a. The pupa considered in reference to its adaptation to its surroundings and its relation to phylogeny
The form of the pupa is a very variable one, as even in Lepidoptera it is not entirely easy to draw the line between a pupa libera and a pupa obtecta (Fig. 578); and though the period is one of inactivity, yet when they are not in cocoons or in the earth in subterranean cells, their form is more or less variable and adapted to changes in their surroundings. Even in the obtected pupa of butterflies, there is, as every one knows, considerable variability of shape and of armature, which seems to be in direct adaptability to the nature of their environment. Scudder has well shown that in certain chrysalids, such as those of the Nymphalidæ, which are variously tuberculated, and hang suspended by the tail, and often hibernate, these projections serve to protect the body. All chrysalids with projections or ridges on different parts of the body, being otherwise unprotected, move freely when struck by gusts of wind, hence “the greater the danger to the chrysalis from surrounding objects, the greater its protection by horny tubercles and roughened callous ridges.” The greater the protection possessed in other ways, as by firm swathing or a safe retreat, the smoother the surface of the body and the more regular and rounded its contours. The tendency to protection by tubercles is especially noticeable in certain South American chrysalids of nymphalid butterflies. This response to the stimuli of blows or shocks is also accompanied by a sensitiveness to the stimulus of too strong light.
Previously Scudder[[103]] had made the important suggestion that the smooth crescent-shaped belt of the “glazed eye” or “eyepiece” of chrysalids is, as an external covering of the eye, midway between that of the caterpillar and the perfect insect, and he asks: “May it not be a relic of the past, the external organ of what once was? And are we to look upon this as our hint that the archaic butterfly in its transformations passed through an active pupal stage, like the lowest insect of to-day, when its limbs were unsheathed, its appetite unabated?” etc. Scudder also shows that “the expanded base of the sheath covering the tongue affords protection also to the palpi which lie beneath and beside the tongue.”
All this tends to show the importance of studying the structure of the pupa, in order to ascertain how the pupal structures have been brought about, with the final object of discovering whether the pupæ of the holometabolic insects are not descended from active nymphs, and if so, the probable course of the line of descent.
b. Mode of escape of the pupa from its cocoon
Fig. 587.—Pupa of Micropteryx purpuriella, front view: md, mandibles; mx. p, maxillary palpus, end drawn separately; mx.′ p, labial palpi; lb, labrum.
“In all protected pupæ,” as Chapman says, “the problem has to be faced, how is the imago to free itself from the cocoon or other envelope protecting the pupa.” In the Coleoptera and Hymenoptera the imago becomes perfected within the cocoon or cell, as the case may be, and as Chapman states, “not only throws off the pupal skin within the cocoon, but remains there till its appendages have become fully expanded and completely hardened, and then the mandibles are used to force an outlet of escape,” and he calls attention to the fact that “in many cases, even in some entire families, they are of no use whatever to the imago except in this one particular,” and he cites the Cynipidæ as perhaps the most striking instance of this circumstance.
In those Neuroptera which spin a silken cocoon, e.g. the Hemerobiidæ, the Trichoptera, and in Micropteryx (Fig. 588), the jaws used by the pupa for cutting its way out of the cocoon are even larger in proportion than in the pupa of caddis-flies (Fig. 588), being of extraordinary size.
