin also consists largely of carbon, its excretion may be of importance in separating carbonaceous products from the blood.

Metamorphosis of Blowfly.

The phenomena of metamorphosis are displayed to their greatest extent in the transformations and physiological processes of the Muscid Diptera, of which the common blowfly is an example. We will briefly consider the information that has been obtained on this subject.

The development of the embryo in the egg of the blowfly is unusually rapid, occupying only a period of twenty to twenty-four hours. After its first moult the blowfly larva grows rapidly during a period of about ten to fourteen days, during which it undergoes moults, the number of which appears not to be definitely ascertained. After becoming full-fed the larva loses its active state, and passes for a period into a condition of comparative quiescence, being spoken of in this state as a resting larva. This quiet period occurs in most full-grown larvae, and is remarkable for the great variation that may occur in its duration, it being in many Insects subject to prolongation for months, in some cases possibly even for years, though in favourable circumstances it may be very short. Lowne informs us that in the blowfly this period of the life is occupied by very great changes in the internal organs, which are undergoing very extensive processes of destruction and rebuilding. After some days the outer skin of the resting larva shrivels, and is detached from the internal living substances, round which it hardens and forms the sort of cocoon or capsule that is so well known. This using of the cast larval skin as a cocoon is, however, limited to certain of the two-winged flies, and perhaps a few other Insects, and so must be considered an exceptional condition. The capsule conceals from view a most remarkable state, known to the old naturalist Réaumur as the "spheroidal condition," but called by more recent writers the pronymph. The pronymphal state may be looked on as being to a great extent a return of the animal to the condition of an egg, the creature becoming an accumulation of soft creamy matter enclosed in a delicate skin. This spheroidal condition, however, really begins in the resting larva, and Van Rees and others think that the delicate membrane enclosing the substance of the pronymph is really the hypodermis of the integument of the larva. Although this seems probable, from the resemblance this condition would in that case present to the phenomena usual in ecdysis, it is not generally admitted, and there is much difficulty in settling the point. Lowne is of a contrary opinion, looking on the limiting membrane as a subsequent formation; he calls it the paraderm. The process of forming the various organs goes on in the pronymph, till the "nymph" has completed its development, the creature having then again taken on a definite form which apparently corresponds to the pupa of Hymenoptera. Great doubt, however, exists as to this equivalence, and indeed as to any exact correspondence between the metamorphic stadia of different Insects, a view which long since was expressed by Sir John Lubbock[^[96]] and Packard. The term nymph is used in this case not because there is any resemblance to the condition similarly named in Insects with less complete metamorphosis, but because the term pupa is applied to the outer case together with the contained nymph. The transformation of the nymph into the perfect blowfly occupies a period very variable according to the temperature.

Histolysis.—The processes by which the internal organs of the maggot are converted into those of the fly are of two kinds,—histolysis or breaking down, histogenesis or building up, of tissue. The intermediary agents in histolysis are phagocytes, cells similar to the leucocytes or white corpuscles of the blood: the intermediary agents in histogenesis are portions of tissue existing in the larval state incorporated with the different organs, or preserving a connexion therewith even when they are to a great extent separated therefrom. In this latter case they are called imaginal discs, though Professor Miall prefers to term them imaginal folds.[^[97]] The two processes of histolysis and histogenesis, though to some extent mutually dependent (for the material to be built up has to be largely obtained by previous destruction), do not go on pari passu, though they are to a great extent contemporaneous. In the resting larva histolysis is predominant, while in the nymph histogenesis is more extensive. Microscopic observation shows that the phenomena connected with the histolysis of the muscular tissue are scarcely distinguishable from those of an inflammatory process, and Viallanes[^[98]] dilates on this fact in an instructive manner. The phagocytes attach themselves to, or enter, the tissues which are to be disintegrated, and becoming distended, assume a granular appearance. By this pseudo-inflammatory process the larval structures are broken down into a creamy substance; the buds, or germs, from which the new organs are to be developed being exempt from the destruction. These buds, of which about sixty or upwards have already been detected, undergo growth as they are liberated, and so the new creature is formed, the process of growth in certain parts going on while destruction is being accomplished in others. Considerable discrepancy prevails as to the extent to which the disintegration of some of the tissues is carried.

Fig. 87.—Imaginal discs of Muscidae in process of development: A, Brain and ventral ganglion of a larva 7 mm. long of M. vomitoria; v, ventral ganglion; c, cephalic ganglion; h, head rudiment; vc, portion of ventral chain; pd, prothoracic rudiment; vc₃, third nerve; md, mesothoracic rudiment: B, mesothoracic rudiment, more advanced, in a pupa just formed of Sarcophaga carnaria, showing the base of the sternum and folds of the forming leg, the central part (f) representing the foot: C, the rudimentary leg of the same more advanced; f, femur; t, tibia; f₁, f₅, tarsal joints: D, two discs from a larva 20 mm. long of Sarcophaga, attached to tracheae; msw, mesonotal and wing-rudiment; mt, metathoracic rudiment: E, r, mesothoracic rudiment of a 7 mm. long larva attached to a tracheal twig. (After Weismann and Graber.)

According to Kowalevsky[^[99]] it would appear that after the phagocytes have become loaded with granules they serve as nutriment for the growing tissues, and he thinks they become blood-cells in the imago. The process of histolysis has been chiefly studied in the blowfly, and not much is known of it in other Insects, yet it occurs to a considerable extent, according to Bugnion[^[100]] and others, in the metamorphosis of Lepidoptera. Indeed it would almost seem that the processes of histolysis and histogenesis may be looked on as exaggerated forms of the phenomena of the ordinary life of tissues, due to greater rapidity and discontinuity of tissue nutrition.

Imaginal Discs.—The imaginal discs are portions of the larval hypoderm, detached from continuity with the main body of the integument, but connected therewith by strings or pedicels which may be looked on as portions of the basement membrane. Whether these discs, or histoblasts as they are called by Künckel d'Herculais,[^[101]] are distinguished by any important character from other buds or portions of regenerative tissue that, according to Kowalevsky,[^[102]] Korschelt and Heider,[^[103]] and others, exist in other parts of the body, does not appear to be at present ascertained.

We give some figures, taken from Weismann and Graber, of the imaginal rudiments existing in the larvae of Muscidae. Although by no means good, they are the best for our purpose we can offer to the reader. Other figures will be found in Lowne's work on the blowfly now in course of publication. Weismann's paper[^[104]] is now thirty years old, and, when it was written, he was not aware of the intimate connexion the rudiments have with the integument; this has, however, now been demonstrated by several observers. Pratt states[^[105]] that the formation of the imaginal discs in Melophagus ovinus takes place in the later stages of the embryonic development, and after the manner formerly suggested by Balfour, viz. invagination of the ectoderm.

Fig. 88.—Median longitudinal section through larva of blowfly during the process of histolysis. (After Graber.) Explanation in text.

Both the regenerative buds and the rudimentary sexual glands are known to be derived directly from the embryo; neither of them undergoes any histolysis, so that we have in them embryonic structures which exist in a quiescent condition during the period in which the larva is growing with great rapidity, and which when the larva has attained its full growth and is disintegrating, then appropriate the products of the disintegration so as to produce the perfect fly.

Our Fig. 88, taken from Graber, represents a longitudinal median section of a full-grown larva of Musca, in which the processes of metamorphosis are taking place. The position of some of the more important imaginal rudiments is shown by it: b¹, b², b³, rudiments of the three pairs of legs of the imago; an, of antennae; between an and w, rudiment of eye; w, of wings; h, of halteres; f, fat-body; d, middle of alimentary canal; n, ventral chain; st, stigma; 6, 7, sixth and seventh body segments.

Physiology of Metamorphosis.

Many years ago, Harvey perceived the probable existence of a physiological continuity between the earlier and later stages of the Insect's life. Modern investigation has shown that in the blowfly a remarkable analogy exists between the conditions of the pupa and the egg. The outer shell of the pupa corresponds to the chorion or egg-shell, and the delicate outer membrane of the pronymph to the oolemn or lining membrane of the egg; the creamy matter corresponds with the yolk, and the regenerative buds are analogous to the formative portions of the developing egg. The process of histolysis as carried out by the phagocytes of the later life appears also to find a parallel in the vitellophags of the embryonic life.[^[106]] It appears probable that the physiological processes of the post-embryonic metamorphosis may be essentially a repetition—or an interrupted continuation—of those of the embryonic period.

