THE PUPA STATE
The word pupa is from the Latin meaning baby. Linnæus gave it this name from its resemblance to a baby which has been swathed or bound up, as is still the custom in Southern Europe. The term pupa should be restricted to the resting inactive stage of the holometabolous insects.
Lamarck’s term chrysalis was applied to the complete or obtected pupa of Lepidoptera and of certain Diptera, and mumia, a mummy, to the pupæ of Coleoptera, Trichoptera, and most Hymenoptera. Latreille (1830) also restricted the term pupa to the “oviform nymph,” or puparium, of Diptera. Brauer applies the term nymph to the pupa of metabolous insects.
Fig. 578.—Pupa obtecta: a, of Sesia, with its cocoon-cutter on the head; b, of Tortrix vacciniivorana.
The typical pupa is that of a moth or butterfly, popularly called a chrysalis. A lepidopterous pupa in which the appendages are more or less folded close to the body and soldered to the integument, was called by Linnæus a pupa obtecta; and when the limbs are free, as in Neuroptera, Mecoptera, Trichoptera, and the lepidopterous genus Micropteryx it is called a pupa libera (Fig. 579). When the pupa is enclosed in the old larval skin, which forms a pupal covering (puparium), the pupa was said by Linnæus to be coarctate. The pupa of certain Diptera, as that of the orthoraphous families, is nearly as much obtected as that of the tineoid families of moths, especially as regards the appendages of the head; the legs being more as in pupæ liberæ (Fig. 580).
Fig. 579.—Pupa libera of neuropterous insects a, Corydalus cornutus; b, Sialis; c, Hemerobius.
The male Coccid anticipates the metabolous insects in passing through a quiescent state, when, as Westwood states, it is “covered by the skin of the larva, or by an additional pellicle.” The body appears to be broad and flat, the antennæ and fore legs resting under the head, while the two hinder pairs of legs are appressed to the under side of the body. There is but a slight approach to the pupa libera of a metabolous insect.
Riley states that the male larva of Icerya purchasi forms a cocoon waxy in character, but lighter, more flossy, and less adhesive than that of the female egg-cocoon. It melts and disappears when heated, proving its entirely waxy nature. When the mass has reached the proper length, the larva casts its skin, which remains in the hind end of the cocoon, and pushes itself forward into the middle of the cocoon. The pupa (Fig. 581) is of the same general form and size as the larva. All the limbs are free and slightly movable, so that they vary in position, though ordinarily the antennæ are pressed close to the side, as are the wing-pads; the front pair of legs are extended forward. “If disturbed, they twist and bend their bodies quite vigorously.” The pupa state lasts two or three weeks. A similar pupa is that of Icerya rosæ. (Riley and Howard.)
Fig. 580.—Pupa obtecta of Diptera: a, Ptychoptera; b, Tabanus atratus; c, Proctacanthus philadelphicus; d, Midas clavatus.
Fig. 581.—Pupa libera of Icerya purchasi, ventral view.—After Riley, Insect Life.
The metamorphosis of Aspidiotus perniciosus is of interest. The male nymph differs much after the first moult from the female, having large purple eyes, while the female nymph loses its eyes entirely. It passes into what Riley terms the pro-pupa (Fig. 582, b), in which the wing-pads are present, while the limbs are short and thick. The next stage is the “true pupa” (Fig. 582, c, d), in which the antennæ and legs are much longer than before. There is no waxy cocoon, but only a case or scale composed of the shed larval skin, i.e. “with the first moult the shed larval skin is retained beneath the scale, as in the case of the female; with the later moultings the shed skins are pushed out from beneath the scale,” and when they transform into the imago they “back out from the rear end of their scale.”
Fig. 582.—Aspidiotus perniciosus, development of male insect: a, ventral view of larva after first moult; b, the same, after second moult (pro-pupa stage); c and d, true pupa, ventral and dorsal views. All greatly enlarged.—After Riley.
