HYPERMETAMORPHISM
When an insect passes through more than the three normal stages of metamorphosis, i.e. the larval, pupal, and imaginal, it is said to undergo a hypermetamorphosis. The best-known examples are the supernumerary stages of Meloë, Stylops, etc.
Fig. 637.—Hypermetamorphosis of male of Aspidiotus nerii: 1, freshly hatched larva; 2, larva shortly before pupating; b, rudiments of the legs; fl, of the wings; 3, pupa before moulting; 4, the same after moulting; 6, larva farther advanced than in 2; a, antennal rudiments; b, rudiments of legs; v, stomach; OG, brain; M. Fl, rudiments of the elevator and depressor muscles of the wing; M. Th, rudiments of the dorsal muscles; H, rudiments of the testes; 7, pupa shortly before entering upon the imago state (5); A, eyes; a, antenna; o, mouth; WD, wax-glands; BG, ventral nervous cord; Sb, caudal setæ; tr, tracheæ; p, genital armature.—After Schmidt.
As has already been observed, Schmidt has shown that in the male of the Coccidæ, there is a true hypermetamorphosis, as shown by Fig. 637. In Aspidiotus nerii there are five stages, there being two larval (1, 2) and two pupal stages (3, 4, 7). Stage 3 (Fig. 637, 2) may be compared with the pro-pupa stage of Riley (Fig. 581).
Fig. 638.—Mantispa interrupta, and side view of the same without wings: natural size.—Emerton del. a, freshly-hatched campodeoid larva of Mantispa styriaca, enlarged; b, the same, but older, before the first moult; enlarged.—Brauer.
We have already, on page 602, described the hypermetamorphosis of the neuropterous insect Mantispa (Fig. 638).
Fig. 639.—Triungulin (a) of a Californian Meloë: b, the three triungulin claws; c, antenna; d, maxillary palpus; e, labial palpus; f, mandible; g, an abdominal joint; h, imago, ♀; i, antenna of ♂.—After Riley.
In Meloë the freshly hatched larva, or “triungulin” (Fig. 639, a), is an active Campodea-like larva, which runs about and climbs up flowers, from which it creeps upon the bodies of bees, such as Anthophora and Andrena, who carry it to their cells, wherein their eggs are situated. The triungulin feeds upon and destroys the eggs of its hostess. Meanwhile its inactive life in the bee’s cell reacts upon the organism; after moulting, the-second larval form (Fig. 640, b) is attained, and now the body is thick, cylindrical, soft, and fleshy, and it resembles a lamellicorn larva, with three pairs of rather long thoracic legs. This is Riley’s carabidoid stage. This second larva feeds upon the honey stored up for the young or larval bees. After another moult, there is another entire change in the body; it is motionless, the head is mask-like without movable appendages, and the feet are represented by six tubercles. This is called the semipupa or pseudo-pupal stage. This form moults, and changes to a third larval form (c), when apparently, as the result of its rich, concentrated food, it is overgrown, thick-bodied, without legs, and resembles a larval bee.
Fig. 640.—Oil-beetle: a, first larva; b, second larva; c, third larva; d, pupa.
Fig. 641.—History of Sitaris: a, triungulin or 1st larva; g, anal spinnerets and claspers of same; b, 2d larva; e, pseudo-pupa; f, 3d larva; c, true pupa; d, imago, ♀.—After V. Mayet, from Riley.
After thus passing through three larval stages, each remarkably different in structure and in the manner of taking food, it transforms into a pupa of the ordinary coleopterous shape (d).
The history of Sitaris, as worked out by Fabre and more recently by Valery-Mayet, is a similar story of two strikingly different adaptational larval forms succeeding the triungulin or primitive larval stage. The first larva (Fig. 641, a) is in general like that of Meloë, the second (b) is thick, oval, fleshy, soft-bodied, and with minute legs, evidently of no use, the larva feeding on the honey stored by its host. The pseudo-pupal stage is still more maggot-like than in the corresponding stage of Meloë, and the third larva (f) is thick-bodied, with short thoracic legs.
