LITERATURE ON THE ORGANS OF TASTE
Meinert, F. Bidrag til de danske myrers naturhistorie. (Kgl. Dansk Vidensk. selsk. skrifter. Kjoebenhavn. Raekke 5 Naturvid. og math. Afd., v, 1861, pp. 273–340.)
Wolff, O. J. B. Das Riechorgan der Biene. (Nova acta d. K. Leop.-Carol. Akad., xxxviii, 1875, pp. 1–251, 8 Taf.)
Joseph, G. Zur Morphologie des Geschmacksorganes bei Insekten. (Amtlicher Bericht der 50. Versammlung deutscher Naturforscher und Aerzte in München, 1877, pp. 227–228.)
Künckel et Gazagnaire. Du siège de la gustation chez les insectes Diptères. Constitution anatomique et physiologique de l’epipharynx et l’hypopharynx. (Comptes-rend. Acad. Sc., Paris, 1881, xcv, pp. 347–350.)
—— Recherches sur l’organisation et le développement des Diptères et en particulaire des Volucelles, i, 1875, ii, 1881, 26 Pls.
Kraepelin, K. Zur Kenntniss der Anatomie und Physiologie des Rüssels von Musca. (Zeitschr. f. wissens. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)
Kirbach, P. Ueber die Mundwerkzeuge der Schmetterlinge. (Zool. Anzeiger, 1883, pp. 553–558, 2 Figs.)
Will, F. Das Geschmacksorgan der Insekten. (Zeitschr. f. wissens. Zool., 1885, xlii, pp. 674–707, 1 Taf.)
Gazagnaire, J. Du siège de la gustation chez les insectes Coléoptères. (Comptes-rend. Acad. Sc., Paris, 1886, cii, pp. 629–632; Ann. Soc. Ent. France, Sér. 6, Bull., pp. 79–80.)
Forel, A. Expériences et remarques critiques sur les sensations des insectes. 2me Part. (Recueil Zool. Suisse, iv, 1887, pp. 161–240.)
Reuter, Enzio. Ueber den “Basalfleck” auf den Palpen der Schmetterlinge. (Zool. Anzeiger, 1888, pp. 500–503.)
—— Ueber die Palpen der Rhopaloceren, etc., 6 Taf. Helsingfors, 1896, pp. 1–578. (Acta Soc. Sc. Fennicæ, xxii, 1896.)
Packard, A. S. On the occurrence of organs, probably of taste, in the epipharynx of the Mecoptera (Panorpa and Boreus). (Psyche, 1889, v, pp. 159–164.)
—— Notes on the epipharynx, and the epipharyngeal organs of taste in mandibulate insects. (Psyche, 1889, v, pp. 193–199, 222–228.)
Also Lubbock’s Senses, etc., of animals, and the writings of Briant, Breithaupt (titles on p. 85).
d. The organs of hearing
Although it has been denied by Forel that insects have the sense of hearing, yet the majority of writers and experimenters agree that insects are not deaf. On general grounds if, as we know, many insects produce sounds, it must follow that they have ears to hear, for there is every reason to suppose that the sounds thus made are, as in other animals, either for attracting the sexes, for a means of communication, or to express the emotions. We will begin by briefly describing the structures now generally supposed to be auditory in function, and about which there can be no reasonable doubt, and then consider the more problematical organs, closing with an account of the extremely various means of producing sounds and cries.
The ears or tympanal and chordotonal sense-organs of Orthoptera and other insects.—The ears or tympana of locusts (Acrydiidæ) are situated one on each side, on the basal joint of the abdomen, just behind the first abdominal spiracle. That this is a true ear was first suggested by J. Müller, and his opinion was confirmed by Siebold, Leydig, Hensen, Graber, Schmidt, Lubbock, etc.[[49]]
Fig. 290.—Ear of a locust (Caloptenus italicus), seen from the inner side: T, tympanum; TR, its border; o, u, two horn-like processes; bi, pear-shaped vesicle; n, auditory nerve; ga, terminal ganglion; st, stigma; m, opening, and m′, closing, muscle of the same; M, tensor muscle of the tympanum membrane.—After Graber.
The apparatus consists of a tense membrane, the tympanum, surrounded by a horny ring (Fig. 290). “On the internal surface of this membrane are two horn-like processes (o, u), to which is attached an extremely delicate vesicle (bi) filled with a transparent fluid, and representing a membranous labyrinth. This vesicle is in connection with an auditory nerve (n) which arises from the third thoracic ganglion, forms a ganglion (ga) upon the tympanum, and terminates in the immediate neighborhood of the labyrinth by a collection of cuneiform, staff-like bodies, with very finely pointed extremities (primitive nerve-fibres?), which are surrounded by loosely aggregated ganglionic globules” (Siebold’s Anatomy of the Invertebrates).
