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.)