Popular Science Monthly/Volume 53/September 1898/The Case Moths
|THE CASE MOTHS.|
IT seems an incontrovertible fact in natural history that there is not a single character which has been used to distinguish any group of considerable extent from which some one or more of the members thereof may not depart. In that great division of the animal kingdom characterized by the possession of articulated limbs, many species are met which are entirely wanting in those organs, and, similarly, the secondary division of the Annulosa, marked by the presence of wings in the final state—the Ptilota of Aristotle—contains species that, throughout life, never acquire instruments of flight. Of wingless insects, indeed, examples might be drawn from most of the orders, and in the majority of cases the females only are thus deprived. Rarely, however, both the great characteristics are absent. Yet certain moths do not possess articulated feet in the wingless state.
Consequently, if we took into consideration merely the adults of these females, this group must be regarded as among the most degraded instances of apiropodous insects. But such a conclusion can not be maintained, as shown by an examination of the early stages of the moths, for these, we find, exhibit as high an amount of organization as those of any of the other insects appertaining to the order. The truth is, these females have become degenerate—very different from the creatures they once were. Their peculiarity consists in this, that whereas, as a whole, winged insects always undergo a gradual evolution of structure, by which ultimately legs and wings are developed, these individuals gradually lose their powers of evolution, and not only this, but suffer a process of deterioration, by which the limbs which they at first possess diminish, and at length dwindle altogether away, until the animal becomes a mere short, inert vermiform bag, having not only no distinct trace of legs and wings, but also the sense-organs, the antennæ, and the organs of the mouth are almost or entirely obliterated, and even the articulated condition of the body has almost disappeared. In these extreme forms it is hardly possible for the degeneration of the female to proceed further, and in all, doubtless, the change has occupied an immense period.
Than these extraordinary moths, familiar to German entomologists under the name of Sackträgers, perhaps no more curious and interesting examples occur among the whole of the insect races; certainly in structure of the female, and in habit, they are the strangest and most abnormal of all Lepidoptera. They belong to the Psychidæ, a portion of the remarkable silk-spinning family of the Bombycidcæ, but offer many points distinct in themselves, which entitle them to rank, as recent lepidopterists agree, as a separate and well-defined tribe.
Their geographical distribution is extensive, since they are found in Europe, in North and South America, the West Indies and Mexico, in northern India and Ceylon, in China, the South Sea Isles, and Australia, being most abundant in subtropical regions. Wonderfully few species are described as natives of the United States, while in California, unfortunately, three have been discovered solely in the larval state, the more mature conditions of the species as yet eluding detection. But there, as in various other parts of the globe, probably greater numbers await the industry of observers.
Among English-speaking folk, the common appellations for the moths originate in the same circumstance as the popular term in Germany—house-builders, sack-bearers, basket-carriers, basket-worms, case moths; by these names they pass in England, America, and Australia, on account of the singular habitations or sacks they weave for the well-being of the caterpillars in the early stages of their growth. Through the whole of their larval life they carry the protecting structure about with them; and as regards the apterous female, she never leaves this home in which she dwelt while in larva—one of the oddest incidents in this odd economy—but reaching maturity, and bringing forth her young, dies at last, without once quitting her self-constructed prison. She deposits her ova, an immense number, within the body of the case, closely enveloped in some species in a short silky down, and almost as soon as hatched the larvæ force their way out of the puparium which had served for the defense of the eggs, deserting their early abode, and going into the world to follow independent lives. Escaping in crowds from the lower end of the tube, to some twig or leaf, they immediately commence to prepare for themselves each a separate case, arranged in every respect as the larger ones, even before they have taken food.
Particles of wood or bark, leaves, sticks, straws, lichens, mosses, and other vegetable substances form, among the different species, the outer covering or decorative fortification of the house; the interior is lined with soft silk, and interwoven silky threads likewise bind together the external fragments. In the building materials chosen, and their arrangement, Metura elongata is a most interesting architect. Strengthening the large elongate ovate bag of silk, and worked into it, irregularly, numerous rows of short sticks appear, rather distantly separated, and about half an inch long, generally speaking, but toward the lower end there are usually several sticks from one to four inches long, in the center of which the lower end of the silken bag protrudes, free from sticks and very flexible; it has a charming silky softness, and is of a gray, ash, or mouse color; of this beautiful tissue the upper or head extremity is also composed, forming a tube half an inch wide.
As a larva grows, needing more accommodation, it splits the habitation at the sides, weaving into the opening portions of the vegetable substances selected, and adding to the exterior fresh pieces of stick, straw, or leaves, as it requires. So with Saunders's case moth, when any accident happens to the nest, the caterpillar, with incredible expedition, repairs the damage received, employing the same silky stuff to fill up the hole, and with a nicety so perfect that the severest scrutiny can not detect what was the extent of the injury.
Under the protection, then, of the substantial and somewhat formidable case the larva lives. At each end there is an opening, and through the anterior one it emerges to feed and change its position. Commonly it only protrudes the head and the first three or four segments of the body, or sufficient to use its six true legs for locomotion when feeding; and if wishful to remain quiet, it usually takes the precaution of fastening a portion of the edge of the aperture by fibers of silk temporarily to the branch upon which it is, that, if alarmed, it can suddenly recede completely into the case, very rapidly drawing in the flexible part after it, by means of its mandibles and fore legs, and contracting the aperture, so as to exclude all enemies; thus hid, it stays in security, suspended only by a few threads. Were the nature of the hanging, tight-closed, strong, tough sack unknown, it would never be suspected of containing an active, voracious larva. Exceedingly wary and timid are these insects in retreating at the approach of danger. On a desire for removal the suspending threads are bitten off close to the case.