The inquiry as to what are the determining causes of the metamorphic changes of the blowfly and other Insects has as yet but little advanced. Why does the larva grow up to a certain period with great rapidity, then cease its appropriating power and break up the parts that have been so rapidly and recently formed? And why do the imaginal buds remain quiescent till the other tissues are being disintegrated, and then, instead of sharing the general condition of disintegration, commence a career of development? To these questions no satisfactory answer has yet been given, though the remarkable studies, already referred to, of Bataillon on the later larval life of the silkworm suggest the direction in which knowledge may be found, for they show that the physiological conditions of the later larval life are different from those of the earlier life, possibly as the direct result of the mere aggregation of matter, and the consequent different relations of the parts of the organism to atmospheric and aqueous conditions.

If we wish to understand metamorphosis, we must supplement the old opinion that ecdysis is merely an occurrence to facilitate expansion, by the more modern conception that it is also an important physiological process. That shedding the skin is done solely to permit of enlargement of size is a view rendered untenable by many considerations. The integument can increase and stretch to an enormous extent without the aid of moulting; witness the queen-termite, and the honey-bearers of the Myrmecocystus ants. Many moults are made when increase of size does not demand them, and the shedding of the skin at the time of pupation is accompanied by a decrease in size. And if moulting be merely connected with increase of size, it is impossible to see why Cloëon should require two dozen moults, while Campodea can do with one, or why a collembolon should go on moulting during the period of life subsequent to the cessation of growth.

The attention of entomologists has been chiefly directed to the ecdyses connected with the disclosure of the pupal and imaginal instars. Various important transformations may, however, occur previous to this, and when they do so it is always in connexion with ecdyses. Caterpillars frequently assume a different appearance and change their habits or character at a particular ecdysis; and in Orthoptera each ecdysis is accompanied by a change of form of the thoracic segments; this change is very considerable at one of the intermediate ecdyses.

The assumption of the pupa state is the concomitant of an ecdysis, and so also is the appearance of the imago; but the commencement of each of these two stages precedes the ecdysis, which is merely the outward mark of the physiological processes. The ecdysis by which the pupa is revealed occurs after the completion of growth and when great changes in the internal organs have occurred and are still taking place; the ecdysis by which the imago appears comes after development has been quite or nearly completed.

Although the existence of a pupa is to the eye the most striking of the differences between Insects with perfect and those with imperfect metamorphosis, yet there is reason for supposing that the pupa and the pupal period are really of less importance than they at first sight appear to be. In Fig. 85 we showed how great is the difference in appearance between the pupa and the imago. The condition that precedes the appearance of the pupa is, however, really the period of the most important change. In Fig. 89 we represent the larva and pupa of a bee; it will be seen that the difference between the two forms is very great, while the further change that will be required to complete the perfect Insect is but slight. When the last skin of the larva of a bee or of a beetle is thrown off, it is, in fact, the imago that is revealed; the form thus displayed, though colourless and soft, is that of the perfect Insect; what remains to be done is a little shrinking of some parts and expansion of others, the development of the colour, the hardening of certain parts. The colour appears quite gradually and in a regular course, the eyes being usually the first parts to darken. After the coloration is more or less perfected—according to the species—a delicate pellicle is shed or rubbed off, and the bee or beetle assumes its final form, though usually it does not become active till after a farther period of repose.

Fig. 89.—Larva and pupa of a bee, Xylocopa violacea: A, larva; B, pupa, ventral aspect; C, pupa, dorsal aspect. (After Lucas.)

CHAPTER VI

CLASSIFICATION—THE NINE ORDERS OF INSECTS—THEIR CHARACTERS—PACKARD'S ARRANGEMENT—BRAUER'S CLASSIFICATION—CLASSIFICATIONS BASED ON METAMORPHOSIS—SUPER-ORDERS—THE SUBDIVISIONS OF ORDERS.

Classification.

We have already alluded to the fact that Insects are the most numerous in species and individuals of all land animals: it is estimated that about 250,000 species have been already described and have had scientific names given to them, and it is considered that this is probably only about one-tenth of those that really exist. The classification in a comprehensible manner of such an enormous number of forms is, it will be readily understood, a matter of great difficulty. Several methods or schemes have since the time of Linnaeus been devised for the purpose, but we shall not trouble the reader to consider them, because most of them have fallen into disuse and have only a historical interest. Even at present there exists, however, considerable diversity of opinion on the question of classification, due in part to the fact that some naturalists take the structure of the perfect or adult Insect as the basis of their arrangement, while others prefer to treat the steps or processes by which the structure is attained, as being of primary importance. To consider the relative values of these two methods would be beyond our scope, but as in practice a knowledge of the structures themselves must precede an inquiry as to the phases of development by which the structures are reached; and as this latter kind of knowledge has been obtained in the case of a comparatively small portion of the known forms,—the embryology and metamorphosis having been investigated in but few Insects,—it is clear that a classification on the basis of structure is the only one that can be at present of practical value. We shall therefore for the purposes of this work make use of an old and simple system, taking as of primary importance the nature of the organs of flight, and of the appendages for the introduction of food to the body by the perfect Insect. We do not attempt to disguise the fact that this method is open to most serious objections, but we believe that it is nevertheless at present the most simple and useful one, and is likely to remain such, at any rate as long as knowledge of development is in process of attainment.

Orders.

The great groups of Insects are called Orders, and of these we recognise nine, viz. (1) Aptera, (2) Orthoptera, (3) Neuroptera, (4) Hymenoptera, (5) Coleoptera, (6) Lepidoptera, (7) Diptera, (8) Thysanoptera, (9) Hemiptera. These names are framed to represent the nature of the wings; and there is some advantage in having the Orders named in a uniform and descriptive manner. The system we adopt differs but little from that proposed by Linnaeus.[^[107]] The great Swedish naturalist did not, however, recognise the Orders Orthoptera and Thysanoptera; and his order Aptera was very different from ours.

These Orders may be briefly defined as follows,—the reader being asked to recall the fact that by a mandibulate mouth we understand one in which the mandibles, or the maxillæ, or both, are fitted for biting, crushing, or grasping food; while the term suctorial implies that some of the mouth parts are of a tubular form or are protrusible as a proboscis, which assists, or protects, a more minute and delicate sucking apparatus:—

1. Aptera (ἀ without, πτερόν a wing). Wingless[^[108]] Insects; mouth mandibulate or very imperfectly suctorial. Metamorphosis very little.

2. Orthoptera (ὀρθός straight, πτερόν a wing). Four wings are present, the front pair being coriaceous (leather-like), usually smaller than the other pair, which are of more delicate texture, and contract in repose after the manner of a fan. Mouth mandibulate. Metamorphosis slight.

3. Neuroptera (νεῦρον nerve, πτερόν a wing). Four wings of membranous consistency, frequently with much network; the front pair not much, if at all, harder than the other pair, the latter with but little or no fanlike action in closing. Mouth mandibulate. Metamorphosis variable, but rarely slight.

4. Hymenoptera (ὑμήν membrane, πτερόν a wing). Four wings of membranous consistency; the front pair larger than the hind, which are always small and do not fold up in repose. Mouth mandibulate, sometimes provided also with a tubular proboscis. Metamorphosis very great.

5. Coleoptera (κολεός sheath, πτερόν a wing). Four wings; the upper pair shell-like in consistency, and forming cases which meet together over the back in an accurate line of union, so as to entirely lose a winglike appearance, and to conceal the delicate membranous hind pair. Mouth mandibulate. Metamorphosis great.

6. Lepidoptera (λεπίς scale, πτερόν a wing). Four large wings covered with scales. Mouth suctorial. Metamorphosis great.

7. Diptera (δίς double, πτερόν a wing). Two membranous wings. Mouth suctorial, but varying greatly. Metamorphosis very great.

8. Thysanoptera (θύσανος fringe, πτερόν a wing). Four very narrow fringed wings. Mouth imperfectly suctorial. Metamorphosis slight.

9. Hemiptera (ἡμι half, πτερόν a wing). Four wings; the front pair either leather-like with more membranous apex, or entirely parchment-like or membranous. Mouth perfectly suctorial. Metamorphosis usually slight.

We must again ask the reader to bear in mind that numerous exceptions exist to these characters in most of the great Orders; for instance, wingless forms are not by any means rare in several of the Orders.

Before remarking further on this system we will briefly sketch two other arrangements of the Orders of Insects, for which we are indebted to Packard and Brauer.

Packard's Classification.

Packard has devoted much attention to the subject, and has published two or three successive schemes, of which the following is the most recent:[^[109]] the definitions are those of the author himself, but the information in brackets is given to institute a concordance with the system we adopt:—

1. Thysanura. Wingless; often with a spring (equivalent to our Aptera).

2. Dermaptera. Front wings minute, elytra-like (= Forficulidae, a part of our Orthoptera).

3. Orthoptera. Wings net-veined; fore wings narrow, hind wings folded (= our Orthoptera after subtraction of Dermaptera).