The pupæ of Coleoptera and of Hymenoptera, though there is, apparently, no near relationship between these two orders, are much alike in shape, and, as Chapman pertinently suggests, those of both orders are helpless from their quiescence, and hence have resorted for protection to some cocoon or cell.
But it is quite otherwise with the pupæ of Lepidoptera and Diptera, which vary so much in adaptation to their surroundings, and hence afford important taxonomical and phylogenetic characters. This, as regards the Lepidoptera, was almost wholly overlooked until Chapman called attention to the subject, and showed that the pupæ had characters of their own, of the greatest service in working out the classification, and hence the phylogeny, of the different lepidopterous groups. We have, following the lead of Chapman, found the most striking confirmation of his views, and applied our present knowledge of pupal structures to dividing the haustellate Lepidoptera into two groups,—Paleolepidoptera and Neolepidoptera.
The pupæ of the Neuroptera, Coleoptera, and Hymenoptera differ structurally from the imago, in the parts of the head and thorax being less differentiated. Thus in the head the limits or sutures between the epicranium and clypeus, and the occiput and gula, are obscurely marked, while the tergal and pleural sclerites of the imago are not well differentiated until the changes occurring just before the final ecdysis.
It is easy, however, to homologize the appendages of the pupæ with those of the imago of all the holometabolous orders except in the case of the obtected pupa of the Lepidoptera (and probably of the obtected dipterous pupæ), where the cephalic appendages are soldered together.
That the appendages of the lepidopterous pupa are, as generally supposed, merely cases for those of the imago has been shown by Poulton to be quite erroneous. He says: “If we examine a section of a pupal antenna or leg (in Lepidoptera), we shall find that there is no trace of the corresponding imaginal organ until shortly before the emergence of the imago. In the numerous species with a long pupal period, the formation of imaginal appendages within those of the pupa is deferred until very late, and then takes place rapidly in the lapse of a few weeks. This also strengthens the conclusion that such pupal appendages are not mere cases for the parts of the imago, inasmuch as these latter are only contained within them for a very small proportion of the whole pupal period.” On the other hand, Miall and Hammond claim that there is a strong superficial contrast as to the formation of the imaginal organs, between Lepidoptera and tipularian Diptera, the appendages, wings, and compound eyes being substantially those of the imago. “With the exception of the prothoracic respiratory appendages and the tail-fin, there is little in the pupa of Chironomus which does not relate to the next stage.”
The exact homology of the “glazed eye” of the lepidopterous pupæ and of the parts under the head, situated over the maxillæ, is difficult to decide upon, and these points need farther examination. In the dipterous pupa it is interesting to observe that the halteres are large and broad, which plainly indicates that they are modified hind wings. The number and arrangement of the spiracles is different in pupæ from those of the larva and imago.
Fig. 583.—Simulium piscicidium: a, larva; b, c, d, pupa; e, thoracic leg; f, row of bristles at end of body. A, S. pecuarum, pupa; a, b, c, adminicula.—After Riley.
There are also secondary adaptive structures peculiar to the pupa, which are present and only of use in this stage. These are the thoracic, spiracular, or breathing appendages of the aquatic Diptera (Fig. 583), the various spines situated on the head or thorax, or on the sides, or more often at the end of the abdomen, besides also the little spines arranged in more or less circular rows around the abdominal segments, the cocoon-breaker, and the cremaster of many pupæ.
In the pupa of certain Diptera, there is a terminal cremaster-like spine, as in that of Tipula eluta (Fig. 584), Tabanus lineola (Fig. 585), besides adminicula or locomotive spines like those of lepidopterous pupæ (Fig. 580, a, b, c).
Fig. 584.—Pupa of Tipula eluta.
Fig. 585.—Pupa of Tabanus lineola.—This and Fig. 584 after Hart.
Fig. 586.—Pupa of Galerita lecontei, and of Adelops hirtus (a, b, c).—After Hubbard.
