34 ENTOMOLOGY FOR MEDICAL OFFICERS 



Between the eyes lie the antennae. The space between the 

 eyes above the antennae is the front ; that between the root 

 of the antennae and the upper margin of the mouth is the 

 face ; that behind and below the eye is the cheek. In addition 

 to the compound eyes many flies possess small simple eyes, 

 or ocelli, which are generally three in number and are set in 

 a triangle, apex forwards, on the top of the head near the 

 occipital margin. 



The antennae (Fig. i) are one of the principal means of 

 classifying flies. In one great group, of which the mosquito 

 is a good example, the antennae, like those of most insects, 

 are long and slender, and are composed of numerous segments 

 all of which, except one or two at the base, are alike. In 

 another great group, of which the common house-fly is a 



FiQ. 1.— AntennsE.- a, Mosquito; 6, House-fly; c, Gadfly; d, Asilid. 



good example, each antenna consists of 3 dissimilar seg- 

 ments, the terminal one being much produced ventrally, 

 and bearing far back on its true dorsal surface a feathered 

 bristle, or arista (Lat. arista = the awn of an ear of corn). 

 Between these two extremes there is a series of gradations ; 

 for instance, in the gadfly the antenna consists of 3 dis- 

 similar segments, the third of which is compounded of several 

 firmly united rings ; and in certain other flies the antenna is 

 formed of 3 dissimilar segments, the third of which may 

 carry a terminal or subterminal bristle, or a slender stfle 

 which may be compound. 



At the lower front of the head is the mouth-cavern, which 

 is usually roofed in by the clypeus. The visible mouth-parts 

 are the "proboscis" and the maxillary palps, and in some flies 

 these can be withdrawn almost out of sight into the mouth- 

 cavern. There is one family of flies — the CEstrid<^—\x\. which 