4. Platyptera. Four net-veined wings; mouth parts adapted for biting (= Termitidae and Mallophaga, parts of our Neuroptera).

5. Odonata. Wings net-veined, equal (= Odonata, a division of our Neuroptera).

6. Plectoptera. Wings net-veined, unequal (= Ephemeridae, a part of our Neuroptera).

7. Thysanoptera. Mouth beaklike but with palpi (= our Thysanoptera).

8. Hemiptera. Mouth parts forming a beak for sucking. No palpi (= our Hemiptera).

The above eight Orders form the group Ametabola, while the following eight constitute the Metabola:—

9. Neuroptera. Wings net-veined; metamorphosis complete (= a small part of our Neuroptera).

10. Mecaptera. Wings long and narrow (for a small part of our Neuroptera; the Panorpatae of Brauer).

11. Trichoptera. Wings not net-veined (= our division of Neuroptera with the same name).

12. Coleoptera. Fore wings sheathing the hinder ones (= our Coleoptera).

13. Siphonaptera. Wingless, parasitic. Flea (= a division of Diptera).

14. Diptera. One pair of wings (= our Diptera after subtraction of Siphonaptera).

15. Lepidoptera. Four wings (and body) scaled (= our Lepidoptera).

16. Hymenoptera. Four clear wings; hinder pair small; a tongue (= our Hymenoptera).

Although this system of the Orders of Insects has some valuable features it is open to very serious objections, to which we can only briefly allude. The Order Hemiptera with its extensive divisions, Heteroptera, Homoptera, Coccidae, and Anoplura exhibiting great differences in structure and considerable divergence in metamorphosis, is treated as only equivalent to the little group Panorpatae (scorpion-flies); these latter being considered a distinct order, although they are not very different in structure or metamorphosis from the Orders he calls Neuroptera and Trichoptera. The arrangement appears to be specially designed with the view of making the Orders adopted in it fall into the two groups Ametabola and Metabola. The propriety of such a course is more than doubtful since very few of the Ametabola are really without metamorphosis, in the wide sense of that term, while the Metabola include Insects with various kinds of metamorphosis. Indeed if we substitute for the term Ametabola the more correct expression, "Insects with little metamorphosis," and for Metabola the definition, "Insects with more metamorphosis but of various kinds," we then recognise that the arrangement is, like all others, a quite artificial one, while it is of little value, owing to the development of so few Insects being hitherto fully ascertained.

Brauer's Classification.

Professor Brauer has recently proposed[^[110]] to adopt 17 Orders or chief groups of Insects, arranging them as follows:—

I. Apterygogenea (with one order).

1. Synaptera (= Aptera of our system).

II. Pterygogenea (= all the other Insects of our arrangement).

2. Dermaptera (= Orthoptera, Fam. Forficulidae in our arrangement).

3. Ephemeridae (= a division of Neuroptera in our arrangement).

4. Odonata (= a division of Neuroptera in our arrangement).

5. Plecoptera (= Neuroptera, Fam. Perlidae in our arrangement).

6. Orthoptera (= our Orthoptera - Forficulidae and + Embiidae).

7. Corrodentia (= the families Termitidae, Psocidae, and Mallophaga, of our Neuroptera).

8. Thysanoptera (as with us).

9. Rhynchota (= Hemiptera with us).

10. Neuroptera (= the families Hemerobiidæ and Sialidæ of our Neuroptera).

11. Panorpatae (= the family Panorpidae of our Neuroptera).

12. Trichoptera (= the division Trichoptera of Neuroptera).

13. Lepidoptera (= as with us).

14. Diptera (= our Diptera - Aphaniptera).

15. Siphonaptera (= Aphaniptera, a division of Diptera with us).

16. Coleoptera (= Coleoptera).

17. Hymenoptera (as with us).

The chief characters on which Brauer bases his system are: (1) The existence or absence of wings. (2) The condition of the mouth, and whether it undergoes radical changes in the ontogeny, arriving thus at the categories Menognatha, Metagnatha, and Menorhyncha, as we have mentioned on p. [161]. (3) The metamorphosis; the grouping adopted being Ametabola, Hemimetabola, Metabola. (4) The number of the Malpighian tubules; Oligonephria, Polynephria. (5) The nature of the wings, the relative proportions of the thoracic segments, and some other characters.

Brauer's treatise is accompanied by a valuable and in many respects very sagacious consideration of the generalised characters of the Insecta; as a classification based partly on generalisations and partly on structures, it is, so far as the present condition of our knowledge goes, a good one. But it is of little value as a practical guide, and as a basis for theoretical speculation it cannot be treated as of importance, because the generalisations it makes use of are premature, owing to the small proportion of the forms that have been examined. And even now the groups adopted are known to be subject to many exceptions.

Thus it begins by a division of Insecta into winged and wingless; but the winged division is made to comprehend an enormous number of wingless Insects, whole subdivisions of Orders such as the Mallophaga being placed in the winged series, although all are without wings. This first division is indeed entirely theoretical; and if a classification on generalisations were adopted, it would be more natural to begin with the old division into Homomorpha and Heteromorpha, and treat the Order Aptera as the first division of the Homomorpha, while the Heteromorpha would commence with the Ephemeridae and Odonata, in which, though the individual in the early part of the ontogeny is very different from the perfect Insect, there is no marked division of the later larval and the pupal stages. Brauer's system is also defective inasmuch as it takes no account of the embryological or oogenetic processes, though these are of equal importance with the later phases of the Ontogeny. Even as regards the division into Orders, it is far from being free from reproach; for instance, the separation of the Dermaptera from the Orthoptera, while Rhynchota remains intact, although including a more extensive series of heterogeneous forms; the division of the Neuroptera into widely separated groups, each of which is treated as equivalent to the great Orders, such as Coleoptera (in which Strepsiptera are included), Hymenoptera, and Diptera, is not reasonable. The association of Mallophaga and Termitidae, while Dermaptera are separated from Orthoptera, is also undeniably arbitrary, and other similar disparities are to be seen on scrutinising the details of the system.

On comparing the three arrangements we have outlined, it will be seen that the chief discrepancies they present come under two heads: (1) The treatment of the Neuroptera, opinions differing as to whether these Insects shall be grouped as a single Order, or shall be divided into numerous Orders; and as to what, if this latter course be adopted, the divisions shall be. (2) The treatment of the parasitic groups Mallophaga, Aphaniptera, etc. It must be admitted that whichever of the three systems we have sketched be adopted, the result is, as regards both these points, open to criticism. The Order Neuroptera, if we take it in the broad sense, differs from the other Orders in the greater variety of metamorphosis exhibited by its members; while if, on the contrary, it be dismembered, we get a number of groups of very unequal extent and not distinguished from one another by the same decisive and important characters as are the other Orders of which they are considered equivalent. The discrepancy exists in nature, and can scarcely be evaded by any system. A similar observation may be made as to the parasitic groups, viz. Mallophaga, Anoplura, Aphaniptera, and Strepsiptera. If these be treated as separate Orders the result is not satisfactory; while, if they be associated with the larger groups to which they are respectively nearest allied, it is almost equally unsatisfactory.

We may mention that Packard and Brauer have in their treatises discussed the question of super-orders, and have gone so far as to propose names for them. These two authorities do not however agree in their conclusions; and as the names proposed are of little practical value, and are but rarely met with, we need not explain them or discuss the comparative merits of the two systems.

The divisions of inferior value to the Order are, after repeated scrutiny by many naturalists, becoming of a more satisfactory character, and notwithstanding various anomalies, may be, many of them, considered fairly natural.[^[111]] Unfortunately entomologists have not been able to agree on a system of terminology, so that for these subdivisions terms such as sub-order, series, legion, section, tribe, etc., are used by different authorities in ways so various as to cause much confusion. In the following pages the terms sub-order and series will be used in a somewhat vague manner, the term sub-order being preferred where the group appears to be an important one and of a fairly natural character, while the word series will be adopted when the groups are connected in a conventional manner. The designation "family" will be used for groups of subordinate importance; and as regards this term we may remark that systematic entomologists are making genuine efforts to define the "families" in an accurate and comprehensible manner. The endeavour to make these systematic families dependent throughout the Class Insecta on characters of similar morphological value has, however, scarcely been entered on, and it is perhaps not desirable, seeing how very small a portion of the Insects of the world have been critically examined, that much effort should be yet expended on an attempt of the kind. It must be admitted that the species of Insects should be obtained before they can be satisfactorily classified, and it is estimated[^[112]] that at least nine-tenths of the Insects of the world are still unknown to entomologists.

