384 



INSECTA. 



a layer of small cells (/), perhaps derived from the imaginal discs, 

 as well as by a gelatinous envelope, forms the so-called yellow 

 body which, until its disintegration, remains lying in the pupal 



intestine. The larval muscular coat (Im) 

 remains intact so long as the imaginal 

 mid-gut is not completely developed, 

 but is afterwards attacked by phagocytes 

 and destroyed. The permanent muscular 

 layer develops from single cells lying on 

 the outer surface of the imaginal discs 

 (Figs. 189, im and 190, m), which must 

 be described as special imaginal cells of 

 the intestinal muscles. 



The metamorphosis of the fore-gut is 

 commenced by the degeneration of the 

 proventriculus and of the sucking stomach. 

 The proventriculus or gizzard (Fig. 191, 

 pr), which seems to be formed by a 

 process of infolding or intussusception of 

 the fore -gut, degenerates through the 

 flattening out of this fold. The sucking 

 stomach also degenerates in a similar 

 way, retreating more and more into the 

 oesophagus, so that, in place of the 

 original diverticulum, there now only 

 remains a widening of the lumen of the 

 oesophagus. At the same time, this part 

 of the gut is attacked and disintegrated by phagocytes, while the 

 disorganised parts are replaced by the gradually extending imaginal 

 portions of the wall. The imaginal ring of the fore-gut (Fig. 191, 

 im), which, according to Kowalevsky, undertakes the formation of 

 a great part of the permanent oesophagus, closes posteriorly, so that 

 the communication with the mid-gut seems to be interrupted. 



The transformation of the hind -gut takes place in a similar 

 manner. Here also the imaginal ring extends so as to form a tube 

 which, growing round the opening of the Malpighian vessels, closes 

 towards the mid-gut, while posteriorly it is connected with the 

 disintegrating proctodaeum. In a similar way, the territory of 

 the proctodaeum is circumscribed by an imaginal tube 



Fig. 191. — Longitudinal section 

 through the larval proventri- 

 culus of one of the Muscidae 

 (after Kowalevsky). im, 

 stomodaeal imaginal riDg ; oe, 

 oesophagus ; pn, proventri- 

 culus. 



growing 



from behind, formed by one of the imaginal discs found in the 

 neighbourhood of the anal aperture, till finally, when the whole of 



