EXTERNAL ANATOMY. 3 



1 . EXTERNAL ANATOMY OF A FLY. 



A. Terminology.* 

 The Head. 



The upper part of the head lying between the eyes is known 

 as the vertex, and an imaginary line dividing it from the back 

 of the head, or occiput, is called the vertical margin. Sometimes 

 a difference of colour clearly defines the limits of these areas, but 

 more frequently such is not the case. 



Thefro)is, or front, is the whole space between the eyes from 

 the vertical margin to an imaginary transverse line above the 

 base of the antennae. The vertex is really therefore simply the 

 upper part of the frous, but it is often occupied by a somewhat 

 conspicuous tubercle, or by a more or less triangular impressed 

 space with rounded corners, in which are situated the three small 

 simple eyes known as the ocelli, placed in a triangle, the apex 

 of the latter pointing downwards, that is towards the antennae. 

 These ocelli are in many families absent, in others reduced to 

 two in number, and they may, in a few groups, be placed more or 

 less in a straight line across the upper part of the frons. 



Those Diptera in the males of which the eyes touch each other 

 in front for any considerable distance, are termed holoptic ; those 

 in which the eyes in both sexes are separated by a broad frons, 

 are termed dichoptic. Although many cases occur in which it is 

 difficult to decide in which category to enrol them, the vast 

 majority of Diptera can be allotted at first sight to either the 

 holoptic or dichoptic class.t 



In holoptic Diptera, through the eyes not being contiguous 

 absolutely the whole distance from the vertex to the antennae, 

 there is always a small inverted triangle just above where the eyes 

 meet, its apex pointing downwards, and this is called the vertical 

 triangle. At the point where the eyes separate again, a little 

 above the antennae, is another more or less triangular space with 

 its apex pointing upwards, and this is called the frontal triangle. 

 Obviously, in flies with the eyes wide apart, both triangles are 

 merged in the frons and in such cases cannot be differentiated 

 from it. 



Cyclorrhaphic Diptera technicaUy possess what is known as 

 the frontal suture, and the presence or absence of this suture is 



* This section defines the principal general terms in constant use; the more 

 specialized ones are explained at more suitable places throughout the text, but 

 all are, without exception, to be found in the Glossary. 



t The majority of the BRACHYCERA, except ASILID^E and DOLICHOPID*, 

 speaking of the larger groups only, are holoptic, as are also the Calyptrate 

 MUSCIDJS as a rule, and the KHYPHID.E, BIBIONID.E, and some BLEPHAHOCERI.E 

 in the NEMATOCERA. The remainder of the NEMATOCERA, with the ASILID^, 

 DOLICHOPID^E and Acalyptrate MUSCID^E, are usually dicboptic, at least in the 

 more extensive groups. 



B2 