The pupæ of Coleoptera are variously spined or hairy (Fig. 586). Those of Hydrophilus and of Hydrobius are provided with stout spines on the prothorax and abdomen which support the body in its cells, so that, as Lyonet first showed, though surrounded on all sides by moist earth, it is kept from contact with it by the pupal spines; other pupæ of beetles, such as that of the plum weevil, which is also subterranean, possess similar spines. The abdomen of many coleopterous pupæ, such as those of Carabidæ, end in two spines, to aid them in escaping from their cells in wood or in the earth; others have stiff bristles, and others spines along each side of the abdomen (Fig. 586). All these structures are the result of a certain amount of activity in what we call quiescent pupæ, but most of these are for use at the end of pupal life, at the critical moment when by their aid the insect escapes from its cocoon or subterranean cell, or if parasitic, bores out of its host.
If we are to account for the causes of their origin, we are obliged to infer that they are temporary deciduous structures due to the need of support while the body is subjected to unusual strains and stresses in working its way out of its prison in the earth, or its cell within the stems and trunks of plants and similar situations. They are pupal inheritances or heirlooms, and well illustrate the inheritance of characters acquired during a certain definite, usually brief, period of life, and transmitted by the action of synchronous heredity.
The pupæ of certain insects are quite active, thus that of Raphidia, unlike that of Sialis, before its final ecdysis regains its activity and is able to run about. (Sharp, p. 448.)
a. The pupa considered in reference to its adaptation to its surroundings and its relation to phylogeny
The form of the pupa is a very variable one, as even in Lepidoptera it is not entirely easy to draw the line between a pupa libera and a pupa obtecta (Fig. 578); and though the period is one of inactivity, yet when they are not in cocoons or in the earth in subterranean cells, their form is more or less variable and adapted to changes in their surroundings. Even in the obtected pupa of butterflies, there is, as every one knows, considerable variability of shape and of armature, which seems to be in direct adaptability to the nature of their environment. Scudder has well shown that in certain chrysalids, such as those of the Nymphalidæ, which are variously tuberculated, and hang suspended by the tail, and often hibernate, these projections serve to protect the body. All chrysalids with projections or ridges on different parts of the body, being otherwise unprotected, move freely when struck by gusts of wind, hence “the greater the danger to the chrysalis from surrounding objects, the greater its protection by horny tubercles and roughened callous ridges.” The greater the protection possessed in other ways, as by firm swathing or a safe retreat, the smoother the surface of the body and the more regular and rounded its contours. The tendency to protection by tubercles is especially noticeable in certain South American chrysalids of nymphalid butterflies. This response to the stimuli of blows or shocks is also accompanied by a sensitiveness to the stimulus of too strong light.
Previously Scudder[[103]] had made the important suggestion that the smooth crescent-shaped belt of the “glazed eye” or “eyepiece” of chrysalids is, as an external covering of the eye, midway between that of the caterpillar and the perfect insect, and he asks: “May it not be a relic of the past, the external organ of what once was? And are we to look upon this as our hint that the archaic butterfly in its transformations passed through an active pupal stage, like the lowest insect of to-day, when its limbs were unsheathed, its appetite unabated?” etc. Scudder also shows that “the expanded base of the sheath covering the tongue affords protection also to the palpi which lie beneath and beside the tongue.”
All this tends to show the importance of studying the structure of the pupa, in order to ascertain how the pupal structures have been brought about, with the final object of discovering whether the pupæ of the holometabolic insects are not descended from active nymphs, and if so, the probable course of the line of descent.
b. Mode of escape of the pupa from its cocoon
Fig. 587.—Pupa of Micropteryx purpuriella, front view: md, mandibles; mx. p, maxillary palpus, end drawn separately; mx.′ p, labial palpi; lb, labrum.
