374 



IXSECTA. 



the body, the peripodal cavity appears closed and the peripodal 

 membrane is connected with the hypodermis merely by a delicate 

 thread-like stalk (Fig. 184 A, is; Fig. 185 A, st). These connective 

 stalks, which were recognised by Dewitz* (No. 102), who correctly 

 grasped their significance, have a fine lumen, as was shown by Van 

 Rees (No. 121), who has recently studied these structures more 



closely. Although the first 

 development of the imaginal 

 discs in the embryo of the 

 Muscidae is still unknown, 

 Ave shall not err in tracing 

 them back, like those of 

 Corethra, to hypodermal 

 invaginations. We must 

 then regard the stalk-like 

 connection as the loner 

 drawn out neck of this 

 invagination. 



In other respects the 

 development of the ex- 

 tremities (Fig. 184) takes 

 just the same course as in 

 Corethra. The rudiments 

 of the legs increase in size 

 and early show traces of 

 the later segmentation. 

 They appear packed into 

 the peripodal cavity in 

 such a way that the differ- 

 ent joints of the limbs 

 telescope into each other 

 " like the rings of a traveller's drinking cup " (as Van Rees 

 appropriately expresses it). The evagination of the developed 

 limb-rudiments, which occurs on the first day after the commence- 

 ment of the pupal condition, takes place by the shortening of the 

 stalk of the imaginal disc (Figs. 184 B, and 185 B) and the widen- 

 ing of its lumen, which allows the extremity, as in Corethra, to 

 emerge finally through the widening aperture of the peripodal 

 invagination (Figs. 184 C, and 186 A). While the latter at the 

 same time gradually disappears, the peripodial membrane is utilised 



* Kunckel d'Herculais (No. 113) also recognised these strands. 



Fio. 1S4. — Diagrammatic transverse section through 

 the larva and pupa of Musca, to illustrate the 

 development of the wings, the legs, and the imaginal 

 hypodermis (from Lang's Text-book), b, limb- 

 rudiments ; ft., wing-rudiments ; ihy, imaginal hypo- 

 dermis, in D extending from the base of the 

 imaginal discs ; iid, imaginal discs of the wings ; 

 H-v, imaginal discs of the legs; is, strand connecting 

 the rudiment with the hypodermis; IK, chitinous 

 integument of the larva ; Ihy, larval hypodermis 

 (indicated by two thin parallel outlines, while the 

 imaginal hypodermis is represented by thick black 

 lines). 



