42 THE LIFE-STORY OF INSECTS [CH. 



five-segmented foot project into the cavity of the larval 

 leg. Hence we understand that the amputation of 

 the latter by the old naturalists truncated only and 

 did not destroy the imaginal limb. In the blow-fly 

 maggot, Weismann, B. T. Lowne (1890) and J. Van 

 Rees (1888) have shown that the imaginal discs of the 

 legs (fig. 1 1 1, 2, 3) grow out from deep dermal inpush- 

 ings. Simple at first, these outgrowths by partial 

 splitting, become differentiated into thigh and shin. 



Similarly the feelers and jaws of the butterfly are 

 developed from imaginal discs, and this fact explains 

 how it comes to pass that they differ so widely from 

 the corresponding structures in the caterpillar. The 

 larval feelers (fig. 3 At) are short and stumpy, those 

 of the butterfly long and many-jointed. The maxilla 

 of the larva (fig. 3 MX) consists of a base carrying 

 two short jointed processes; in the butterfly a certain 

 portion of the maxilla, the hood or galea, is modified 

 into a long, flexible grooved process, capable of 

 forming with its fellow the trunk through which the 

 insect sucks its liquid food (fig. 2). Nothing but 

 some such provision as that of the imaginal discs 

 could render possible the wonderful replacement of 

 the caterpillar's jaws, biting solid food, into those of 

 the butterfly sipping nectar from flowers. 



A curious segmental displacement of the imaginal 

 discs with regard to the larva is noticeable in 

 some Diptera. In the larva of the harlequin-midge 