“In all protected pupæ,” as Chapman says, “the problem has to be faced, how is the imago to free itself from the cocoon or other envelope protecting the pupa.” In the Coleoptera and Hymenoptera the imago becomes perfected within the cocoon or cell, as the case may be, and as Chapman states, “not only throws off the pupal skin within the cocoon, but remains there till its appendages have become fully expanded and completely hardened, and then the mandibles are used to force an outlet of escape,” and he calls attention to the fact that “in many cases, even in some entire families, they are of no use whatever to the imago except in this one particular,” and he cites the Cynipidæ as perhaps the most striking instance of this circumstance.
In those Neuroptera which spin a silken cocoon, e.g. the Hemerobiidæ, the Trichoptera, and in Micropteryx (Fig. 588), the jaws used by the pupa for cutting its way out of the cocoon are even larger in proportion than in the pupa of caddis-flies (Fig. 588), being of extraordinary size.
Fig. 588.—Mandibles (md) of Micropteryx purpuriella, enlarged.—Author del. A, pupal head of a hydropsychid caddis-fly, showing the large mandibles.—After Reaumur, from Miall.
In Myrmeleon the pupa pushes its way half out of the cocoon, and then remains, while the imago ruptures the skin and escapes (Fig. 589, a).
Thus in the Neuroptera and Trichoptera we have already established the more fundamental methods of escape from the cocoon, which we see carried out in various ways in the more generalized or primitive Lepidoptera.
The most primitive method in the Lepidoptera of escaping from the cocoon seems to be that of Micropteryx.
Fig. 589.—Larva of Myrmeleon with (a) its cocoon and cast pupa-skin.
“In this genus,” says Chapman, “though it is nominally the pupa that escapes from the cocoon, it is in reality still the imago, the imago clothed in the effete pupal skin. To rupture the cocoon it uses not its own jaws, but those of the pupal skin, energizing them, however, in some totally different way from ordinary direct muscular action, their movements being the result of the vermicular movements of the pupa, acting probably by fluid pressure on the articular structure of the jaws, by some arrangement not altogether different perhaps from the frontal sac of the higher Diptera. In the Micropteryges the jaws of the pupa not only rupture the cocoon, but appear to be the most active agents in dragging the pupa through the opening in the cocoon and through any superincumbent earth, being merely assisted by the vermicular action of the abdominal segments, and we find in accordance with this circumstance that the pupal envelope is still very thin and delicate, and has little or no hardening or roughness by which to obtain a leverage against the walls of the channel of escape.” (Trans. Ent. Soc. London, 1896, pp. 570, 571.)
Fig. 590.—Pupa of Talæporia: a, cocoon-cutter; with vestiges of four pairs of abdominal legs, and the cremaster.
Some sort of a beak or hard process, more or less developed, according to Chapman, adapted for breaking open the cocoon exists in nearly all the Lepidoptera with incomplete pupæ (pupæ incompletæ), except the limacodid and nepticulid section. “In all these instances the pupa emerges from the cocoon precisely as in the Micropteryges, that is, the moth it really is that emerges, but does so encased in the pupal skin. To achieve this object, it seems to have been found most efficient to have three, four, or five abdominal segments capable of movement, but to have the terminal sections (segments) soldered together.”
This cocoon-breaker, as we may call it, is especially developed in Lithocolletis hamadryadella. As described by Comstock, it forms a toothed crest on the forehead which enables it to pierce or saw through the cocoon.
“Each pupa first sawed through the cocoon near its juncture with the leaf and worked its way through the gap, by means of the minute backward-directed spines upon its back, until it reached the upper cuticle of the leaf. Through this cuticle it sawed in the same way that it did through the cocoon. The hole was in each case just large enough to permit the chrysalis to work its way out, holding it firmly when partly emerged. When half-way out it stopped, and presently the skin split across the back of the neck and down in front along the antennal sheaths, and allowed the moth to emerge.”[[104]]
We have observed and figured the cocoon-breaker in Bucculatrix, Talæporia (Fig. 590, a), Thyridopteryx, and Œceticus, and rough knobs or slight projection answering the purpose in Hepialidæ, Megalopyge, Zeuzera, and in Datana.[[105]] See also the spine on the head of Sesia tipuliformis (Fig. 578).