Fig. 291.—Fore tibia of Locusta viridissima. td, cover of the drum; tr, fissure between the drum and its cover.—After Graber, from Lang.
In the green grasshoppers, katydids, and their allies, the ears are situated on the fore tibiæ, where these organs can be found after a careful search (Figs. 291, 292).
The presence of the structure is indicated by the oval disc, the drum, which is a thin tense membrane covering the auditory apparatus of nerves, ganglion cells, and auditory rods beneath.
The tympana, or drums, are not present in all Locustidæ and Gryllidæ, and, as Lubbock states, it is an additional reason for regarding them as auditory organs, that in those species which possess no stridulating organs the tympana are also wanting. In many of the Locustidæ the tympana are covered or protected by a fold of the skin projecting over them. These covered ones are, Graber thinks, derived from the open ones.
Fig. 292.—A, fore tibia of a European grasshopper (Meconema), containing the ear: Ty, tympanum or outer membrane; Tr 1, Tr 2, tracheæ. B, diagrammatic cross-section through the tibia and ear of the same; Ty, tympanum; Ct, cuticula; CM, hypodermis: A, the auditory organ connecting with the tympanum; B, supra-tympanal auditory organ; GZ, the ganglion-cell belonging to them; Hst, the auditory rod connecting with the ganglion-cells.—After Graber, from Judeich and Nitsche.
On examining the apparatus within the leg under the drum, it is seen to consist of the trachea, the auditory vesicles and rods, ganglion cells, and acoustic nerve. The trachea is greatly modified (Fig. 292, Tr 1). On passing into the tibia the trachea enlarges and divides into two branches, which reunite lower down. The spiracles supplying the air to this enlarged trachea are considerably enlarged, while in the dumb species it is of the normal size. The enlarged trachea passes close to the tympanum, which thus has air on both sides of it: the open air on the outer, the air of the trachea on its inner surface. In fact, as Lubbock states, “the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit the atmospheric vibrations.”
Fig. 293.—The auditory apparatus in the tibia of a grasshopper, showing the tympanal nerve-endings in situ: EBI, terminal vesicles of Siebold’s organ; SN, nerve of the organ of Siebold; Gr, group of vesicles of same; SO, nerve-endings of the same; vT, front tympanum; vTr, front branch of the trachea; hT, hinder tympanum; hTr, hinder branch of the trachea; Sp, space between the tracheæ; go, supra tympanal ganglion; rN, connecting nerve-fibrils between the ganglion cells and the terminal vesicles; R, upper, n-S, lower, root of the transparent covering membrane. (Other lettering not explained by author.)—After Graber.
Fig. 294.—Auditory rod of Gryllus viridissimus: fd, auditory rod; ko, terminal piece.—After Graber, from Lubbock.
These tracheæ, says Graber, though formed on a similar plan, present many variations, corresponding to those of the tympana, and showing that the tympana and the tracheæ stand in intimate connection with one another. For instance, in those species where the tympana are equal, the tracheæ are so likewise; in Gryllotalpa, where the front tympanum only is developed, though both tracheal branches are present, the front one is much larger than the other; and where there is no tympanum, the trachea remains comparatively small, and even in some cases undivided (Lubbock, ex Graber).
The acoustic nerve, which next to the optic is the thickest in the body, divides soon after entering the tibia into two branches, one almost immediately forming a ganglion, the supra-tympanal ganglion, the other passing down to the tympanum, where it expands into an elongated flat ganglion, the organ of Siebold (Fig. 293), and closely applied to the anterior tracheæ.
At the upper part of the ganglion is a group terminating below in a single row of vesicles, the first few of which are approximately equal, but which subsequently diminish regularly in size. Each of these vesicles is connected with the nerve by a fibril (Fig. 293, vN), and contains an auditory rod (Fig. 294). They are said by Graber to be brightly refractive, hollow (thus differing from the retinal rods, which are solid), and terminate in a separate end-piece (ko). The rods were first discovered by Siebold, and, as Lubbock remarks, may be regarded as specially characteristic of the acoustic organs of insects.
Fig. 295.—Chordotonal organ in nymph of a white ant.—After Müller, from Sharp.