In the complicated life-history of another cantharid, Epicauta vittata, as worked out by Riley (Fig. 642), we have the same acquisition of new habits and forms after the first larval stage, which evidently were at the outset the result of an adaptation to a change of food and surroundings. The female Epicauta lays its eggs in the same warm, sunny situation as that chosen by locusts (Caloptenus) for depositing their eggs. On hatching, the active minute carnivorous triungulin, ever on the search for eggs, on happening upon a locust egg gnaws into it, and then sucks the contents. A second egg is attacked and its contents exhausted, when, owing to its comparatively inactive habits and rich nourishing food after a period of inactivity and rest, the skin splits along its back, and at about the eighth day from beginning to take food the second larva appears, with much smaller and shorter legs, a much smaller head, and with reduced mouth-parts. This is the carabidoid stage of Riley. After feeding for about a week in the egg a second moult occurs, and the change of form is slight, though the mouth-parts and legs are still more rudimentary, and the body assumes “the clumsy aspect of the typical lamellicorn larva.” This Riley denominates the scarabæidoid stage of the second larva.
Fig. 642.—Epicauta cinerea: a, end of 2d larval stage; b, portion of dorsal skin; c, d, coarctate larva; e, f, pupa.—After Riley.
After six or seven days there is another transformation, the skin being cast, and the insect passes into another stage, “the ultimate stage of the second larva.” The larva, immersed in its rich nutritious food, grows rapidly, and after about a week leaves the now addled and decaying locust eggs, and burrows into the clear sand, where it lies on its side in a smooth cell or cavity, and where it undergoes an incomplete ecdysis, the skin not being completely shed, and assumes the semipupa stage, or coarctate larval stage of Riley.
In the spring the partly loose skin is rent on the top of the head and thorax, and then crawls out of it the “third larva,” which only differs from the ultimate stage of the second larva “in the somewhat reduced size and greater whiteness.” The insect in this stage is said to be rather active, and burrows about in the ground, but food is not essential, and in a few days it transforms into the true pupa state.
These habits and the corresponding hypermetamorphosis are probably common to all the Meloidæ, though the life-history of the other species has yet to be traced.
In the genus Hornia described by Riley, the wings of the imago are more reduced than in any other of the family, both sexes having the elytra as rudimentary as in the European female glow-worm (Lampyris noctiluca). These, with the simple tarsal claws and the enlarged heavy abdomen, as Riley remarks, “show it to be a degradational form.”
Its host is Anthophora, and the beetle itself lives permanently in the sealed cells of the bee, and Riley thinks it is subterranean, seldom if ever leaving the bee gallery. The triungulin is unknown, but the ultimate stage of the second larva, as well as the coarctate larva, is like those of the family in general, the final transformations taking place within the two unrent skins, in this respect the insect (Fig. 643) approaching Sitaris.
It appears, then, that as the result of its semi-parasitic mode of life the Campodea-form or triungulin larva of these insects, which has free-biting mouth-parts like the larvæ of Carabidæ and other carnivorous beetles, instead of continuing to lead an active life and feeding on other insects; living or dead, and then like other beetles directly transforming into the normal pupa, moults as many as five times, there being six distinct stages before the true pupa stage is entered upon. So that there are in all eight stages including the imaginal or last stage.
One cannot avoid drawing the very obvious conclusion that the five extra stages constituting this hypermetamorphosis, as it is so well styled, are structural episodes, so to speak, due to the peculiar parasitic mode of life, and were evidently in adaptation to the remarkable changes of environment, so unlike those to which the members of other families of Coleoptera, the Stylopidæ excepted, have been subjected. The fat overgrown body and the atrophied limbs and mouth-parts are with little doubt due to the abundant supply of rich food, the protoplasm of the egg of its host, in which the insect during the feeding time of its life is immersed. Since it is well known that parthenogenesis is due to over, or at least to abundant nutrition, or to a generous diet and favoring temperature, there is little reason to doubt that the greatly altered and abnormally fat or bloated body of the insect in these supernumerary stages is the result of a continuous supply of rich pabulum, which the insect can imbibe with little or no effort.
