STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 515 
lateral muscles (/. m.) situated next to the hypodermis and 
attached in a dorso-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 (/. 2. m.); these by their contraction bring about a 
aecrease 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. J.); 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 larvee of the calyptrate muscidz are necessary before we 
can arrive at any definite conclusions concerning the com- 
position of the bodies of these larval forms. 
