II] GENERAL MORPHOLOGY 13 



possess a distinctly segmented abdomen with the exception 

 of the Pupipara, a very aberrant group which has become 

 modified for an exclusively parasitic mode of existence. 



General Morphology. 



The head is connected with the thorax by an extremely 

 slender neck and in consequence is very mobile, being capable 

 of undergoing semi-rotation. The majority of the exposed 

 surface is occupied by the large faceted eyes, which are usually 

 much larger in the male than in the female, probably owing 

 to the fact that the former has to seek out its mate. The 

 eyes may actually meet in the middle line, in which case they 

 are termed holoptic, in contradistinction to when they are 

 separate, or dichoptic. The holoptic condition is especially 

 characteristic of the male but occasionally is found in both 

 sexes. 



In certain families, e.g. Muscidae, the head shews anteriorly 

 a small depression, the lunula, which is bounded by an arched 

 suture passing over the bases of the antennae. This structure 

 is the remnant of a peculiar organ, the ptilinum, which, in 

 the fly emerging from the pupal case, appears as a bladder- 

 like expansion of the front of the head. The ptilinum can 

 be expanded and retracted and consequently it is of . great 

 use in helping the fly to break through the pupal case, and 

 also to force its way through any material in which the latter 

 may be buried. It is only present during the first few hours 

 after the fly has emerged and subsequently it becomes com- 

 pletely introverted, forming the inconspicuous lunula of the 

 mature fly (Figs. 1-3)'. 



The antennae are of great use to the systematist as they 

 offer a simple means of classification, the number of segments 

 being of considerable importance. 



In the more primitive families of the Order, e.g. Culicidae, 

 there are a series of segments of approximately equal size, 

 the number of which varies from eight to sixteen. In some 

 cases the antennae of the male are larger and composed of more 

 segments than those of the female. The majority of flies, 