Geological Record.—Although Insects have a very long pedigree, it is as yet a very imperfect one. The remains of creatures that can be referred to the Class Insecta have been found, it is said, in Silurian strata; only one or two of these very early forms are at present known, and the information about them is by no means satisfactory; if Insects at all—as to which some doubt exists—they apparently belong to very different forms, though, like all the earliest fossil Insects, they are winged. In the strata of the Carboniferous epoch numerous Insects have been detected, in both Europe and North America. These earlier Insects are by Scudder called Palaeodictyoptera, and separated from the Insects around us on the ground that he considers there existed amongst these palaeozoic Insects no ordinal distinctions such as obtain in the existing forms, but that the primeval creatures formed a single group of generalised Hexapods. Brauer does not accept this view, considering that the earlier Insects can be referred to families existing at the present time and forming parts of the Orthoptera, Neuroptera, and Hemiptera. The discrepancy between these two authorities depends to a great extent on the different classifications of existing Insects that they start from; Scudder treating the wings as of primary importance, while Brauer assigns to them only a subordinate value. From the point of view taken in the present work Scudder's view appears to be in the main correct, though his expression as to the primary fossil Insects forming a single homogeneous group is erroneous. The Neuroptera, still in existence, certainly form a heterogeneous group, and it is clear that the Palaeozoic fossils represent a more diverse assemblage than the present Neuroptera do.[^[113]]

In the more recent rocks Insect remains become comparatively numerous, and in Mesozoic strata forms that can satisfactorily be referred to existing Orders are found, the Palaeodictyoptera of Goldenberg and Scudder having mostly disappeared; the Blattidae or cockroaches do not apparently present any great discontinuity between their Palaeozoic and Mesozoic forms. The Tertiary rocks afford us fairly satisfactory evidence to the effect that Insects were then more numerous in species than they are at the present day. At Florissant in Colorado the bed of an ancient lake has been discovered, and vast quantities of Insect remains have been found in it, the geographical conditions indicating that the creatures were not brought from a distance, but were the natural fauna of the locality; and if so we can only conclude that Insects must have been then more abundant in species than they are now.

Scudder has informed us[^[114]] that not only were Insects abundant in the Tertiaries, but that their remains indicate conditions of existence very similar to what we find around us. "Certain peculiarities of secondary sexual dimorphism accompanying special forms of communistic life, such as the neuters and workers in Hymenoptera and the soldiers among the Termitina, are also found, as would be expected, among the fossils, at least through the whole series of the Tertiaries. The same may be said of other sexual characteristics, such as the stridulating organs of the Orthoptera, and of peculiarities of oviposition, as seen in the huge egg-capsules of an extinct Sialid of the early Tertiaries. The viviparity of the ancient Aphides is suggested, according to Buckton, by the appearance of one of the specimens from the Oligocene of Florissant, while some of the more extraordinary forms of parasitism are indicated at a time equally remote by the occurrence in amber of the triungulin larva of Meloe, already alluded to, and of a characteristic strepsipterous Insect; not only, too, are the present tribes of gall-making Insects abundant in the Tertiaries, but their galls as well have been found."

CHAPTER VII

THE ORDER APTERA–DEFINITION–CHIEF CHARACTERISTICS–THYSANURA–CAMPODEA–JAPYX–MACHILIS–LEPISMA–DIVERSITY OF INTERNAL STRUCTURE IN THYSANURA–ECTOTROPHI AND ENTOTROPHI–COLLEMBOLA–LIPURIDAE–PODURIDAE–SMYNTHURIDAE–THE SPRING–THE VENTRAL TUBE–ABDOMINAL APPENDAGES–PROSTEMMATIC ORGAN–TRACHEAL SYSTEM–ANURIDA MARITIMA–COLLEMBOLA ON SNOW–LIFE-HISTORIES OF COLLEMBOLA–FOSSIL APTERA–APTERYGOGENEA–ANTIQUITY AND DISTRIBUTION OF CAMPODEA.

Order I. Aptera.

Small Insects with weak outer skin, destitute throughout life of wings or their rudiments, but with three pairs of legs; antennae large or moderate in size.

The above definition is the only one that can at present be framed to apply to all the Insects included in our Aptera. Unfortunately it is far from diagnostic, for it does not enable us to distinguish the Aptera from the larvae or young individuals of many Insects of other Orders. There are, however, certain characters existing in many species of Aptera that enable their possessors to be recognised with ease, though, as they are quite wanting in other members, they cannot correctly be included in a definition applying to the whole of the Order.

We are thus brought in view of two of the most important generalisations connected with the Aptera, viz. that these Insects in their external form remain throughout their life in a condition resembling the larval state of other Insects, and that they nevertheless exhibit extreme variety in structural characters.

The more important of the special characters alluded to above as being possessed by some but not by all members of the Order are (1) a remarkable leaping apparatus, consisting of two elongate processes at the under side of the termination of the body; (2) a peculiar ventral tube, usually seen in the condition of a papilla with invaginated summit, and placed on the first abdominal segment (see Fig. 100, p. [194]); (3) the scales covering the body; (4) the existence of abdominal appendages in the form of long cerci or processes at the termination of the body, or of short processes on the sides of the under surface of the abdominal segments.

Throughout the Order the general shape approximates to that of a larva; this is shown by the diagrammatic section of the body of Machilis (Fig. 90). There is a succession of rings differing little from one another, except so far as the head is concerned; even the division of thorax from abdomen is but little evident, and although in some of the forms the three thoracic segments may differ considerably among themselves, yet they never assume the consolidated form that they do to a greater or less extent in the imago stage of the other Orders. Fig. 90 shows the larva-like structure of the body, and also exhibits the inequalities in size between some of the dorsal and the corresponding ventral plates. This phenomenon is here displayed only to a small extent, so that the true relations of the dorsal and ventral plates can be readily detected; but in the higher Insects want of correspondence of this kind may be much more extensive.

Fig. 90.--Section of body of Machilis: o, ovipositor. (After Oudemans.)

The respiratory system is in many of these Insects very inferior in development, and may even be, so far as tracheae and spiracles are concerned, entirely absent, but in other members of the Aptera it is well developed. In the other internal organs there is also great variety, as there is in the external structure.

A brief explanation as to the term Aptera, which we have adopted as the name of this Order, is necessary. This name was used by Linnaeus for our Insects, but as he associated with them various other heterogeneous forms which were afterwards separated, his "Aptera" became completely broken up and ceased to be recognised as an Order of Insects. The term was, however, revived by Haeckel and Balfour several years since, and applied quite properly to the Insects we have in view. Subsequently Packard and Brauer, recognising the claims of these Insects to an isolated position, proposed for them the names Synaptera and Apterygogenea, and Packard has also used the term Cinura. There is, however, clearly an advantage in retaining the termination "ptera" for each of the Orders of Insects; and as the fact that "Aptera" of Linnaeus included many Insects is not a sufficient reason for refusing to apply the term to a portion of the forms he used it for, we may, it is clear, make use of the Linnaean name with propriety, it being explicitly stated that the Order does not include by any means all the apterous forms of Insects.

The Order includes two sub-orders, viz. (1) Thysanura, in which the hind body (abdomen) is composed of ten segments, and there is no ventral tube on its first segment; and (2) Collembola, in which the hind body consists of not more than six segments, the first of which is furnished beneath with a peculiar tube or papilla.

Thysanura.

Our knowledge of this important sub-order has been recently much increased by the works of Grassi[^[115]] and Oudemans.[^[116]] Very little is known, however, of the extra-European forms, there being great difficulties in the way of collecting and preserving specimens of these Insects in such a way as to render them available for study and accurate comparison. Grassi and Rovelli[^[117]] recognise four families among the few European species of Thysanura, viz. Campodeidae, Japygidae, Machilidae, Lepismidae. Campodeidae is perhaps limited to a single species, only one having been satisfactorily established, though several descriptions have been made of what are supposed to be other species.

This Insect (Campodea staphylinus) is, so far as external form goes, well known, from its having been figured in many works on natural history on account of its having been supposed to be the nearest living representative of a primitive or ancestral Insect. The creature itself is but little known even to entomologists, although it is one of the commonest of Insects over a large part of Europe. It is numerous in the gardens and fields about London and Cambridge, and abounds in damp decaying wood in the New Forest; if there be only one species, it must possess an extraordinary capacity for adapting itself to extremes of climate, as we have found it at midsummer near the shores of the Mediterranean in company with the subtropical white ants, and within a day or two of the same time noticed it to be abundant on the actual summit of Mount Canigou, one of the higher Pyrenees, where the conditions were almost arctic, and it was nearly the only Insect to be found. The species is said to exist also in North America and in East India. It is a fragile, soft Insect of white colour, bending itself freely to either side like a Myriapod; the legs are rather long, the antennae are long and delicate, and the two processes, or cerci, at the other extremity of the body are remarkably similar to antennae. It has no eyes and shuns the light, disappearing very quickly in the earth after it has been exposed. If placed in a glass tube it usually dies speedily, and is so extremely delicate that it is difficult to pick it up even with a camel's hair brush without breaking it; so that we may fear it to be almost hopeless to get enough specimens from different parts of the world to learn what differences may exist amongst the individuals of this so-called primitive Insect. Meinert, a very able entomologist, considers that there is really more than one species of Campodea.