Fig. 643.—1, Egg-pod of Caloptenus differentialis with the mouth torn open, exposing the newly hatched larva of Epicauta vittata (1 a) eating into an egg and the passage which it made through the mucous covering; natural size. 2, dorsal view of the 1st larva, or triungulin, of E. vittata; 2 a, one side of the head of same from beneath, greatly enlarged so as to show the mouth-parts; 2 b, terminal joint of maxillary palpus, showing imbrications and flattened inner surface armed with stout points; 2 c, leg, showing more plainly the tarsal spines; 2 e, labrum; 2 d, one of the abdominal joints from above, showing stout points, stigmata, and arrangement of spinous hairs. 3, eggs of E. vittata, the natural size indicated at side. 4, dorsal view of the carabidoid stage of the 2d larva of E. vittata: 4 a, its antenna; 4 b, its right maxilla; 4 c, its leg; 4 d, side view of same, showing its natural position within the locust-egg mass. 5, lateral view of the ultimate or full-grown stage of the 2d larva of E. vittata; 5 a, portion of the dorsal skin, showing short setaceous hairs. 6, third head, or that from the scarabæidoid stage of the 2d larva of E. vittata from beneath, showing the reduction of mouth-parts as compared with the first head (2 a); 6 a, antenna of same; 6 b, maxilla of same; 6 c, mandible of same. 7, fourth head, or that of the full-grown larva of E. vittata, from above; 7 a, leg of same; 7 b, the breastplate or prosternal corneous piece. 8, lateral view of the pseudo-pupa or coarctate larva of E. vittata, with the partially shed skin adhering behind: 8 a, dorsal view of same; 8 b, its head, from the front; 8 c, same from side; 8 d, tuberculous leg; 8 e, raised spiracle; 8 f, anal part of same, 9. lateral view of the true pupa of Epicauta cinerea Forst: 9 a, ventral view of same. 10, Epicauta vittata (lemniscata or trivittate var.). 11, Epicauta cinerea Forst. (= marginata Fabr.). 12, antenna of the triungulin of Epicauta pennsylvanica: 12 a, maxilla of same; 12 b, labial palpus of same. 13. ♂ Hornia minutipennis, dorsal view; 13 a, lateral view of same; 13 b, simple claw of same; 13 c, coarctate larva; 13 d, leg of ultimate stage of 2d larva.—After Riley.
Fig. 644.—Triungulin stage of Stylops childreni.
The life-history of the Stylopidæ is after the same general fashion, though we do not as yet know many of the most important details. The females are viviparous, the young hatching within the body of the parent, as we once found as many as 300 of the very minute triungulin larvæ issuing in every direction from the body of what we have regarded as the female of Stylops childreni in a stylopized Andrena caught in the last of April. The larvæ differ notably from those of the Meloidæ in the feet being bulbous and without claws, yet it is in general Campodea-like and in essential features a triungulin (Fig. 644). The intestine ends in a blind sac, as in the larvæ of bees, and this would indicate that its food is honey. The complete life-history of no Stylopid is completely known. It is probable that, hatched in June from eggs fertilized in April, the larvæ crawl upon the bodies of bees and wasps; finally, after a series of larval stages as yet unknown,[[119]] penetrating within the abdomen of its host before the latter hibernates, and living there through the winter. The females, owing to their parasitic life, retain the larval form, while the free males are winged, not leading in the adult stage a parasitic life, though passing their larval and pupal stages in the body of their host, and are so unlike ordinary beetles as to be referred by good authorities to a distinct order (Strepsiptera).
Fig. 645.—Stylops childreni, ♂: a, abdomen of Andrena with ♀ Stylops (b).
The triungulin stage of these insects corresponds in general to the form of the larval Staphylinidæ and allied families, such as the Tenebrionidæ, which are active in their habits, running about and obtaining their food in a haphazard way, often necessarily suffering long fasts. In the external-feeding, less active coleopterous larvæ, like the phytophagous species, which have an uninterrupted supply of nutritious food, we see that the body is thick and fleshy. So also in the larvæ of the Scarabæidæ, Ptinidæ, and the wood-boring groups. In internal feeders, like the larval weevils and Scolytidæ, which live nearly motionless in seeds, fruits, and the sap-wood of plants and trees, with a constant supply of nourishing, often rich food, the eruciform body is soft, thick, and more or less oval-cylindrical. So it is with the larvæ of Hymenoptera, especially in the parasitic forms, and in the ants, wasps, and bees, which are nearly if not quite motionless, at least not walking about after their food.
