70 ON THE ANATOMY OF THE FLY. 



only very little lateral movement is allowed, from the manner in which 

 the muscles are inserted into it. 



The muscles which move the coxae and femora arise from the back, and 

 form the vertical groups of muscles, which occupy the lateral regions of 

 the thorax. A pair are inserted by fleshy insertions into each portion of 

 the coxa, and another pair terminate in tendons, and are inserted into the 

 proximal extremity of each femur, one into its upper or outer, and one 

 into its lower or inner margin, the former being a flexor and the latter 

 an extensor of the limb. The tendons of the anterior femoral muscles 

 receive fleshy slips, which arise within the anterior coxae, and are 

 inserted into them in a bipenniform manner. 



Each of the other joints, except those of the tarsus, gives rise to a pair 

 of bipenniform muscles, a flexor and extensor of the succeeding joint. The 

 muscle, which moves the terminal joint of the tarsus, arises in the upper part 

 of the femur, and is connected with the last tarsal joint by a long thread- 

 like apode'me. The apode'me is connected with the proximal extremity of 

 each tarsal joint by a membranous slip, and terminates in a triangular 

 plate, which bears the pulvilli and claws. This plate is grooved below by 

 a number of parallel grooves on either side of the mesial line. These 

 are seen very frequently in the feet of insects. It terminates in a long 

 blunt spine, and supports the pulvilli beneath, as well as the claws above. 



The last four tarsal joints are occupied, as has been already stated,* by a 

 sac, which secretes a viscid fluid. As I have only succeeded in seeing 

 this sac in the manner already indicated, I cannot say whether it is single 

 or double ; neither can I tell whether the fluid secreted by it is conveyed 

 into the pulvilli by ducts or by an opening in the neck of the sac, or 

 whether it passes into it simply by transudation. The great delicacy of 

 the secreting sac and the opacity of the tarsus renders it exceedingly 

 difficult, if not impossible, to make out these details in the fly. 



Section VIII. The Abdominal Segment*. 



The abdominal segments are far less complicated than those 

 of the thorax. They are nine in number, but the last four are 

 modified to form the sexual organs, these will be described in 

 their proper places.f 



* Page 19. f Page 104. 