Fig. 91.—Campodea staphylinus. (After Lubbock, × 15.)

Campodeidae as a family may be briefly defined as Thysanura with the trophi buried in the head and with the body terminated by antenna-like processes. We shall consider some of the anatomical peculiarities of this interesting Insect after we have briefly reviewed some of the external characters of the other Thysanura.

The second family (Japygidae) consists of one genus Japyx, of which there are, no doubt, several different species in various parts of the world, such having already been detected in tropical Africa, in Malasia, and in Mexico, as well as in Madeira and Europe. The commoner species of the latter continent, Japyx solifugus, lives in moss or in shady places on the edges of woods. It possesses a great resemblance to a newly-hatched earwig, and the writer has found it in France under a stone in company with a number of the tiny creatures it was so much like. This species has been found as far north as Paris, but has not been met with in Britain. The family Japygidae is, like the Campodeidae, entotrophous, and is distinguished by the body being terminated behind by a pair of forceps instead of antennary organs.

The other two families of Thysanura, Machilidae and Lepismidae, are ectotrophous—that is, the parts of the mouth are not buried in the head, but are arranged in the fashion usual in mandibulate Insects.

Only one genus of Machilidae is known, but it is no doubt very numerous in species, and probably is distributed over most of the globe. Machilis maritima is common in some places on the coast of England. Another species (M. polypoda) occurs amongst dead leaves in the New Forest, and we have also observed a species of the genus under the loose stones that frequently form the tops of the "dykes" or piled walls in Scotland. In more southern Europe species of Machilis are commonly met with on the perpendicular faces of very large stones or rocks, over which they glide with wonderful facility. The scales on the bodies of these rock-inhabiting species form pretty patterns, but are detached with such facility that it is almost impossible to obtain specimens in satisfactory condition for examination.

In Machilidae the dorsal plates of the hind body are reflexed to the under surface so as to form an imbrication covering the sides of the ventral plates, and the eyes are largely developed; by which characters the family is distinguished from the Lepismidae. The pair of large compound eyes (Fig. 92, O) is a remarkable feature, being indeed unique in the Aptera. The structures (o, o′) that Oudemans considers to be simple eyes have, in external appearance, a resemblance to the fenestrae of the Blattidae; Grassi states, however, that not only are they eyes, but that they are of almost unique structure, being, in fact, intermediate between simple and compound eyes.

The mode of development of the compound eyes of Machilis is of considerable interest, but unfortunately very little is known about it, even the period at which the eyes appear being uncertain. Judging from analogy with the Orthoptera, we should suppose them to be present when the Insect leaves the egg, and Oudemans apparently considers this to be the case, but Bolivar states[^[118]] that in the early stages of Machilis the eyes are only simple eyes; these being replaced by compound eyes in the later life. The writer has observed very young individuals of Machilis polypoda, and found the eyes to be evidently compound.

Fig. 92.—Head of Machilis maritima (after Oudemans): A, base of antenna; C, clypeus; F, vertex; P, fold; O, eye; o, o′, supposed simple eye; M, mandible; m, maxilla; L, upper lip; l, lower lip; T, portion of maxillary palp; t, of labial palp. × 20.

Fig. 93.—Lepisma cincta. (After Oudemans.) × 4. (The line indicates the natural length.)

The remaining family of Thysanura, the Lepismidae, is in certain respects the most highly developed of the Order. The covering of scales found on the body is very remarkable in some of the species, especially in the genus Lepisma (Fig. 93, L. cincta); the thoracic segments are different from one another and from those of the abdomen, and the tracheal system is more highly developed than it is in the Machilidae. Several genera are known, but only two members of the family have yet been detected in Britain. One of them (Lepisma saccharina), occurs only in houses, and is sometimes called the silver fish; it is, when full grown, less than half an inch long, and is covered with scales that give it a feebly metallic lustre. Like the other Thysanura, its movements are very perfect. It is said that it is occasionally injurious by nibbling paper, but the writer's observations lead him to doubt this; its usual food is doubtless farinaceous or saccharine matter. Thermobia furnorum, our other British Lepismid, has only recently been discovered; it is found in bakehouses at Cambridge and elsewhere. The bakers call these Insects fire-brats, apparently considering them to be fond of heat.

Much valuable information as to the anatomy of Thysanura has been obtained by Grassi and Oudemans, and is of great interest. Taking four genera, viz. Campodea, Japyx, Machilis, and Lepisma, to represent the four families constituting the sub-order, we will briefly enumerate some of the more remarkable of the characters of their internal anatomy. Campodea has a very inferior development of the tracheal system; there are three pairs of spiracles, which are situate on the thoracic region; the tracheae connected with each spiracle remain distinct, not uniting with those coming from another spiracle; there are thus six separate small tracheal systems, three on each side of the body. Japyx solifugus has eleven pairs of spiracles, of which four are thoracic; the tracheae are united into one system on each side by means of lateral tubes; thus there are two extensive tracheal systems situate one on each side of the body, there being a single transverse tube, placed near the posterior extremity, uniting the two lateral systems. In Machilis there are nine pairs of stigmata, two of them thoracic, seven abdominal; the tracheae from each spiracle remain unconnected, so that there are eighteen separate tracheal systems, some of which are considerably more developed than others. The Lepismidae have ten pairs of stigmata, and the tracheae connected with them are completely united into one system by longitudinal and transverse tubes. Besides these differences there are others, of considerable importance, in the position of the stigmata.

All the Thysanura possess salivary glands. In Campodea there are about sixteen extremely short Malpighian tubules, or perhaps glands representing these organs; Japyx is destitute of these structures; Machilis maritima has twenty elongate tubules; in Lepisma also they are long, and apparently vary in number from four to eight in different species. The proportions of the three divisions of the alimentary canal differ extremely; there is a very large proventriculus in Lepisma, but not in the other families; coecal diverticula are present on the anterior part of the true stomach in Machilis and in Lepisma, but are wanting in Campodea and in Japyx.

The dorsal vessel seems not to present any great differences in the sub-order. Grassi says there are no alary muscles present, but Oudemans describes them as existing in Machilis, but as being excessively delicate.

The ventral chain of nerve-ganglia consists in Campodea of one cephalic ganglion, one sub-oesophageal (which clearly belongs to the ventral series of ganglia), three thoracic, and seven abdominal. In the other families there are eight instead of seven abdominal ganglia.

The structure of the internal sexual organs is very remarkable in the Thysanura. In Campodea there is one extremely large, simple tube on each side of the body. In Japyx there are seven small tubes on each side, placed one in each of the successive abdominal segments, and opening into a common duct. In Machilis there are also seven tubes opening into a common duct, but the arrangement is no longer a distinctly segmental one. In Lepisma there are five egg-tubes on each side, the arrangement being segmental in the young state but not in the adult condition. In Campodea nutrient cells alternate with the eggs in the tubes, but this is not the case in the other families. Fig. 94 shows the ovaries in three of the Thysanura; in the drawing representing this part in Machilis (C), one of the two ovaries is cut away for the sake of clearness.

The male organs in Campodea are very similar in size and arrangement to the ovaries, there being a single large tube or sac and a short vas deferens on each side of the body. In Japyx there is a sac on each side, but it is rendered double by a coecum at its base, and there are long and tortuous vasa deferentia. In Lepisma there are three pairs of coeca on each side, segmentally placed and opening into a common duct. In Machilis there are three retort-shaped sacs on each side opening near one another into a common duct, the vasa deferentia are elongate, and are very curiously formed, being each double for a considerable length, and the separated portions connected at intervals by five transverse commissural ducts.

Fig. 94.—Ovaries of Thysanura: A, of Campodea; B, of Japyx; C, of Machilis. (After Grassi and Oudemans.)

One of the characteristic features of Insect structure is the restriction of articulated legs to the thoracic region. In the Thysanura there exist appendages occupying a position on the hind body somewhat similar to that of the legs on the thorax. These appendages are quite small bodies, and are placed at the hind margins of the ventral plates of the abdomen, one near each side; they are connected by a simple joint to the sternite and are provided with muscles. They are found in Campodea on segments 2 to 7; in Lepisma on 8 and 9, in the allied Nicoletia on 2 to 9; in Japyx on 1 to 7, being, however, more rudimentary than they are in Campodea. In Machilis they attain perhaps their greatest development and exist on segments 2 to 9; moreover, in this genus such appendages occur also on the coxae of the second and third pairs of thoracic legs. Oudemans thinks they help to support the abdomen, and that they also assist in leaping; Grassi considers that they are supporting agents to some extent, but that they are essentially tactile organs. He calls them false legs "Pseudozampe."