The imago of the attacine moths cuts or saws through its cocoon by means of a pair of large, stout, black spines (sectores coconis), one on each side of the thorax at the base of the fore wings (Fig. 591), and provided with five or six teeth on the cutting edge (C, D).
Fig. 591.—Cocoon-cutter of the Luna moth: front view of the moth with the shoulders elevated and the rudimentary wings hanging down: s, cocoon-cutter; p, patagium. B, represents another specimen with fully developed wings: ms, scutum; st, scutellum of the mesothoracic segment; s, cocoon-cutter, which is evidently a modification of one of the pieces at the base of the fore wings; it is surrounded by membrane, allowing free movement. C and D, different views of the spine, magnified, showing the five or six irregular teeth on the cutting edge.
Fig. 592.—Larva and pupa of a wood-wasp (Rhopalum), enlarged: h, temporary locomotive tubercles on head of pupa.—Trouvelot del.
Our attention[[106]] was drawn to this subject by a rustling, cutting, and tearing noise issuing from a cocoon of Actias luna. On examination a sharp black point was seen moving to and fro, and then another, until both points had cut a rough irregular slit, through which the shoulder of the moth could be seen vigorously moving from side to side. The hole or slit was made in one or two minutes, and the moth worked its way at once out of the slit. The cocoon was perfectly dry. The cocoon-cutter occurs in all the American genera, in Samia cynthia, and is large and well marked in the European Saturnia pavonia-minor and Endromis versicolora. In Bombyx mori the spines are not well marked, and they are quite different from those in the Attaci. There are three sharp points, being acute angles of the pieces at the base of the wing, and it must be these spines which at times perform the cutting through of the threads of the cocoon described by Réaumur, and which he thought was done by the facets of the eyes. It is well known that in order to guard against the moths cutting the threads, silkraisers expose the cocoon to heat sufficient to destroy the enclosed pupa. In Platysamia the cocoon-cutters, though well developed, do not appear to be used at all, and the pupa, like that of the silkworm and other moths protected by a cocoon, moistens the silk threads by a fluid issuing from the mouth, which also moistens the hairs of the head and thorax, together with the antennæ. It remains to be seen whether these structures are only occasionally used, and whether the emission of the fluid is not the usual and normal means of egress of the moth from its cocoon. Dr. Chapman remarks that throughout the obtected moths “there are many devices for breaking through the cocoon: specially constructed weak places in the cocoon, softening fluid, applied by the moth, assisted by special appliances of diverse sorts, such as in Hybocampa[[107]] and Attacus,” etc.
As to the fluid mentioned above, Trouvelot states that it is secreted during the last few days of the pupa state, and is a dissolvent for the gum so firmly uniting the fibres of the cocoon. “This liquid is composed in great part of bombycic acid.” (Amer. Naturalist, i, p. 33.)
The pupa of the dipterous genus Sciara (S. ocellaris O. S.) resembles a tineid pupa, and before transforming emerges for about two-thirds of its length from the cocoon; the pupa-skin remaining firmly attached in this position.[[108]]
Certain hymenopterous pupæ are provided with temporary deciduous conical processes. Thus we have observed in the pupa of Rhopalum pedicellatum two very prominent acute tubercles between the eyes (h, Fig. 592). As the cocoon is very slight, these may be of use either in extracting itself from the silken threads or in pushing its way along before emerging from the tunnel in the stem of plants. (See also p. 611.)
c. The cremaster
Although this structure is in general confined to lepidopterous pupæ, and is not always present even in them, since it is purely adaptive in its nature, yet on account of its singular mode of development from the larval organs, and the accompanying changes in the pupal abdomen, it should be mentioned in this connection. The cremaster is the stout, triangular, flattened, terminal spine of the abdomen, which aids the pupa in working its way out of the earth when the pupa is subterranean, or in the pupa of silk-spinning caterpillars its armature of secondary hooks and curved setæ enables it to retain its hold on the threads of the interior of its cocoon after the pupa has partially emerged from the cocoon, restraining it, as Chapman well says, “at precisely that degree of emergence from the cocoon that is most desirable.” He also informs us that while in the “pupæ incompletæ the cremaster is attached to an extensible cable, which always allows some emergence of the pupa, in the pupæ obtectæ there is no doubt but that in such cases as the Ichthyuræ, Acronyctæ, and many others, it retains the pupal case in the same position within the cocoon that the living pupa occupied; this is also very usually the case in the Geometræ and in the higher tineids (my pyraloids).”