Fig. 296.—Right half of 8th body-segment of Corethra plumicornis: g, ganglion of ventral cord; lm, longitudinal muscle; cn, chordotonal nerve; cl, chordotonal ligament; cg, chordotonal ganglion; cs, rod of chordotonal organ; cst, terminal cord; tb, tactile setæ; hn, out-going fibres of the integumental nerves.—After Graber, from Lang.
As will be seen in Fig. 293, at the upper part of the tibial organ of Ephippigera there is a group of cells, and below them a single row of cells gradually diminishing in size from above downwards. “One cannot but ask oneself,” says Lubbock, “whether the gradually diminishing size of the cells in the organ of Siebold may not have reference to the perception of different notes, as is the case with the series of diminishing arches in the organ of Corti of our own ears.”
These organs were supposed to be restricted to the Orthoptera, but in 1877 Lubbock discovered what seems to resemble the supra-tympanal auditory organ of Orthoptera in the tibia of the yellow ant (Lasius flavus). Graber confirmed Lubbock’s account, and also discovered these organs in the tibia of a Perlid (Isopteryx apicalis), and Fritz Müller has detected them in the fore tibiæ of the nymph of Calotermes rugosus (Fig. 295). To these structures Graber gave the name of chordotonal organs.
He has also detected these organs in all the legs of other insects (Trichoptera, Pediculidæ), and auditory rods have been discovered in the antennæ of Dyticus and of Telephorus by Hicks, Leydig, and Graber. Graber classifies the chordotonal organs into truncal and membral. In Coleoptera and Trichoptera they may occur on several joints of the leg; others are more localized,—thus he distinguishes femoral (Pediculidæ), tibial (Orthoptera, Perlidæ, Formicidæ), and tarsal organs (Coleoptera).
A type of chordotonal organ, observed in the body-segments of the larvæ of several insects by Leydig, Weismann, Graber, Grobben, and Bolles Lee, is to be seen in the transparent larva of Corethra (Fig. 296), where the auditory organ extends to the skin. It contains at the point cs two or three auditory rods. In the opposite direction a fine ligament (cl) passes from cg to the skin; in this way the auditory organ is suspended in a certain state of tension, and is favorably situated to receive even very fine vibrations. A similar apparatus has been detected in the larva of Ptychoptera.
Antennal auditory hairs.—It is not at all improbable that the antennæ of different insects contain auditory as well as olfactory structures. Lubbock has suggested that the singular organs which have only been found in the antennæ of ants and certain bees, and to which he gives the name of “Hicks’ bottles” (Fig. 281), may act as microscopic stethoscopes, while Leydig also regards them as chordotonal organs.
That, however, some of the antennal hairs of the mosquito, as first suggested by Johnson and afterwards proved experimentally by Mayer, are auditory, seems well established. Fastening a male mosquito down on a glass slide, Mayer then sounded a series of tuning-forks. With an Ut4 fork of 512 vibrations per second, some of the hairs were seen to vibrate vigorously, while others remained comparatively at rest. The lower (Ut3) and higher (Ut5) harmonics of Ut4 also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano.
Mayer then made large wooden models of these hairs, the one corresponding to the Ut3 hair being about a metre in length, and on counting the number of vibrations they made when they were clamped at one end and then drawn on one side, he found that it “coincided with the ratio existing between the numbers of vibrations of the forks to which covibrated the fibrils,” or hairs. It should be observed that the song of the female mosquito corresponds nearly to this note, and would consequently set the hairs in vibration. Mayer observed that the song of the female vibrates the hairs of one of the antennæ more forcibly than those of the other. Those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence from the position of the antennæ and the hairs a sound will be loudest or most intense if it is directly in front of the head. If, then, the song of the female affects one antenna more than another, the male turns his head until the two antennæ are equally affected, and is thus able to fly straight towards the female. From his experiments Mayer found that the male can thus guide himself to within 5° of the direction of the female. Hence he concludes that “these insects must have the faculty of the perception of the direction of sound more highly developed than in any other class of animals.” (Also see Child’s work.)
Special sense-organs in the wings and halteres.—Organs of a special sense, which Hicks supposed to be those of smell, were found by him near or at the base of the wings of Diptera, Coleoptera, and less perfect ones in Lepidoptera, Neuroptera, and Orthoptera, with a trace of them in Hemiptera; but these were considered by Leydig to be auditory organs, since he found the nerves to end in club-shaped rods, like those of Orthoptera.
Hicks found, as to the halteres and their sense-organs, that the nerve in the halter is the largest in the insect, except the optic nerve; and that at the base of the halteres is a number of vesicles arranged in four groups, to each of which the nerve sends a branch. Afterwards Bolles Lee discovered that the vesicles, undoubtedly perforated, contain a minute hair, those of the upper groups being protected by hoods of chitin. He regarded them as olfactory organs, while Lubbock seems inclined to consider them as auditory structures. Graber also regards the vesicles of Hicks as chordotonal organs.