Now the change from the active triungulin stage to the series of secondary, nearly legless, sedentary, inactive stages is plainly enough due to the change of station and to the change of food. From being an independent, active, roving triungulin, the young insect becomes a lodger or boarder, fed at the expense of its host, and the lack of bodily exertion, coupled with the presence of more liquid food than is actually needed for its bare existence, at once induces rotundity of body and a loss of power in the limbs, followed by their partial or total atrophy.
That this process of degeneration may even occur in one and the same stage of larval existence is very well illustrated by what we know of the life-history of the wasp-parasite of Europe, Rhipiphorus paradoxus. Thanks to the very careful and patient observations of Dr. T. A. Chapman, we have a nearly complete life-history of this beetle, the representative of a family in many respects connecting the Meloidæ and Stylopidæ.[[120]] Where Rhipiphorus lays her eggs is unknown. Dr. Chapman, however, found a solitary specimen of the young larva in the triungulin stage. He describes it as “a little black hexapod, about 1
50 inch (.5 mm.) in length, and 1
120 inch in breadth, broadest about the fourth segment, and tapering to a point at the tail; a triangular head with a pair of three-jointed antennæ nearly as long as the width of the head, with legs very like those of Meloë larvæ; the tibiæ ending in two or three claws, which are supported and even obscured by a large transparent pulvillus or sucker of about twice their length; this was marked by faint striæ radiating from the extremity of the tibiæ, giving it much the aspect of a lobe of a fly’s proboscis. Each abdominal segment had a very short lateral spine pointing backwards; the last segment terminated by a large double sucker similar to those of the legs; and the little animal frequently stood up on this, and pawed the air with its feet, as if in search of some fresh object to lay hold of.”
This almost microscopic larva finds a wasp grub and bores into its body, probably entering at a point near the back of the first or second segment behind the head. Dr. Chapman succeeded in finding the larva of the beetle within that of the wasp, before the latter had spun up. “Assuming that the wasp larva lives six days in its last skin before spinning up, I should guess that the youngest of these had still two or three days’ feeding to do. The Rhipiphorus larvæ were but a little way beneath the skin of the back, about the fourth and fifth segments [counting the head as the first], and indifferently on either side. The smallest of these was 1
16 inch in length, and, except its smaller size, was precisely like the larger ones I am about to refer to, having the same head, legs, plates, etc. These were of the same size as those of the larger larvæ, the difference in size of the latter being due to the expansion of the intermediate colorless integument.”
After the wasp grub has spun the silken covering of its cell the larva of Rhipiphorus may still be detected in some of them, being rendered visible by its black legs and dark dorsal and ventral plates. “On extracting this larva, it bears a general resemblance in size and outline to the youngest larva of Rhipiphorus that I had found feeding externally on the wasp grub, but with the very notable exception of the already mentioned black marks. These are, in fact, a corneous head, six-jointed legs, and a dorsal and ventral series of plates. I immediately recognized the head and legs as identical with those of the little black mite already described, but presenting a ludicrous appearance in being widely separated from each other by the white skin of the larva. I have no doubt that the dorsal and ventral series of black marks are the corresponding plates of the mite-like larva floated away from each other by the expansion of the intervening membrane. By measurement also they agree exactly in size, although the larva extracted from the wasp grub is ten times the length and six times the width of the little Meloë-like larva. In length it is ⅙ inch (4.5 mm.), and 1
28 inch in breadth.”
The remarkable changes thus described in the larva of this beetle after it has begun its parasitic life within the body of its host are especially noteworthy because the great increase in size and difference in shape, as well as in habits, all take place before the insect has moulted. The rapid development in size, and consequent distension of the body and the separation of the sclerites of the segments behind the head, are paralleled, as Chapman says, by the greatly swollen abdominal region of the body in Sarcopsylla penetrans and in the female of the Termitidæ. In those insects this distension is due to the enlargement of the ovaries and of the eggs contained within them, but in the Rhipiphorus it is due to the comparative inactivity of the larva, and to its being gorged with an unending supply of rich food, the blood and fat of its host. It follows, then, that if a sedentary life and over, or at least abundant, nutrition will have this effect within the short period covered by the single first larval stage of the Rhipiphorus, it is reasonable to infer that the hypermetamorphosis is also due to the same factors.