As long as the caterpillar is small, and the house of no great weight, it is borne nearly erect; but soon, as a rule, the incumbent mass lies flat, owing to increased weight, and is dragged along in that attitude. The abdominal and anal legs of the larva are furnished with a series of small points or hooks, with which it moves in the tube, laying hold of the interior of the lining, to which it can adhere with great pertinacity; so firm the hold retained, it is impossible to remove the creature without injury.
Having attained full growth, and being about to change to pupa, the larva of Metura Saundersii firmly fixes itself, by means of silken fibers spun for the purpose, to a branch or trunk of a tree, or paling, drawing together and permanently closing the head opening. It reverses its position in the case, so that the head is where the tail used to be—pointed toward the posterior or unattached end—and envelops itself in a soft silken cocoon of a yellowish color; allowing itself to hang perpendicularly, head downward, it awaits the pupal sleep.
From the facts just stated it need hardly be said that, when the time arrives, the perfect insect emerges from the posterior portion of the tube. At this particular time the male pupa becomes endowed with the power of stretching out its segments, to enable it to work its way out of the extremity. Through the opening of the posterior end it pushes the anterior half of its length, by a slight elongation and contraction of the body, which, with the assistance of a transverse series of minute sharp spines or hooks, directed backward, on some of the •segments, is in this way forced out head foremost, in like manner as the pupæ of the goat moths and the large swifts are made to emerge from timber and the earth when the moth is ready to escape. The pupæ are prevented from being thrust entirely out of the case by two strong anal hooks. After the issue of the imago the segments remain in their stretched-out condition; cases having belonged to males are often seen with the empty pupa skin sticking rather more than half out of the lower aperture, hanging head downward, as left by the moth.
The males of these moths are swift fliers, of extraordinary activity, dashing themselves wildly, almost to pieces, among the branches of the trees. A fiery little creature has no sooner arisen from his pupal slumber than he begins his violent fluttering, and as the wings are delicate in structure, in many instances nearly transparent, his beauty has generally disappeared before the entomologist can secure him; therefore specimens in good order are rare in collections. With slight exception, we find no homogeneousness in the perfect state of the insects of this group, but much variation of form presented by the different species. The general shape of the body varies from one greatly elongated, as in Metura elongata, to a short and robust as well as to a short and slender form. In like manner the wings vary from a long, narrow, and sharp-pointed wing, as in Metura, to those of short, broad, and ample proportions; and, again, may either be densely squamose, or colorless, of beautiful hyaline texture, almost or completely destitute of scales or hairs. The antennae may be deeply pectinated only at the base, in others they are feathered to the tip, and in the number of joints offer striking variations. But the males of nearly all Psychidæ are characterized by a uniform dull dark color, of a brown or gray tint; there is an almost total absence of bright color or of pattern. Yet these moths are in nearly all cases day-flying. Probably the beauty of the males disappears when the females become degenerate, and the conditions which produced it are then at an end.
The larval cases of these moths are among the "common objects" in Australia, meeting the eye everywhere suspended to trees and shrubs, such as the different kinds of Leptospernum, Melaleuca, etc., by their anterior end, and swinging loose otherwise. When unusually abundant, so as to look like a good crop of some seed or fruit, the pendent berths are particularly conspicuous, and attract the attention of the least curious of mortals. The most striking examples of the group belong to Metura Saundersii, whose cases are sometimes over five inches long; those of the male are one third smaller.
Considering this abundance, the insects are singularly rare in the moth state; not one case in a hundred will be found to produce a moth, owing partly to the destructive effects of attacks on the larvæ of ichneumonideous and dipterous parasites. From the same cause, nothing is harder, nay, more nearly impossible, than to rear these creatures in confinement; the caterpillars of a species may be collected persistently for years, and watched with incessant care, and yet never reach the perfect stage; hence there are already imperfectly known species of which the more mature conditions await discovery; and when success does attend our efforts at protection, many examples are probably observed of the depredations of the insidious parasites. Not that failure to attain perfection is always due to infestation of parasitic insects, as undoubtedly the somewhat ponderous houses of the larvæ render them to a high degree impervious to the onslaughts of insect enemies; the cause of death must be looked for elsewhere. Death usually occurs after the larva has undergone metamorphosis, the pupa gradually shriveling up after assuming its proper form, nor can anything be done, apparently, to avert the calamity.
To return to the case moth's metamorphoses. The female insect, as we have seen, unlike the male, is destined never to desert the larval home. For her no hour of emergence ever comes. When the pupa has slept the appointed time, the unwieldy and almost motionless moth feels little of the movement of oncoming life then experienced by her lithe and lively partner; the animal, still resident within the habitaculum formed by the larva, splits asunder the pupa skin, and her transformations are complete; in some, at least, of the species the female imago is continually inclosed in the pupa case. Here, therefore, we have an insect which in its adult state is forever excluded from the light, and never even beholds its mate.
Having filled the bottom of their puparium with their ova, packed in the down rubbed from their own body, these females do not long survive. The moth is then literally nothing but thin skin. Reduced to a shriveled, dried, and scarcely animated morsel of this matter, she either presses herself through the opening of the case or, exhausted, the last feeble flicker of life burned out, expires within.