The post-embryonic development of these Insects is amongst the most remarkable of modern entomological discoveries. The first steps were made by Newport in 1851,[[140]] and the subject has since been greatly advanced by Fabre, Riley, and others. As an example of these peculiar histories, we may cite Riley's account[[141]] of Epicauta vittata (Fig. 140), a blister-beetle living at the expense of North American locusts of the genus Caloptenus. The locust lays its eggs underground, in masses surrounded by an irregular capsule, and the Epicauta deposits its eggs in spots frequented by the locust, but not in special proximity to the eggs thereof. In a few days the eggs of the blister-beetle hatch, giving rise to little larvae of the kind called triungulin (Fig. 140, A), because each leg is terminated by three tarsal spines or claws. In warm, sunny weather these triungulins become very active; they run about on the surface of the ground exploring all its cracks, penetrating various spots and burrowing, till an egg-pod of the locust is met with; into this the triungulin at once eats its way, and commences to devour an egg. Should two or more triungulins enter the same egg-pod, battles occur till only one is left.
Fig. 140.—Hypermetamorphosis of Epicauta vittata. North America. (After Riley.) A, Young larva or triungulin; B, Caraboid instar or second larva; C, coarctate larva, or instar between the Scarabaeoid and Scolytoid larva; D, Scarabaeoid larva, from which the Scolytoid, or sixth, instar differs but little; E, pupa; F, imago.
After a few days passed in devouring a couple of eggs, the triungulin sheds its skin and appears as a different larva (Fig. 140, B), with soft skin, short legs, small eyes, and different form and proportions; a second moult takes place after about a week, but is not accompanied by any very great change of form, though the larva is now curved, less active, and in form like a larva of Scarabaeidae; when another moult occurs the fourth instar appears as a still more helpless form of larva (Fig. 140, D), which increases rapidly in size, and when full grown leaves the remains of the egg-pod it has been living on, and forms a small cavity near by; here it lies on one side motionless, but gradually contracting, till the skin separates and is pushed down to the end of the body, disclosing a completely helpless creature that has been variously called a semi-pupa, pseudo-pupa, or coarctate larva (Fig. 140, C); in this state the winter is passed. In spring the skin of the coarctate larva bursts, and there crawls out of it a sixth instar which resembles the fourth (Fig. 140, D), except in the somewhat reduced size and greater whiteness. It is worthy of remark that the skin it has deserted retains its original form almost intact. In this sixth instar the larva is rather active and burrows about, but does not take food, and in the course of a few days again moults and discloses the true pupa (Fig. 140, E). As usual in Coleoptera this instar lasts but a short time, and in five or six days the perfect beetle appears (Fig. 140, F). It is extremely difficult to frame any explanation of this complex development; there are, it will be noticed, no less than five stages interposed between the first larval instar and the pupal instar, and the creature assumes in the penultimate one a quasi-pupal state, to again quit it for a return to a previous state. It is possible to look on the triungulin and the pupal instars as special adaptations to external conditions; but it is not possible to account for the intermediate instars in this way, and we must look on them as necessitated by the physiological processes going on internally. Nothing, however, is known as to these. It may be well to mention that, after describing and figuring (loc. cit.) this series of instars, Riley changed his views as to their nomenclature.[[142]] The following summary of the metamorphosis, to which we have added the two nomenclatures of Riley—the original one, when different from the amended one, being given in square brackets—may therefore be useful, viz.—Egg; 1, triungulin-larva—moult; 2, Caraboid larva [second larva, Caraboid stage]—moult; 3, Scarabaeoid larva [second larva, Scarabaeoid stage]—moult; 4, Scarabaeoid larva [second larva, ultimate stage] (large amount of food and much growth)—moult; 5, coarctate larva [pseudo-pupa, or semipupa]; 6, Scolytoid larva [third larva] (active, but little or no food taken)—moult; 7, pupa—moult; 8, perfect Insect.