Chapman then goes on to say that finally, within six hours of the time of spinning up of the wasp grub, the Rhipiphorus larva at the end of Stage 1., which is “usually in motion, and for its situation might be called tolerably active, is seen to lay hold of the interior of the skin with its anterior legs, and keeps biting and scratching with its strong and sharp jaws until it is able to thrust through its head, when, in less than a quarter of an hour, it completely emerges by a vermiform movement; and at the same time it casts a skin, together with the black head, legs, plates, etc.”
The larva, now in its second stage, passes forward and seizes hold of the upper or lateral aspect of the prothoracic segment of the wasp grub. On emerging it becomes shorter and thicker, “and very soon loses length by that curving forward of its head which is so marked in the full-grown larva, and which does not exist before its emergence.” The larva is now found “lying like a collar immediately under the head of the wasp grub, and is attached to it by the head, though not very firmly.” At this stage the feeding of the young Rhipiphorus is rather sucking than eating.
Fig. 646.—First larva (a) of Bruchus fabæ, greatly enlarged; b, thoracic processes; c, head, from front; d, from side; e, antenna; f, thoracic leg; g, rear view of tarsus; h, same, front view.—After Riley.
When about 6 mm. in length it moults a second time, and the full-grown larva closely though superficially resembles a Crabro or Pemphredon larva, the small head being bent over forwards. By the time it is ready to pupate it has wholly eaten the wasp larva, and the temperature of the cell being high, a larva 5 mm. long grows large enough in two days to fill the top of the cell of its host, and the larva is ready to pupate about a week after hatching, so that its development is very rapid. The beetles themselves do not live in the cells. Chapman thinks they hibernate, and that the eggs are laid in the spring or summer.
We thus have in this insect three larval stages, the triungulin, and two later stages, the great differences between the first and the last two being apparently due to their parasitic mode of life, the larva spending its second stage within its host, involving an existence in a cell with a high temperature, an uninterrupted supply of rich, stimulating food, and a comparatively sedentary mode of life compared with that of the triungulin at the beginning of its existence. It is quite obvious that the hypermetamorphosis is primarily due to a great change in its surroundings, i.e. the parasitic mode of life of the beetle, habits of very rare occurrence in the Coleoptera, numerous in species as they are.
Fig. 647.—First larval stage of Bruchus pisi: a, egg in pea-pod; b, cross-section of opening of mine; c, young larva and opening on inside of pod by which it has entered, enlarged; d, d, d, eggs, natural size; e, 1st larval stage; f, a leg of same; g, prothoracic spinous processes.—After Riley.
In this connection attention may be drawn to a supernumerary larval stage observed by Riley in the pea- and bean-weevils (Figs. 646 and 647). The larva on hatching has long slender legs, though differing from those of an ordinary coleopterous larva in having but three joints (j, g, h). This stage is very short, and the legs temporary, as, after entering the bean or pea, it casts its skin, losing its legs, and assuming the vermiform shape of the second larval stage. In this case the change from a pedate to an apodous larva is plainly enough due to the change from an external feeder, like a chrysomelid larva, to a larva leading a boring, internal, almost quiescent life.
Certain ichneumons also appear to have two distinct larval stages, as Ratzeburg inferred that in Anomalon there are four larval stages (Fig. 648).
Fig. 648.—History of Anomalon circumflexum: A, 1st instar or stage. B, 2d instar. C, larva in the 3d or encysted stage removed from its cyst. D, mature larva. E, pupa.—After Ratzeburg, from Sharp.
In another ichneumon, Klapálek detected what he calls the “sub-nymph.” The insect pupates within the case of a caddis-fly, Silo (Fig. 649).
In the Proctotrypidæ there is also a hypermetamorphosis, though the remarkable precocious stages they pass through are rather embryonic than larval.
In a species of Platygaster which is parasitic in the larva of Cecidomia, the first larva (Cyclops stage) is of a remarkable shape, not like an insect, but rudely resembling a parasitic Copepod crustacean. In this condition it clings to the inside of its host by means of its hook-like jaws, moving about, as Ganin says, like a Cestodes embryo with its well-known six hooks. In this stage it has no nervous, vascular, or respiratory system, and the digestive canal is a blind one (Fig. 651).
