331 



Tli€ post-defaecation period is marked by a continued 

 orowth. of these imaginal rudiments, the completion of the 

 process being marked by the last larval moult. 



The head integument bulges outwards, and the biseg- 

 mental condition disappears (fig. 12). The head grows, especi- 

 ally in height, while above and below the posterior head 

 integument grows inwards to form the nearly vertically 

 sloping occiput. As a result of these changes, and probably 

 also on account of the pressure exerted upon it by the over- 

 lying larval cuticle, the head adopts the curious retracted 

 attitude so characteristic of the insect. The second segment 

 may also be observed, at the end of larval life to be partly 

 invaginated into the third. In Calliphora this condition is 

 much more pronounced. 



The antennae have meanwhile been growing greatly in 

 size. Originally forward outgrowths from the upper region 

 of the first segment, the pressure of the larval cuticle soon 

 forces them to turn back upon themselves and downwards ; 

 in the post-defaecation period they grow greatly in length 

 and thickness, and at the time of pupation, are in the form 

 of two thick appendages, lying ventrally and extending two- 

 thirds the distance down the thorax (fig. 7). 



The mouth parts meanwhile continue to^ grow in size, 

 the turning downw^ards of the head, as already described, 

 forcing these into the position in which we see them in the 

 imago. Shortly before the larva undergoes its last moult they 

 cease to grow, and develop a cuticle on their surface. They 

 are now large thick ungainly structures (fig. 14), in no way 

 resembling the neat, specialized mouth parts of the imago. 

 The labium is a triangular, irregular flap overhanging the 

 mouth; the mandibles are a pair of irregular, ''shapeless" 

 masses, each bearing its palp, w^hich has now, however, 

 greatly degenerated and is little more than a tubercle on 

 the mandible. The maxillae are nearly as large as the 

 mandibles and project forwards ; the palps exceed the maxillae 

 in size and are extremely prominent. The labium is quite 

 large and from its posterior part project the palps. The 

 second antennae have disappeared. 



The remainder of the development of the mouth append- 

 ages, during the pupal period, consists of a very pronounced 

 shrinking of the structures within the cuticle which they have 

 secreted, as a result of which they gradually assume their 

 adult shape (fig. 14). 



This process commences a few hours after pupation, 

 and within t'wenty-four hours is practically complete; seg- 

 mentation of the appendages has become very marked, and 

 bristles are developing on them. The proximal portion of 