Still more remarkable and obscure in function are the vesicles found near the appendages; we figure a pair after Oudemans, showing them in the exserted state. In the retracted state the outer portion of the vesicles is withdrawn into the basal part P (Fig. 95), so that the vesicles are then only just visible, being concealed by the ventral plate. The abdominal appendage is not retractile. In Machilis there are twenty-two of these vesicles, arranged either two or four on one ventral plate of the hind body. They are also present in Campodea, where there are six pairs. They are usually said to be absent in Japyx and in Lepisma, but Haase shows[^[119]] that Japyx possess a pair placed behind the second ventral plate of the abdomen. The vesicles appear to be exserted by the entrance of blood into them, and to be retracted by muscular agency. Much difference of opinion prevails as to their function; it appears probable that they may be respiratory, as suggested by Oudemans.

The scales found on the bodies of the Ectotrophous Thysanura may be looked on as modified hairs, and are essentially similar to those of the Lepidoptera, and they drop off as readily as do those of the Lepidoptera.

Stummer-Traunfels, who has recently published[^[120]] the results of his researches on the mouth-organs of Thysanura and Collembola, confirms the division of the Thysanura into Entotrophi and Ectotrophi, and considers that the Collembola agree with the former group. The German author therefore proposes to divide our Aptera, not into Thysanura and Collembola, but into Ectognathi and Entognathi, the former group consisting of Machilidae and Lepismidae, the latter of Campodeidae, Japygidae and the various families of Collembola. We think it far more natural, however, to retain the older division into Thysanura and Collembola.

Fig. 95.—Abdominal appendage and exsertile vesicles of Machilis. A, appendage; V, vesicles protruded; P, basal portion; R, muscles, × 70.

Collembola.

The sub-order Collembola, which we have defined on p. [182], consists of small Insects, many of which possess the capacity of leaping, or springing suddenly, and when disturbed or alarmed naturally make use of this means of escaping. Their leaps, however, appear to be made quite at random, and very frequently do not have the result of taking the creature into concealment, and in such circumstances they may be rapidly and frequently repeated until the Insect feels itself, as we may suppose, in a position of safety. Three families may be very readily distinguished, viz. (1) Lipuridae, in which no leaping apparatus is present; (2) Poduridae, a leaping apparatus exists near the extremity of the abdomen; the body is subcylindric and evidently segmented; (3) Smynthuridae, a leaping apparatus exists: the body is sub-globular with comparatively large head and abdomen, the intervening thoracic region being small; the segmentation of the body is obscure.

The study of the Collembola is much less advanced than that of the Thysanura, comparatively little having been added to our knowledge of the group since Lubbock's monograph of the British forms was published by the Ray Society in 1873. Why the Collembola should be neglected when the Thysanura attract so much attention is as inexplicable as many other fashions are.

The family Lipuridae consists of a few very small and obscure Insects of soft consistence. They move slowly, and, owing to the absence of any leaping power, attract attention less readily than the other Collembola do. Two genera are generally recognised, and they should probably form separate families; indeed, in Lubbock's arrangement they do so. In one of the genera (Anoura) the mouth is very imperfect, no mandibles or maxillae having been detected, while in the other genus (Lipura) these organs exist.

In the members of the family Poduridae, including the Degeeriidae of Lubbock, a saltatory apparatus is present in the form of appendages attached to the fifth abdominal segment (Degeeriides), or to the fourth (Podurides). These appendages are during life flexed beneath the body, but in dead specimens usually project backwards, having the appearance of a bifid tail. Poduridae are of elongate form, somewhat like small caterpillars, and are frequently prettily marked with variegate colours. Fig. 97 represents an arctic form closely allied to our native genus Isotoma.

Fig. 96.—Lipura burmeisteri. (After Lubbock.)

The peculiar shape of the members of the Smynthuridae is sufficient for their identification. They possess a very convex abdomen, and very near to it a large head, the intervening chink being occupied by the small thorax. The segmentation of the body is not easily distinguished. Nicolet states that the thorax consists of three segments and the abdomen of the same number, and that when the Insect emerges from the egg these divisions can be perceived. In after life the posterior part of the thorax becomes amalgamated with the abdomen, so that it is difficult to trace the divisions, but there appears to be no information as to the manner in which this change occurs. Some of these minute Insects frequent trees and bushes, and their leaping powers are very perfect, so that it is difficult to capture them. The family includes both the Smynthuridae and the Papiriidae of Lubbock.

Fig. 97.—Corynothrix borealis: a, ventral tube; b, the spring. (After Tullberg.)

Fig. 98.—Smynthurus variegatus, with spring extended. (After Tullberg.)

The two most characteristic organs of the Collembola are the spring and the ventral tube. The first of these is an elongate structure attached to the underside of the abdomen near its extremity, either on the penultimate or ante-penultimate segment. It consists of a basal part, and of two appendages attached thereto. It is carried under the Insect bent forwards, and is retained in this position by means of a catch which projects from the under surface of the third segment of the body, descending between the two branches of the spring, and passing under the extremity of its basal segment. It is considered that the spring is elastic, is flexed under the body by muscular action, and, being retained in this position of restraint by the catch, when the latter is removed the spring extends by reason of its elasticity, and the leap is thus executed. Whether this is really the exact method of leaping is, however, doubtful, for Lubbock says that the catch "only exists in certain genera"; while in its structure it does not appear to be well calculated to retain in position an organ that by virtue of its elasticity is constantly exerting a considerable force.

Fig. 99.—Smynthurus fuscus, with exsertile vesicle (a) protruded from ventral tube; b, the spring extended.

The ventral tube is an anomalous and enigmatic structure. In the lower forms, such as Lipura or Anurida, it consists merely of a papilla (Fig. 100, A, a) more or less divided by fissure into two parts. In the Smynthuridae it is more highly developed, and protects two long delicate tubes that are capable of being protruded, as shown in the outline profile of Smynthurus fuscus (Fig. 99), which is taken from specimens preserved in balsam by Mr. J. J. Lister. The nature and use of this ventral tube have given rise to much discussion. Lubbock considered, and others have agreed with him, that it serves to attach the Insect to bodies to which it may be desirable the Insect should, when in the perpendicular position, adhere. Reuter[^[121]] assigns a quite different function to this singular structure. He states that the hairs of the body are hygroscopic, and that the peculiar claws of the Insect having collected the moisture from the hairs, the ventral tube becomes the means of introducing the liquid into the body. These Insects possess, however, a mouth, and there seems to be no reason why a complex apparatus should be required in addition to it for so simple a purpose as the introduction of moisture to the interior of the body. Haase finds[^[122]] that Collembola can crawl on glass without the aid of the ventral tube; he considers its function to be physiological, and that it may probably be respiratory as it has been suggested is the case with the vesicles of Thysanura. The function of the ventral tube is certainly not yet satisfactorily elucidated. The vesicles contained in it are said to be extruded by blood-pressure, and withdrawn by muscular action in a manner similar to that which we have described as occurring in the case of the exsertile vesicles of the Thysanura. The processes in Smynthurus bear glandular structures at their extremities. It has been suggested that the ventral tube of Collembola is the homologue of a pair of ventral appendages. The term Collophore has been applied to it somewhat prematurely, seeing the doubt that still exists as to its function.

Some of the Collembola possess a very curious structure called the prostemmatic or ante-ocular organ; its nature and function have been very inadequately investigated. The ocular organs of the Collembola consist, when they are present, of isolated ocelli placed at the sides of the head like the corresponding organs of caterpillars; the prostemmate is placed slightly in front of the group of ocelli, and has a concentric arrangement of its parts, reminding one somewhat of the compound eyes of the higher Insects. This structure is represented in Fig. 100, B, C; it is said by Sir John Lubbock to be present in some of the Lipuridae that have no ocelli, and he therefore prefers to speak of it as the "post-antennal" organ.

A very characteristic feature in the Collembola is the slight development of the tracheal system. Although writers are far from being in accord as to details, it seems that stigmata and tracheae are usually absent. In Smynthurus there are, however, according to Lubbock,—whose statement is confirmed by Meinert and Tullberg,—a pair of stigmata situate on the head below the antennae, and from these there extends a tracheal system throughout the body. Such a position for stigmata is almost, if not quite unique in Insects; Grassi, however, seems to have found something of the kind existing in the embryo of the bee.