In his elaborate account of the balancers, Weinland concludes that the organs of sense of varying structure occurring at the base of these appendages allow the perception of movements which the halteres perform and which enable the fly to steer or direct its course. The halteres can thus cause differences in the direction of the flight of a fly in the vertical plane. If the balancers act unequally, there is a change in direction.
e. The sounds of insects
Insects have no true voice; but sounds of different intensity, shrill cries, and other noises are produced mechanically by insects, either being love-songs to attract the sexes, to give signals, to communicate intelligence, or perhaps to express the emotions. The loud, shrill cry of the Cicada, or chirp of the cricket, is evidently a love-call, and results in the mating of individuals of separate broods more or less widely scattered, thus preventing too close interbreeding.
The simplest means of making a noise is that of the death-watch (Anobium), which strikes or taps on the wall with its head or abdomen. Longicorn beetles make a sharp sound by the friction of the mesoscutellum against the edge of the prothoracic cavity, the head being alternately raised and lowered, Burying-beetles (Necrophorus) rub the abdomen against the hinder edges of the elytra. Weevils make a loud noise by rapidly rubbing the tips of the abdomen on the ends of the elytra.
Landois offers the following summary of the kinds of noises produced by beetles:
1. Tapping sounds (Bostrycinæ, Anobium). 2. Grating sounds (Elateridæ).
Mutilla makes a rather sharp noise by rubbing one abdominal segment against another. Ants (Ponera) have a stridulating apparatus, and other genera numerous (20) ridges between the segments.
Even certain moths and butterflies emit a rasping or crackling noise. The death’s-head moth and other sphinges cause it by rubbing the palpi against the base of the proboscis. These and certain butterflies are provided with parallel ridges forming a rasp on the “basal spot” of the inner side of the basal joint of each palpus (Reuter). A South American butterfly (Ageronia feronia) can be heard for several yards as it flies with a crackling sound. Hampson finds that the cause of the clicking sound is due to a pair of strong chitinous hooks attached to the thorax, against which play the spatulate ends of a pair of hooks attached to the fore wings. An Australian moth (Hecatesia) flies with a whizzing sound; Vanessa is said to be sonorous.
The males of Orthoptera produce their shrill cries or chirping noises, 1, by rubbing the thighs against the sides of the body (Acrydiidæ); 2, by the friction of the base of the fore wings on each other (Locustidæ); 3, by rubbing the base of the upper on the base of the hinder or under pair (Gryllidæ), in the two last there being a shrilling apparatus consisting of a file on the hind wings, which rubs on a resonant surface on the fore wings. The females are not invariably dumb, both sexes of the European Ephippigera being able to faintly stridulate. Corixa also produces shrill chirping notes. (Carpenter.)
Certain insects also hum, and have what may perhaps be called a voice. The cockchafer, besides humming with the wings, produces a sound almost like a voice. In the large trachea, just behind each spiracle, is a chitinous process, which is thrown into vibrations by the air during respiration, and thus produces a humming noise. (Lubbock.) Such is also the case with flies, the mosquito, dragon-flies, and bees. In flies and dragon-flies the “voice” is caused by the air issuing from the thoracic spiracles; while in the humble-bee the abdominal spiracles are also musical. The sound made by the spiracles bears no relation to that caused by the wings. Landois tells us that the wing-tone of the honey-bee is A′; its voice, however, is an octave higher, and often goes to B″ and C″.
The sounds produced by the wings are constant in each species, except where, as in Bombus, there are individuals of different sizes; in these the larger ones generally give a higher note. Thus the comparatively small male of Bombus terrestris hums on A′, while the large female hums an entire octave higher.
From the note produced the rapidity of the vibrations can be calculated. For example, the house-fly, which produces the sound of F, vibrates its wings 21,120 times in a minute, or 335 times in a second; and the bee, which makes a sound of A′, as many as 26,400 times, or 440 times in a second. On the contrary, a tired bee hums on E′, and therefore, according to theory, vibrates its wings only 330 times in a second. Marey has confirmed these numbers graphically, and found by experiment that the fly actually makes 330 strokes in a second. (Lubbock.)
A different kind of musical apparatus is that of the cicada, which has been elaborately described by Graber. The shrill, piercing notes issue from a pair of organs on the under side of the base of the abdomen of the male, these acting somewhat as two kettle-drums, the membrane covering the depressions being rapidly vibrated.