In many of the more generalized moths there is no cremaster (Micropteryx, Gracilaria, Prodoxus, Tantura, Talæporia, Psychidæ, Hepialidæ, Zeuzera, Nola, Harrisina), though in Tischeria and Talæporia (Fig. 590, but not in Solenobia) and Psychidæ, two stout terminal spines perform the office of a cremaster, or there are simply curved setæ on the rounded, unarmed end of the abdomen, as in Solenobia.
In the obtected Lepidoptera, for example in such a group as the Notodontidæ, where the cremaster is present, though variable in shape, it may from disuse, owing to the dense cocoon, be without the spines and hooks in Cerura, or the cremaster itself is entirely wanting in Gluphisia, and only partially developed in Notodonta. In the butterflies whose pupæ are suspended (Suspensi), the cremaster is especially well developed. Reference might here be made to the temporary pupal structures in certain generalized moths, which take the place of a cremaster, such as the transverse terminal row of spines in Tinea, the two stout spines in Tischeria, and the dense rough integument and thickened callosities of the pupal head and end of abdomen of Phassus, which bores in trees with very hard wood; also the numerous stout spines at the end and sides of the abdomen in Ægerians. These various projections and spines, besides acting as anchors and grappling hooks, in some cases serve to resist strains and blows, and have undoubtedly, like the armature in the larvæ and imagines of other insects, arisen in response to intermittent or occasional pressure, stresses, and impacts.
Mode of formation of the cremaster and suspension of the chrysalis in butterflies.—We are indebted to Riley[[109]] for an explanation of the way the cremaster has originated, his observations having been made on species of over a dozen genera of butterflies (Suspensi).
He shows that the cremaster is the homologue of the suranal plate of the larva.[[110]] The preliminary acts of the larva have been observed by various authors since the days of Vallisneri, i.e. the larva hanging by the end of the abdomen, turning up the anterior part of the body in a more or less complete curve, and the skin finally splitting from the head to the front edge of the metathoracic segment, and being worked back in a shrivelled mass toward the point of attachment. The critical feat, adds Riley, which has most puzzled naturalists, is the independent attachment of the chrysalis and the withdrawal from and riddance of the larval skin which such attachment implies. Réaumur explained this in 1734 by the clutching of the larval skin between sutures of the terminal segments of the chrysalis, and this is the case, though the sutures act in a somewhat different way.
Before pupation the larva spins a mass or heap of silk, the shape of which is like an inverted settee or a ship’s knee, and “one of the most interesting acts of the larva, preliminary to suspension, is the bending and working of the anal parts in order to fasten the back of the (suranal) plate to the inside of the back of the settee, while the crotchets of the legs are entangled in the more flattened position or seat.”
In shedding the larval skin, the following parts are also shed, and have some part to play in the act of suspension: i.e. 1st, the tracheal ligaments (Fig. 593, tl), or the shed tracheæ from the last or 9th pair of spiracles; 2d, the rectal ligament (Fig. 593, rl), or shed intestinal canal; 3d, the Osborne or retaining membrane (membrana retinens, Fig. 593, mr), which is the stretched part of the membrane around the rectum and in the anal legs, and which is intimately associated with the rectal ligament.
Fig. 593.—Shrunken larval skin of Vanessa antiopa, cut open from the back and showing (mr) the retaining membrane, (rl) the rectal ligament, and (tl) the tracheal ligaments.
