(7) 



STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 401 



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 membrane. 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. 



The occipital ring is surrounded by the four plates, which 

 make up the sides and back of the head capsule. On the 

 ventral side, between the occipital ring and the aperture 

 from which the proboscis depends, a median basal plate, the 

 gulo-mental plate, represents the fused gula-and basal por- 

 tions of the greatly modified second maxillae. The occipital 

 segment is bounded laterally by the gense (Lowne's para- 

 cephala) and dorsally by the epicranium. These parts have 

 been divided by systematists into so many regions that a 

 somewhat detailed description will be necessary to make 

 their boundaries clear. 



The genge bear the large compound eyes which occupy 

 almost the whole of the antero-lateral regions of the head. 

 On the posterior flattened surface of the head the gense are 

 flat, and extend from the gulo-mental plate to the epicranial 

 plate, the sutures of the latter being vertical. On the dorsal 

 side each sends a narrow strip between the inner margin of 

 the eye and the epicranium ; this strip surrounds the eye 

 and meets the ventral portion of the geua ; it is of a silver to 

 golden metallic lustre. On the ventral side below the eye 

 each gena bounds the proboscis aperture laterally ; a number 

 of stout bristles arise from this margin and also from its 

 antero-lateral region, which is often spoken of as the " jowl." 

 In the anterior region, where the gense are in contact with 

 the clypeus, there are two prominent ridges bearing strong 

 setae ; these are usually known as the f ' f acialia." 



