38 Till: LII K-STORY OF INSECTS [CH. v 



forming beneath the cuticle of the last larval instar, 

 can they grow outwards. 



Anatomical study of the caterpillar at various 

 stages verifies the conclusions just drawn from 

 superficial observation. A hundred and fifty years 

 ago P. Lyonet in his monumental work (1762) on 

 the caterpillar of the Goat Moth (Cossus) detected, 

 in the second and third thoracic segments, four little 

 white masses buried in the fat-body, and, while 

 doubtful as to their real meaning, he suggested that 

 their number and position might well give rise to 

 the suspicion that they were rudiments of the 

 wings of the moth. But it was a century later that 

 A. Weismann in his classical studies (1864) on the 

 development of common flies, showed the presence 

 in the maggot of definite rudiments of wings, and 

 other organs of the adult rudiments to which he 

 gave the name of imagined discs. We will recur 

 later to these transformations of the Diptera. For 

 the present, we pursue our survey of changes in the 

 life-history of the Lepidoptera and can take to guide 

 us the excellent researches of J. Gonin (1894). 



Careful study of the imaginal discs of the wings 

 in a caterpillar (fig. 10) made by examining micro- 

 scopically sections cut through them, shows that the 

 epidermis is pushed in to form a little pouch ((7, p) 

 and that into this grows the actual wing-rudiment. 

 Consequently the whitish disk which seems to lie 



