MOSQUITOES, MIDGES AND FLIES. 329 



Fig. 6 b), and in a group of flies there is a little 

 winglet called a squama (Plate 40, Fig. 6a), which 

 is at the very l^ase of each wing, and which may 

 cover or partly cover the halteres. The house fly and 

 blow fly have well-developed squama. 



Tlie thorax is very solid and all in one piece — 

 that is, one cannot easily distinguish the three 

 tlioi-acic segments. 



Tlie head is usually well separated from the 

 thorax, and can be moved freely. The eyes are 

 very large, and in most male dipterous insects meet, 

 forming the greater part of the head. The eyes 

 are compound, being formed of numerous facets, 

 which may vary in size according to their position. 

 It is the efficiency of these eyes that makes it fairly 

 hard to catch most flies. Three small ocelli, or 

 single eyes, are also present. 



The antennse are of three types — two widely 

 differing in form (1 and 3), and an intermediate 

 type (2) linking the two extremes. (Plate 40, 

 Figs, b and c.) TJie first type is the plumose, with 

 wliorls of longer or shorter hairs. These can be 

 seen in gall gnats, midges, and mosquitoes. (Plate 

 40, Fig. 7 a.) The third type is curious. There are 

 usually only four segments. The three basal ones 

 are more or less solid, the fourth and upper joint 

 (which is probably formed by the fusing of many 

 smaller joints) is produced in the form of a bristle 

 or arista. In many flies, such as the house fly, 

 stable fly, fruit fly, the third joint enlarges 

 and by unequal growth forms a thick, rounded- 

 oblong segment or lobe, and the bristle is reduced 



