PSYCHE. 



17 



A dissection of an okl larva or a young 

 pupa of either of the insect orders 

 above mentioned would show that these 

 new organs which the insect is to ac- 

 cjuire during its metamorphosis are 

 really already present, not as fully 

 formed organs, however, hut in the form 

 of rudiments or anlagen. In the body 

 cavitv of the caterpillar, for histance, 

 burieil beneath the dorsal meso- and 

 metathoracic integument are two pairs 

 of small disc-like islands of cells. These 

 remain unfunctional and inactive dur- 

 ing the caterpillar's lifetime although 

 growing constantly, but during its meta- 

 morphosis they develop into the two pairs 

 of wings of the butterfl)'.* Similar cell- 

 islands are pi'esent in the larval coleop- 

 teron.f The larvcd hymenopteron also 

 possesses them t ; while beneath its 

 ventral thoracic integument are three 

 other pairs of cell-islands whose fate it is 

 to furnish tlie imaginal legs. In the 

 neniatocera, also, ventral and dorsal 

 pairs of cell-islands are present in the 

 lar\'a, as the observations of Weismann 

 on Corethra \ first showed. This classic 

 investigation demonstrated the presence 

 of three dorsal pairs of cell-islands as 

 well as three ventral pairs. They are sit- 

 uated in the body cavity of the larva just 

 beneath the integiniient, a dorsal and a 

 vential pair in the prothorax, destined 

 to form the pupal spiracles and the 



*Swammerdam. Bibel der Natur. Leipzig 1752, p. 3. Pan- 

 critius. Beitr. z. Keiint. d. Fliigelentw. b. d. Insecten. 

 Konigsberg, 1SS4. 



X Weismaun. Die Metamorphose von Corethra plan 

 i=. Zeit. f. wiss. Zool. 16 Bd. 1S66. p. 45. 



imaginal prothoracic legs, respectively ; 

 a dorsal and a ventral pair in the meso- 

 thorax, destined to form the wings and 

 the mesothoracic legs; and a dorsal and 

 a \'entral pair in the metathorax, des- 

 tined to form the balancers and the 

 metathoracic legs. 



In the brachycera Weismann § was 

 again the first to prove the existence ot 

 these cell-islands. It was in 1S64 that 

 he published in his account of the post- 

 embryonic development of the muscids 

 the first correct and extended observa- 

 tions on these peculiar cell-islands inan}' 

 insect. He called them imaginal discs. 

 He showed that in Musca six pairs are 

 present in the larval thorax, not neai' 

 the surftice as in Corethra, but in the cen- 

 ter of the lar\a, and that their fate is 

 exactly the same as in Corethra. In 

 addition to these thoracic imaginal discs 

 he described two large cephalic discs 

 situated in the forward portion of the 

 thorax and connected with the larval 

 pharynx, the fate of which is to form 

 the imaginal head. Weismann also 

 showed II that but a small portion of the 

 larval body passes directly into the im- 

 aginal body, but that most of it under- 

 goes disintegration so that the different 

 tissues entirely lose their identity, after 

 which the imaginal body is built up from 

 the imaginal discs. To this process, the 

 entire significance of which was not, 

 however, understood until later, he gave 

 the name histolysis. 



§ Weismann, A. Die nacliembryonale Entwick. d. Musci- 

 den nach Beob. an Musca vom. u. Sarcaphoga 

 Zeit. f. wiss. Zool. 14 Bd. Heft 3. 1S64. p. 221. 