At present only a small number of species of the Order Aptera are known; Lubbock recognised about sixty British species, and Finot sixty-five as found in France. The North American forms have not received so much attention as the European, and the Aptera of other countries, though they are probably everywhere fairly numerous, are scarcely known at all. A few have been described from the Indo-Malayan region and some from Chili, and the writer has seen species from the West Indian and Sandwich Islands. All the exotic forms as yet detected are very similar to those of Europe.

The Thysanura are probably not very numerous in species, and appear to be in general intolerant of cold. With the Collembola the reverse is the case. They are excessively numerous in individuals; they are found nearly everywhere on the surface of the ground in climatic conditions like those of our country, while no less than sixteen species have been found in Nova Zembla and one each in Kerguelen and South Georgia. One species, if not more, of Podura, lives on the surface of stagnant waters, on which the minute creatures may frequently be seen leaping about in great numbers after being disturbed.

In 1874 the plain of Gennevilliers in France was copiously irrigated; in the following year the soil was still very damp, and there existed numerous pools of stagnant water, on the surface of which Podura aquatica was developed in such prodigious quantity as to excite the astonishment of the inhabitants of the region.

Accounts have been frequently given of the occurrence on snow and glaciers of Insects spoken of as snow-fleas, or snow-worms. These mostly relate to Poduridae, which are sometimes found in countless number in such situations. The reason for this is not well understood. According to F. Löw,[^[123]] on the 17th of March at St. Jacob in Carinthia, Parson Kaiser observed, on the occurrence of the first thaw-weather, enormous numbers of a Podura (? Achorutes murorum) on the surface of the snow for an extent of about half a mile, the snow being rendered black in appearance by them; eleven days afterwards they were found in diminished numbers on the snow, but in large quantity on the water left by its melting. This account suggests that the occurrence of the Insects on the snow was merely an incident during their passage from the land, where they had been hibernating, to the surface of the water.

One little member of the Lipuridae, Anurida maritima (Lipura maritima of Lubbock), has the habit, very unusual for an Insect, of frequenting salt water. It lives amongst the rocks on the shores of the English Channel, between high and low tide-marks. Its habits have been to some extent observed by Laboulbène[^[124]] and Moniez[^[125]]; it appears to be gregarious, and when the tide is high, to shelter itself against the commotions of the water in chinks of the rocks and other positions of advantage. When the tide is out the Insects apparently delight to congregate in masses on the surface of the rock pools. This Anurida can endure prolonged immersion; but both the observers we are quoting say that it is, when submerged, usually completely covered with a coat of air so that the water does not touch it. The little creature can, however, it would appear, subsist for some time in the pools of salt water, even when it is not surrounded by its customary protecting envelope of the more congenial element. Its food is said, on very slender evidence, to consist of the remains of small marine animals, such as Molluscs. We reproduce some of Laboulbène's figures (Fig. 100); the under-surface shows at a the divided papilla of the ventral tube; B, C represent the peculiar prostemmatic organ, alluded to on p. [193], in its mature and immature states.

Fig. 100.—Anurida maritima: A, under-surface; a, papilla of ventral tube; B, prostemmatic organ of young; C, of adult. (After Laboulbène.)

Very little information exists as to the life-history of the Aptera; as for their food, it is generally considered to consist of refuse vegetable or animal matter. It is usual to say that they are completely destitute of metamorphosis, but Templeton says of Lepisma niveo-fasciata that "the young differ so much from the mature Insect that I took them at first for a distinct species; the thoracic plates are proportionately less broad, and the first is devoid of the white marginal band." As regards the moults, it would appear that in this, as in so many other points, great diversity prevails, Grassi stating that in Campodea there is a single fragmentary casting of the skin; and Sommer informing us that in Macrotoma plumbea the moults are not only numerous, but continue, after the creature has attained its full growth, throughout life.

A very marked feature of the Aptera is their intolerance of a dry atmosphere. Although Campodea can exist under very diverse conditions, it dies very soon after being placed in a dry closed tube; and the same susceptibility appears to be shared by all the other members of the Order, though it is not so extreme in all; possibly it may be due to some peculiarity in the structure of the integument. So far as tolerance of heat and cold goes, the Aptera can apparently exist in any climate, for though some of the species extend to the Arctic regions, others are peculiar to the tropics.

Thysanura are recorded by Klebs and Scudder as occurring commonly in amber; the latter author has described a fossil, supposed to be a Lepisma, found in the Tertiary deposits at Florissant. Scudder has also described another fossil, likewise from Florissant, which he considers to form a special sub-order of Thysanura—Ballostoma—but it is extremely doubtful whether this anomalous creature should be assigned to the Order at all. A still older fossil, Dasyleptus lucasii Brongniart, from the Carboniferous strata in France, is considered to belong to the Order Aptera, but it must be admitted there is some doubt on this point.

The interest aroused in the minds of naturalists by the comparatively simple forms of these purely wingless and therefore anomalous Insects has been accompanied by much discussion as to their relations to other Insects, and as to whether they are really primitive forms, or whether they may perhaps be degenerate descendants from some less unusual states of Insect-life. Mayer and Brauer dissociated our Aptera entirely from other Insects, and proposed to consider the Hexapoda as being composed of two groups—(1) the Apterygogenea, consisting of the few species we have been specially considering; and (2) the Pterygogenea, including all the rest of the immense crowd of Insect forms. They were not, however, able to accompany their proposed division by any satisfactory characters of distinction, and the subsequent progress of knowledge has not supported their view, all the best investigators having found it necessary to recognise the extremely intimate relations of these Insects with the Orthoptera. Meinert thought that Lepisma must be included in the Orthoptera; Grassi proposes to consider the Thysanura as a distinct division of Orthoptera; and Oudemans recognises the close relations existing between Machilis and Orthoptera proper. Finot includes the Aptera in his Orthoptères de la France, and a species of Japyx has actually been described by a competent entomologist as an apterous earwig. At present, therefore, we must conclude that no good distinction has been found to justify the separation of the Aptera from all other Insects.

The taxonomy of the Collembola has not yet been adequately treated, and it is possible that more grounds will be found for separating them as a distinct Order from the Thysanura,—a course that was advocated by Lubbock,—than exist for dividing these latter from the Orthoptera proper. There are apparently no grounds for considering the Aptera to be degenerate Insects, and we may adopt the view of Grassi, that they are primitive, or rather little evolved forms. It must be admitted that there are not at present any sufficient reasons for considering these Insects to be "ancient" or "ancestral." The vague general resemblance of Campodea to many young Insects of very different kinds is clearly the correlative of its simple form, and is no more proof of actual ancestry to them than their resemblances inter se are proofs of ancestry to one another. But even if deprived of its claim to antiquity and to ancestral honours, it must be admitted that Campodea is an interesting creature. In its structure one of the most fragile of organisms, with a very feeble respiratory system, inadequate organs of sense, only one pair of ovarian tubes, very imperfect mouth-organs, and a simple alimentary canal, it nevertheless flourishes while highly-endowed Insects become extinct. In the suburban gardens of London, on the shores of the Mediterranean, on the summits of the higher Pyrenees, in North America even it is said in the caves of Kentucky, and in India, Campodea is at home, and will probably always be with us.

CHAPTER VIII

ORTHOPTERA—FORFICULIDAE, EARWIGS—HEMIMERIDAE

Order II.—Orthoptera.

Insects with the mouth parts conspicuous, formed for biting, the four palpi very distinct, the lower lip longitudinally divided in the middle. The tegmina (mesothoracic wings), of parchment-like consistence, in repose closed on the back of the Insect so as to protect it. The metathoracic wings, of more delicate consistence, ample, furnished with radiating or divergent nervures starting from the point of articulation, and with short cross nervules forming a sort of network; in repose collapsing like a fan, and more or less completely covered by the tegmina (except in certain Phasmidae, where, though the wings are ample, the tegmina are minute, so that the wings are uncovered). In a few forms (winged Forficulidae and some Blattidae) the metathoracic wings are, in addition to the longitudinal folding, contracted by means of one or two transverse folds. The mode of growth of each individual is a gradual increase of size, without any abrupt change of form, except that the wings are only fully developed in the final condition. There is no special pupal instar. Species in which the wings are absent or rudimentary are numerous.

The Orthoptera are Insects of comparatively large size. The Order, indeed, includes the largest of existing Insects, while none are so minute as many of the members of the other Orders are; three millimetres is the least length known for an Orthopterous Insect, and there are very few so small, though this is ten times the length of the smallest beetle. The Order includes earwigs, cockroaches, soothsayers or praying-insects, stick- and leaf-insects, grasshoppers, locusts, green grasshoppers, and crickets.