Fig. 649.—Metamorphosis of Agriotypus: A, larva. B, “sub-nymph.” C, case of the Silo, with the string of attachment formed by Agriotypus. D, section of the case: v1, operculum of case; v2, cocoon; ag, pupa of Agriotypus; e, exuvia of same; w2, wall of cocoon; s, remains of Silo; w1, closure of case.—After Klapálek, from Sharp.
After moulting, the insect entirely changes its form; it is thick oval-cylindrical, nearly motionless, with no appendages, but with a digestive canal and a nervous and vascular system (Fig. 652).
After a second moult the third and last larval stage is attained, and the insect is of the ordinary appearance of ichneumon larvæ.
Not less striking is the life-history of Polynema, which lays its eggs in those of a small dragon-fly (Agrion virgo). The first larval stage is most remarkable. It hatches as a microscopic immovable being, entirely unlike any insect, with scarcely a trace of organization, being merely a flask-shaped sac of cells. After remaining in this state five or six days it moults.
Fig. 650.—Development of Platygaster: A, stalked egg: a central cell giving origin to the embryo. B, g, germ; b, blastoderm cells. C, the same, farther advanced. D, cyclops-like embryo: md, rudiments of mandibles; d, rudimentary pad-like organs, seen more developed in E; st, bilobed tail.
The second stage, or Histriobdella-like form, as Ganin names it, is more like that leech-like worm than an insect.
The third larval form is very bizarre, though more as in insects, having rudimentary antennæ, mouth-parts, legs, and ovipositor. In this condition it lives from six to seven days before pupating (Fig. 653).
The strange history of another egg-parasite (Ophioneurus) agrees in some respects with that of the foregoing forms. It is when hatched of an oval shape, with scarcely any organs, and differs from the genera already mentioned in remaining within its egg-membrane, and not assuming their strange shapes. From the cylindrical sac-like non-segmented larva resembling the second larva of Platygaster it passes directly into the pupa state.
A fourth form, Teleas (Fig. 654, A-D), is an egg-parasite of Gerris, and in America one species oviposits in the eggs of Œcanthus.
Fig. 651.—First larva of Platygaster: m, mouth; at, rudimentary antenna; md, mandibles; d, tongue-like appendages.
Fig. 652.—Second larva of Platygaster: œ, œsophagus; ng, brain; n, nervous cord; ga and g, genital organs; ms, muscular band.
The spindle-shaped larva in its first stage roughly resembles a trochosphere of a worm rather than the larva of an insect so high in the scale as a Hymenopter. It is active, but after moulting the second larva is oval, still without segments. Dr. Ayers gives a profusion of details and figures of the first and second stages of our Teleas, the second strongly resembling the Cyclops stage of Ganin. He describes three stages, and though he did not complete the life-history of the insect, he thinks it changes to an ovoid flattened form which succeeds the Cyclops stage in other Pteromalidæ, and that there are at least four ecdyses.
It is difficult to account for these strange larval forms, unless we suppose that the embryos, by their rich, abundant food, have undergone a premature development, the growth of the body-walls being greatly accelerated, the insects so to speak having been, under the stimulus of over-nutrition and their unusual environment, and perhaps also the high temperature of the egg, hurried into vermian existence on a plane scarcely higher than that of an active ciliated gastrula.
Further observations, difficult though they will be, are needed to enable us to account for the singular prematurity of the embryo of these parasites. That these stages are reversional and a direct inheritance from the vermian ancestors of these insects is not probable, but the forms are evidently the result of adaptation in response to a series of stimuli whose nature is in part appreciable but mostly unknown.
Fig. 653.—Third larva of Polynema: at, antenna; fl, imaginal bud of wing; l, rudimentary legs; tg, buds of one of the three pairs of styles of the ovipositor; fk, fat-body; eg, ear-like process.
Fig. 654.—A-D, development of Teleas; A, stalked egg; B, C, D, the 1st larval stage: at, antenna; md, hook-like mandibles; mo, mouth; b, bristles; m, intestine; sw, the tail; ul, under lip or labium. E, larva of another parasite, Ophioneurus.—This and Figs. 650–653 after Ganin.
It may be noted, however, that the appearance of a primitive band in the second larval stage suggests the origin of these forms, as well as that of insects in general, from a Peripatus-like, and again from an earlier leech-like Annelid ancestor. Hence the first larval or Cyclops stage is due to a precocious development caused by the unusual environment, and is simply adaptational, and not of phylogenetic significance.