CHAPTER II 



THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



Previous to this study the only complete account which has 

 been published on Muscid anatomy was LoAvne's comprehensive 

 monograph (1895) on the blow-fly, Calliphora erythrocephala, 

 which is an elaboration of his smaller and earlier memoir 

 (1870). Not only are many of Lowne's conclusions untenable, 

 but the value of his work as a comparative study would increase 

 with confirmation. 



The Head Capsule. 



The head capsule of M. domestica presents great modifica- 

 tions when compared with the typical insect head. Considerable 

 difficulty is experienced in explaining its structure in the morpho- 

 logical terms employed in the simpler orders of insects. Lowne 

 did not lessen the difficulty in describing the head of the blow-fly 

 by the invention of new terms of little morphological value. The 

 head of the fly is strongly convex in front, the posterior surface 

 being almost flat and slightly conical. For the sake of clearness 

 the composition of the head capsule will be described from behind 

 forwards. The occipital foramen occupies a median slightly ventral 

 position on the posterior surface. It is surrounded by the occipital 

 ring, the inner margin of which projects into the cavity of the 

 head. From the sides of the inner margin of the occipital ring 

 two short chitinous bars bend inwards and approach each other 

 internally, forming a support — the jugum — for the tentorial mem- 

 brane. On each side of the occipital ring below the jugum a 

 small cavity occurs, into which a corresponding process from the 

 prothorax fits, forming a support for the head. 



