10 THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



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, i-epresents the fused gula and basal portions of 

 the greatly modified second maxillae. The occipital segment 

 is bounded laterally by the genae (Lowne's paracephala) and 

 dorsally by the epicranium. These parts have been divided by 

 systeraatists into so many regions that a somewhat detailed de- 

 scription will be necessary to make their boundaries clear. 



The genae bear the large compound eyes which occupy almost 

 the whole of the antero-lateral region of the head. On the pos- 

 terior flattened surface of the head the genae 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 gena ; it is of a silver to golden metallic lustre. On the 

 ventral side below the eye each gena bounds the probo.scis 

 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 

 genae are in contact with the clypeus, there are two promi- 

 nent ridges bearing strong setae ; these are usually known 

 as the "facialia." Berlese (1909) regards the facialia as repre- 

 senting a portion of the fourth (mandibular) segment. They 

 are certainly distinct from the genae, as may be seen in the 

 head of the newly formed nymph (fig. 43). Strictly speaking 

 they are both facialia and peristomalia, facio-peristomial sclerites 

 in fact. 



The epicranium (epicephalon of Lownc) on the posterior sur- 

 face of the head is flat. On the anterior surface it is convex, 

 and divided into a number of regions. On the top of the head 

 between the eyes it is called the vertex. This contains the three 

 ocelli situated on a slightly raised ocellar triangle, which is sur- 

 rounded by a second triangle, the vertical triangle. The median 

 region in front of and below the vertex is the fi'ons. In the 

 middle of this there is a black frontal stripe. The frons appears 