The changes of form that accompany the growth of the individual are much less abrupt and conspicuous than they are in most other Insects. The metamorphosis is therefore called Paurometabolous. It has been supposed by some naturalists that Orthoptera go through a larger portion of their development in the egg than other Insects do. This does not clearly appear to be the case, though it seems that there are distinctions of a general character in the embryology; the period of development in the egg is prolonged, and the yolk is said by Wheeler[^[126]] to be more than usually abundant in comparison with the size of the young embryo. The embryonic development may in tropical countries be accomplished in three weeks (see Mantidae), but in countries where winter supervenes, the period may in some species be extended over seven or eight months.

The external features of the post-embryonic development—a term that is more convenient in connexion with Orthoptera than metamorphosis—are as follows: the wings are never present when the Insect is first hatched, but appear subsequently, and increase in size at the moults; the form and proportions of the segments of the body—especially of the thorax—undergo much change; an alteration of colour occurs at some of the moults, and the integument becomes harder in the adult condition. Neither the development of the internal organs, nor the physiological processes by which the changes of external form are effected, appear to have been studied to any great extent.

Many of the Orthoptera do not possess wings fit for flight, and some species even in the adult state have no trace whatever of such organs. Flight, indeed, appears to be of minor importance in the Order; in many cases where the wings exist they are purely musical organs, and are not of any use for flight. The apterous and the flightless conditions are not confined to one division of the Order, but are found in all the families and in many of their subdivisions. As the front pair of wings in Orthoptera do not really carry out the function of flight, and as they differ in several particulars from the hinder pair, or true wings, it is usual to call them tegmina. The musical powers of the Orthoptera are confined to the saltatorial group of families.

Fig. 101.—Poecilimon affinis ♂. Bulgaria. Alar organs serving only as musical organs. The ear on front tibia and aural orifice of prothorax are well shown.

The Cursoria are dumb or nearly so; it is a remarkable fact that also in this latter division the alar organs, though frequently present, have but little value for flight, and are in some cases devoted to what we may call purposes of ornament or concealment. This is specially the case in the Phasmidae and Mantidae, where the effectiveness of colour and pattern of these parts becomes truly astonishing. The tegmina frequently exhibit an extraordinary resemblance to vegetable structures, and this appearance is not superficial, for it may be seen that the nervures of the wings in their disposition and appearance resemble almost exactly the ribs of leaves. One of the most remarkable of the features of Orthoptera is that a great difference frequently exists between the colours of the tegmina and of the wings, i.e. the front and hind wings; the latter are concealed in the condition of repose, but when activity is entered on and they are displayed, the individual becomes in appearance a totally different creature. In some cases, contrary to what usually occurs in Insects, it is the female that is most remarkable; the male in Mantidae and Phasmidae being frequently a creature of quite inferior appearance and power in comparison with his consort. The musical powers of the saltatorial Orthoptera are, however, specially characteristic of the male sex. There is evidence that these powers are of great importance to the creatures, though in what way is far from clear. Some parts of the structures of the body are in many of these musical species clearly dominated by the musical organs, and are apparently specially directed to securing their efficiency. We find in some Locustidae that the tegmina are nothing but sound-producing instruments, while the pronotum is prolonged to form a hood that protects them without encumbering their action. In the males of the Pneumorides, where the phonetic organ is situated on the abdomen, this part of the body is inflated and tense, no doubt with the result of increasing the volume and quality of the sound. In the genus Methone (Fig. 185) we find a grasshopper whose great hind legs have no saltatorial function, and but little power of locomotion, but act as parts of a sound-producing instrument, and as agents for protecting some parts of the body in repose. Further particulars of these cases must be looked for in our accounts of the different groups.

The eggs of many Orthoptera are deposited in capsules or cases; these capsules may contain only one egg, or a great many.

The Order includes many species of Insects, though in Britain it is poorly represented: we have only about forty species, and this small number includes some that are naturalised. Only a few of the forty extend their range to Scotland. A revision of the species found in Britain has recently been made by Mr. Eland Shaw.[^[127]] In continental Europe, especially in the south, the species become more numerous; about 500 are known as inhabitants of geographical Europe. In countries where the face of nature has been less transformed by the operations of man, and especially in the tropical parts of the world, Orthoptera are much more abundant.

The lowest number at which the species now existing on the surface of the earth can be estimated is 10,000. This, however, is probably far under the mark, for the smaller and more obscure species of Orthoptera have never been thoroughly collected in any tropical continental region, while new forms of even the largest size are still frequently discovered in the tropics.

We shall treat the Order as composed of eight families:—

Series, Cursoria: hind legs but little different from the others.		1. Forficulidae—Tegmina short, wings complexly folded; body armed at the extremity with strong forceps.
		2. Hemimeridae—Apterous: head exserted, constricted behind.
		3. Blattidae—Coxae of the legs large, exserted, protecting the lower part of the body.
		4. Mantidae—Front legs very large, raptorial, armed with spines.
		5. Phasmidae—Mesothorax large as compared with the prothorax.
Series, Saltatoria: hind legs elongate, formed for leaping, their femora usually thickened.		6. Acridiidae—Antennae short, not setaceous, of not more than 30 joints, tarsi three-jointed.
		7. Locustidae—Antennae very long, setaceous, composed of a large number of joints, tarsi four-jointed.
		8. Gryllidae—Antennae very long, setaceous, tarsi two- or three-jointed.

The first five of these subdivisions are amongst the most distinct of any that exist in the Insecta, there being no connecting links between them. The three groups forming the Saltatoria are much more intimately allied, and should, taken together, probably have only the same taxonomic value as any one of the other five groups.

Owing partly to the inherent difficulties of the subject, and partly to the fragmentary manner in which it has been treated by systematists, it has been impossible till recently to form any clear idea of the classification of Orthoptera. During the last twenty years Henri de Saussure and Brunner von Wattenwyl have greatly elucidated this subject. The latter of these two distinguished naturalists has recently published[^[128]] a revision of the system of Orthoptera, which will be of great assistance to those who may wish to study these Insects. We therefore reproduce from it the characters of the tribes, placing the portion relating to each family at the end of our sketch thereof.

Fam. I. Forficulidae—Earwigs.

(DERMAPTERA OR DERMATOPTERA OF BRAUER AND OTHERS)

Insects of elongate form, with an imbricate arrangement of the segments of the body; bearing at the posterior extremity a pair of callipers or more distorted instruments. The hind wings (when present) folded in a complex manner, and covered, except at their tips, by a pair of short wing-covers (tegmina), of a leather-like consistence. Wingless forms are very numerous. The young is very similar to the adult.

Fig. 102.—Pygidicrana hugeli. Java.

Although earwigs are said to be rare in most parts of the world, yet in Europe no Insect is better known than Forficula auricularia, the common earwig, it being very abundant even in gardens and cultivated places. In certain seasons it not unfrequently enters our houses, in which case it too often falls a victim to prejudices that have very little to justify them. This Insect is a good type of the winged earwigs. In the parts of the mouth it exhibits the structures usual in the Orthoptera; there is a large labrum, a pair of maxillae, each provided with two lobes and a palpus consisting of two very short basal joints and three longer joints beyond these; the mandibles are strong, with curvate pointed extremities; in the lower lip there is a ligula exposed in front of a very large mentum; it consists of two pieces, not joined together along the middle, but each bearing on its lateral edge a palpus with two elongate joints and a short basal one; this lip is completed by the lingua, which reposes on the upper face of the part, and completely overlaps and protects the chink left by the want of union along the middle line of the external parts of the lip. The antennae are elongate, filiform, and are borne very near the front of the exserted head. There are rather large facetted eyes, but no ocelli. The three segments of the thorax are distinct, the prothorax being quite free and capable of movement independent of the parts behind it: the meso- and meta-nota are covered by the tegmina and wings; these latter project slightly from underneath the former in the shape of small slips, that are often of rather lighter colour; the wing-covers are short, not extending beyond the insertion of the hind legs, and repose flat on the back, meeting together in a straight line along the middle. These peculiar flat, abbreviated wing-covers, with small slips (which are portions of the folded wings) projecting a little from underneath them, are distinctive marks of the winged Forficulidae.

The legs are inserted far from one another, the coxae being small; each sternum of the three thoracic segments projects backwards, forming a peculiar long, free fold, underlapping the front part of the following segment. The hind body or abdomen is elongate, and is formed of ten segments; the number readily visible being two less in the female than it is in the male. The segments are fitted together by a complex imbrication, which admits of great mobility and distension, while offering a remarkable power of resistance to external pressure: each segment is inserted far forward in the interior of that preceding it, and each also consists of separate upper and lower plates that much overlap where they meet at the sides (see Fig. 103). The body is always terminated by a pair of horny, pincer-like processes, which are differently shaped according to the sex of the individual.