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 STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 515 



lateral muscles (/. w.) situated next to the hypodeimis ar.d 

 attached in a dorse-ventral position; these will assist in draw- 

 ing the dorsal and ventral regions of the segments together 

 and so increase the length of the larva. Between segments 

 four and five and the remaining segments to twelve there is, 

 on the intersegmental ring, a pair of lateral intersegmental 

 muscles (I. i. m.) ; these by their contraction bring about a 

 decrease in the size of the intersegmental ring and so assist 

 the lateral muscles in increasing the length of the larva. 



The muscles of the last segment (xiii) are not regularly 

 arranged as in the preceding segments ; they consist of three 

 main groups : (1) the recti muscles, which assist in contract- 

 ing the segments ; (2) the anal muscles (an. m.), which are 

 attached ventrally to the anal lobes (an. I.) ; and (3) the 

 dorso-ventral muscles (d. v.), which by their contraction 

 assist in lengthening the segment. In addition to these there 

 are certain small muscles in relation with the posterior 

 spiracles. 



In the second and third segments the recti muscles are 

 reduced to four pairs and the attachment of the two lateral 

 and external pairs of muscles has led me to regard the 

 apparently single first post-oral segment as consisting of two 

 segments; it is not a single post-cephalic or pro-thoracic seg- 

 ment as it has been called. There is quite a distinct internal 

 division and the external constriction has been already 

 noticed. This view does not necessarily alter the homology 

 of the third segment, which may still be regarded as pro- 

 thoracic if this is desirable. The segment which I regard as 

 the second body-segment may be a rudiment of the cephalic 

 region which has been almost lost, and this loss, or, as I 

 prefer to regard it, this withdrawal of the head, only serves to 

 make any discussion as to the homologies of these anterior 

 segments with those of the adult extremely difficult, and, I 

 believe, at present valueless. Further, comparative studies 

 of the larvae of the calyptrate muscidse are necessary before we 

 can arrive at any definite conclusions concerning the cpm- 

 position of the bodies of these larval forms. 



