Fig. 570.—a, Erax bastardi; b, pupa.—After Riley.
The larval skin of the coarctate Diptera is retained as a protection for the soft-bodied pupa within, the old larval skin separating from the integument of the semipupa. To this cocoon-like covering of the coarctate pupa we have restricted the term puparium, originally used by Kirby and Spence to designate the pupa. The puparium is usually cylindrical or barrel-shaped, rounded at each end.
Fig. 571.—Puparium of Hypoderma bovis: a, side; b, ventral view, showing exit hole of adult; c, cap which splits off for exit of fly.—After Clark, from Osborn, Bull. 5, Div. Ent. U. S. Dept. Agr.
In the Diptera cyclorhapha, or common house and flesh flies, etc., the puparium remains in vital connection, by means of four tracheæ, with the enclosed pupa, which escapes from the case through a curved seam or lid at the anterior end and not by a slit in the back, as do the orthoraphous families, represented by the horse-fly (Tabanidæ, Asilidæ, Fig. 570), etc., where in some cases the obtected pupa remains within the loose envelope formed by the old larval skin, which Brauer calls a false puparium. The dry, hard puparium is burst open at the cephalic end when the fly emerges, by means of the frontal vesicle, which is distended with fluid (Fig. 571).
The exact mode of spinning the cocoon by caterpillars has been carefully observed by L. Trouvelot in the case of the polyphemus silkworm.
“When fully grown, the worm, which has been devouring the leaves so voraciously, becomes restless and crawls about the branches in search of a suitable place to build up its cocoon; before this it is motionless for some time, holding on to the twig with its front legs, while the two hind pair are detached; in this position it remains for some time, evacuating the contents of the alimentary canal until finally a gelatinous, transparent, very caustic fluid, looking like albumen, or the white of an egg, is ejected; this is a preparation for the long catalepsy that the worm is about to fall into. It now feels with its head in all directions, to discover any leaves to which to attach the fibres that are to give form to the cocoon. If it finds the place suitable, it begins to wind a layer of silk around a twig, then a fibre is attached to a leaf near by, and by many times doubling this fibre and making it shorter every time, the leaf is made to approach the twig at the distance necessary to build the cocoon; two or three leaves are disposed like this one, and then fibres are spread between them in all directions, and soon the ovoid form of the cocoon distinctly appears. This seems to be the most difficult feat for the worm to accomplish, as after this the work is simply mechanical, the cocoon being made of regular layers of silk united by a gummy substance. The silk is distributed in zigzag lines of about one-eighth of an inch long. When the cocoon is made, the worm will have moved his head to and fro, in order to distribute the silk, about 254,000 times.
“After about half a day’s work, the cocoon is so far completed that the worm can hardly be distinguished through the fine texture of the wall; then a gummy resinous substance, sometimes of a light-brown color, is spread all over the inside of the cocoon. The larva continues to work for four or five days, hardly taking a few minutes of rest, and finally another coating is spun in the interior, when the cocoon is all finished and completely air tight. The fibre diminishes in thickness as the completion of the cocoon advances, so that the last internal coating is not half so thick and so strong as the outside ones.” (Amer. Naturalist, i, p. 86.)
The mode of spinning the cocoon of an ichneumon (Microgaster) parasitic on Philampelus has been well described by John P. Marshall, as follows:—