M. Fabre has succeeded in elucidating the history of Sitaris humeralis, a Cantharid that lives at the expense of bees of the genus Anthophora.[[143]] The eggs of the Sitaris are deposited in the earth in close proximity to the entrances to the bees' nests, about August. They are very numerous, a single female producing, it is believed, upwards of 2000 eggs. In about a month—towards the end of September—they hatch, producing a tiny triungulin of black colour; the larvae do not, however, move away, but, without taking any food, hibernate in a heap, remaining in this state till the following April or May, when they become active. Although they are close to the abodes of the bees they do not enter them, but seek to attach themselves to any hairy object that may come near them, and thus a certain number of them get on to the bodies of the Anthophora and are carried to its nest. They attach themselves with equal readiness to any other hairy Insect, and it is probable that very large numbers perish in consequence of attaching themselves to the wrong Insects. The bee in question is a species that nests in the ground and forms cells, in each of which it places honey and lays an egg, finally closing the receptacle. It is worthy of remark that in the case of the Anthophora observed by M. Fabre, the male appears about a month before the female, and it is probable that the vast majority of the predatory larvae attach themselves to the male, but afterwards seize a favourable opportunity, transfer themselves to the female, and so get carried to the cells of the bee. When she deposits an egg on the honey, the triungulin glides from the body of the bee on to the egg, and remains perched thereon as on a raft, floating on the honey, and is then shut in by the bee closing the cell. This remarkable act of slipping on to the egg cannot be actually witnessed, but the experiments and observations of the French naturalist leave little room for doubt as to the matter really happening in the way described. The egg of the bee forms the first nutriment of the tiny triungulin, which spends about eight days in consuming its contents; never quitting it, because contact with the surrounding honey is death to the little creature, which is entirely unfitted for living thereon. After this the triungulin undergoes a moult and appears as a very different creature, being now a sort of vesicle with the spiracles placed near the upper part; so that it is admirably fitted for floating on the honey (Vol. V. Fig. 86, 10). In about forty days, that is, towards the middle of July, the honey is consumed, and the vesicular larva after a few days of repose changes to a pseudo-pupa (11 of the fig. cited) within the larval skin. After remaining in this state for about a month, some of the specimens go through the subsequent changes, and appear as perfect Insects in August or September. The majority delay this subsequent metamorphosis till the following spring, wintering as pseudo-pupae and continuing the series of changes in June of the following year; at that time the pseudo-pupa returns to a larval form (12 of the fig. cited), differing comparatively little from the second instar. The skin, though detached, is again not shed, so that this ultimate larva is enclosed in two dead skins; in this curious envelope it turns round, and in a couple of days, having thus reversed its position, becomes lethargic and changes to the true pupa, and in about a month subsequent to this appears as a perfect Insect, at about the same time of the year as it would have done had only one year, instead of two, been occupied by its metamorphosis. M. Fabre employs the term, third larva, for the instar designated by Riley Scolytoid larva, but this is clearly an inconvenient mode of naming the instar. Sitaris humeralis is now very rare in Britain, but it seems formerly to have been more common, and it is not improbable that its triungulin may have been the "Pediculus melittae," that was believed by Kirby to be a sort of bee-louse. Some species of the genus Meloe are still common in Britain, and the Insects may be seen with heavy distended abdomen grazing on herbage in the spring. The females are enormously prolific, a single one producing, it is believed, about 10,000 eggs. Meloe is also dependent on Anthophora, and its life-history seems on the whole to be similar to that of Sitaris; the eggs are, however, not necessarily deposited in the neighbourhood of the bees' nests, and the triungulins distribute themselves on all sorts of unsuitable Insects, so that it is possible that not more than one in a thousand succeeds in getting access to the Anthophora nest. It would be supposed that it would be a much better course for these bee-frequenting triungulins to act like those of Epicauta, and hunt for the prey they are to live on; but it must be remembered that they cannot live on honey; the one tiny egg is their object, and this apparently can only be reached by the method indicated by Fabre. The history of these Insects certainly forms a most remarkably instructive chapter in the department of animal instinct, and it is a matter for surprise that it should not yet have attracted the attention of comparative psychologists. The series of actions, to be performed once and once only in a lifetime by an uninstructed, inexperienced atom, is such that we should a priori have denounced it as an impossible means of existence, were it not shown that it is constantly successful. It is no wonder that the female Meloe produces 5000 times more eggs than are necessary to continue the species without diminution in the number of its individuals, for the first and most important act in the complex series of this life-history is accomplished by an extremely indiscriminating instinct; the newly hatched Meloe has to get on to the body of the female of one species of bee; but it has no discrimination whatever of the kind of object it requires, and as a matter of fact, passes with surprising rapidity on to any hairy object that touches it; hence an enormous majority of the young are wasted by getting on to all sorts of other Insects; these larvae have been found in numbers on hairy Coleoptera as well as on flies and bees of wrong kinds; the writer has ascertained by experiment that a camel's-hair brush is as eagerly seized, and passed on to, by the young Meloe as a living Insect is.
The histories of several other Cantharids have been more or less completely discovered. Fabre has found the larva of Cerocoma schaefferi attacking the stores of provisions laid up by a fossorial wasp of the genus Tachytes, and consisting of Orthoptera of the family Mantidae. The student who wishes for further information may refer to M. Beauregard's work on this family.[[144]]
Some half-dozen species of the genus Cephaloon found in Siberia, Japan, and North America, have, by some authorities, been separated as the family Cephaloidae. Nothing is known as to the metamorphosis of these rare beetles; and at present it is not necessary to distinguish them from Cantharidæ.
Fam. 76. Trictenotomidae.—Large Heteromera, with powerful free projecting mandibles; the antennae long, but with the terminal three joints short, with angular projections on one side. This family includes only two genera and seven or eight species. They are very remarkable Insects; Autocrates aenea being three inches long. The family is of considerable interest, as it seems to have no affinity with any other Coleoptera. The appearance of the species somewhat reminds one of Lucanidae, or Prionides; but Trictenotomidae have even less relation to those beetles than they have to the members of the Heteromerous series. The Trictenotomidae appear to be found only in the primitive forests of the Indian and Indo-Malayan regions. Nothing is known as to their life-histories.
Series V. Phytophaga.
Tarsi apparently four-jointed, the three basal joints usually densely set with cushion-like pubescence beneath; the third joint different in form, being divided into two lobes, or grooved on its upper surface so as to allow of the fourth joint being inserted near its base instead of at its extremity. Head not forming a definite prolonged beak; its labrum visible, the palpi rarely (and even then not completely) occluded in the mouth.