14 MEDICAL ENTOMOLOGY 



of the body. The appendages of these segments are all highly modi- 

 fied for special functions, the first two in connection with the special 

 senses, the remaining three in connection with the mouth. The head 

 in the Diptera is relatively large, but this is not due entirely to the 

 development of the brain, but to the presence in the head of certain 

 large air sacs which have an important function in connection with the 

 sucking apparatus. In addition to the brain and the air sacs, the head 

 contains the pharynx or sucking pump, and the muscles of the head ap- 

 pendages. The shape varies from that of a flattened dome, as in the 

 Tabanidae, to a more or less regular sphere, as in the Muscidae. The 

 antero-superior surface is rounded, and bears the antennae at its most 

 anterior point. The posterior surface is usually flattened, and may be 

 concave. Its lower portion is occupied by the base of the proboscis, 

 while about its middle there is the occipital foramen, to the boundaries 

 of which the membrane of the neck is attached. The foramen is usually 

 supported by a horseshoe-shaped arch. 



The various parts of the head have received special names for descrip- 

 tive purposes. Unfortunately a good deal of confusion exists in the 

 nomenclature of this region, for many of the terms are merely distinctive 

 names employed in systematic work, and can only be properly applied to 

 particular groups of flies, while others have originated in attempts to 

 determine the homology of the head. 



In the Orthorraphic flies the angle at which the posterior and superior 

 surfaces join one another is termed the occiput. The area in front of 



this, which may be conspicuous, as in the Culicinae, 

 Regions of the head . , J \ , 



is known as the vertex. In front of this, and between 



the eyes, is the frons. The eyes occupy the entire lateral regions of the 

 head ; when they are separated from one another by a definite space the 

 fly is said to be dichoptic, while if there is only a very narrow interval 

 the fly is said to be holoptic. The antennae arise at the anterior end of 

 the frons, and may have at their bases a raised plate, as in the Tabanidae, 

 in which it is known as the callus. Below the antennae there is a 

 median plate, generally produced forwards, and forming the distal limit of 

 the anterior surface ; this is known as the clypeus or face. The region 

 below the eyes and external to the clypeus on each side is the gena, or 

 cheek. The median posterior area extending from the base of the pro- 

 boscis towards the occipital foramen is termed the gular region. It is 

 generally membraneous in the Orthorrapha. 



In the Muscoidean flies, among which are the blood-sucking Muscidae, 

 the shape of the head is rather rounder than in the Orthorrapha, and the 



