174 THE BIOLOGY OF INSECTS 



wall. These '' imaginal discs " as they are now called, have 

 been detected in all metamorphic insects whose develop- 

 ment has been carefully traced. J. Gonin, for example, 

 has shown (1894) ^^^^^ i^ ^^^ White Cabbage Butterfly 

 (Pteris) they arise as thickenings beneath the skin, and grow 

 inwards as they increase in size in such a way as to form 

 little flattened hollow pads lying in thin-walled pouches 

 continuous with the skin whence they originate ; thus, 

 although they are situated within the body, they retain 

 their primitive connection with its outer wall. Branches 

 from the air- tube system grow into them, prefiguring the 

 main features of the nervures in the developed wing. After 

 the last larval cuticle has separated from the skin in prepara- 

 tion for the final moult of the caterpillar, these wing-buds 

 grow very quickly and are thrust out from their pouches ; 

 thus projecting from the surface they become covered with 

 cuticle, and so the wings are apparent when, the moult 

 completed, the pupa is revealed. A similar mode of wing- 

 growth has been traced in the larva of a Lady-bird Beetle 

 {Hippodamia) by Comstock and Needham (1899), some of 

 whose drawings are reproduced here (Fig. 46). Wing- 

 buds of essentially the same type can be demonstrated in 

 the grub of the Honey Bee (Fig. 50,/, h). 



Not only the wings, but all the organs of the winged 

 adult that become apparent in the pupa, arise in the larva 

 as imaginal discs, often sinking within the body but remain- 

 ing connected by strands of tissue with the skin whence 

 they first develop. Up to the third larval stage the leg of a 

 caterpillar may be cut off without damage to the corre- 

 sponding limb of the adult, but if such mutilation be 

 perpetrated later in the course of development, the tip of 

 the shin and the foot of the imaginal leg will be removed, 

 as these then project into the cavity of the larval leg, though 

 the basal region of the limb is sunk in a lateral depression 

 of the body. The transformation of the internal organs 

 differs in nature and degree in the various systems of organs 

 and in the various orders of insects. Generally it may be 

 stated that the nervous system, the heart, and the ovaries 



