NATURE OF INSECT METAMOEPHOSIS. 357 



own illustrious entomologists Kirby and Spence, believed that even at 

 the birth of the caterpillar all these skins existed ready-formed one 

 beneath the other, and that the most external being removed at intervals 

 displayed in succession the skins placed underneath. Surely the advo- 

 cates of this extraordinary theory could scarcely have reflected upon the 

 real object of the moults in question (namely, to provide a succession of 

 larger coverings proportioned to the continually increasing bulk of the 

 larva) when they advocated this strange doctrine, alike at variance 

 with observation and sound physiological principles. The epidermis and 

 all cuticular structures are mere secretions from the subjacent cutis or 

 true skin ; and it can be no more necessary to suppose the pre-exist- 

 ence of so many skins in order to explain the moults of a larva, than to 

 imagine that because, when in our own persons the cuticle is removed by 

 the application of a blister, a new layer of epidermis is again and again 

 produced, man should possess as many skins, one beneath the other. 

 Nothing, in fact, can be more simple and free from the miraculous than 

 the whole process : at certain periods, when the old cuticle has grown 

 too small for the rapidly-enlarging dimensions of the insect, it becomes 

 gradually loosened and separated from the vascular and living skin or 

 cutis by which it was originally secreted ; and, a new secretion of corneous 

 matter taking place, a fresh and more extensive layer of cuticle is slowly 

 formed, and then the old, dry and dead epidermis, being quite detached, 

 is split by the exertions of the larva, and the newly-secreted layer 

 placed beneath it appears. When the old skin is at length completely 

 thrown off, the newly-formed one soon hardens by exposure, and the re- 

 clothed caterpillar assumes again its former activity and habits. 



(923.) Neither is the change from the larva to the pupa or chrysalis 

 less easily explained, although regarded by our forefathers as being so 

 mysterious and astonishing a phenomenon. According to the hypothesis 

 above alluded to, after removing three or four skins in the embryo larva, 

 the anatomist ought to have arrived at the totally different pupa-case 

 ready-formed, and only waiting for the removal of the coats above it to 

 exhibit its characteristic form. Leaving, however, such visionary notions, 

 let us examine the real nature of this portion of the metamorphosis. 

 The reader will bear in mind that, whatever the form of the exterior or 

 epidermic crust, it is merely a dead and extra-vascular secretion, un- 

 changeable when once deposited. But the living skin or cutis, beneath 

 it, is, during the whole process of the metamorphosis, undergoing great 

 and important changes increasing only in size during the larva condi- 

 tion, but when perfectly organized, developing itself at different points, 

 and expanding into variously-shaped organs which did not previously 

 exist. In the Dragon-fly, for example (fig. 147), when the cutis had 

 become expanded to its mature larva condition, it secreted from its 

 surface the external epidermic crust which gives form to the larva (B) : 

 this outward integument remains, of course, unchanged when once 