The structures in the chrysalis are, first, the cremaster, with its dorsal (Fig. 594, dcr) and ventral (vcr) ridges, and the cremastral hook-pad (chp), said by Riley to be “thickly studded with minute but stout hooks, which are sometimes compound or furnished with barbs, very much as are some of our fishing-hooks, and which are most admirably adapted to the purpose for which they are intended.”
Fig. 594.—Ideal representation of the anal subjoint of Vanessa antiopa, from behind, with the spines removed, and all parts forced apart by pressure so as to show the homologies of the parts in the chrysalis which are concerned in pupation: homologies indicated by corresponding letters in Fig. 595, except that r (the rectum) corresponds with pr in Fig. 595.
Fig. 595.—Anal parts of chrysalis of Vanessa antiopa, just prior to final extraction from shrunken larval skin: c, cremaster; chp, cremastral hook-pad; h, one of the hooks, more enlarged; vcr, ventral cremastral ridge; dcr, dorsal cremastral ridge; lr, larval rectum; pr, pupal rectum; rp, rectal plate; sr, sustentor ridges; mr, membrana retinens; rl, rectal ligament; tl, tracheal ligament; the 11th or last spiracle-bearing joint and the 12th joint being numbered.
Secondly, there are the other structures, viz., the sustainers (sustentors), two projections which Riley states “homologize with the soles (plantæ) of the anal prolegs, which take on various forms (3), but are always directed forward so as easily to catch hold of the retaining membrane.” These sustentors are, however, as Jackson[[111]] has shown, and as we are satisfied, the vestiges of the anal legs.
Fig. 596.—A, chrysalis of Terias. B, posterior end of chrysalis of Paphia. C, posterior end of chrysalis of Danais. E, one of the sustainers of Terias, greatly enlarged to show its hooked nature. All the parts of subjoint lettered to correspond with Fig. 595.
Thirdly, the sustentor ridges, which, as Riley states, may be more or less obsolete in some forms, in Paphia (Fig. 596, B) and Limenitis form “quite a deep notch, which doubtless assists in catching hold of the larval skin in the efforts to attach the cremaster.”
Fig. 597.—Pupation of butterflies: a, attachment of larva of Danais archippus; p, attachment of larva of Paphia glycerium; b, ideal larva soon after suspension; d, ideal larva a few hours later, the needle (n) separating the forming membrane from the sustainers; l, ideal larva just before splitting of larval skin, with retaining membrane loosened from the sustainers and showing its connection both with the larval and pupal rectum. In all the figures the joints of the body are numbered; the forming chrysalis is shaded in transverse lines; the intervening space between it and larval skin is dotted: h, is the hillock of silk; hl, hooks of hind legs; ap, anal plate; lr, larval rectum; pr, pupal rectum; mr, retaining membrane; c, cremaster; s, sustainers.—This and Figs. 593–596 after Riley.
“It is principally,” adds Riley, “by the leverage obtained by the hooking of the sustainers in the retaining membrane, which acts as a swimming fulcrum, that the chrysalis is prevented from falling after the cremaster is withdrawn from the larval skin. It is also principally by this same means that it is enabled to reach the silk with the cremastral hook-pads.”
“Dissected immediately after suspension, the last abdominal segment of the larva is found to be bathed, especially between the legs and around the rectum, in an abundance of translucent, membranous material.”
“An hour or more after suspension the end of the forming chrysalis begins to separate from the larval skin, except at the tip of the cremaster (Fig. 597, b). Gradually the skin of the legs and of the whole subjoint (10th segment) stretches, and with the stretching, the cremaster elongates, the rectal piece recedes more and more from the larval rectum, and the sustentor ridges diverge more and more from the cremaster, carrying with them, on the sustainers, a part of the soft membrane.” The rectal ligament will sustain at least 10 or 12 times the weight of the chrysalis. That of Apatura seems to rely almost entirely on the rectal ligament, assisted by the partial holding of the delicate larval skin.