4 INTRODUCTION. 



theoretically the only decisive character by which to distinguish 

 members in the imago state, of the CYCLOERHAPHA or OETHO- 

 BEHAPHA respectively ; but as three of the principal families do 

 not possess the suture when they should theoretically do so, the 

 value of the character from a practical point of view is very 

 greatly diminished. This suture, when present (it is never found 

 in the OBTHOEEHAPELA), is a more or less oval, or most frequently 

 crescent-shaped, narrow groove, sometimes reduced to an impressed 

 line, and is situated immediately above the base of the antennae, 

 and the small space it encloses is termed the frontal lunule. 

 Through this space is thrust out during the pupal stage, the 

 ptilinum, an inflatable organ by means of which the perfect insect 

 springs off the cap of the pupa-case to enable it to emerge. 

 Occasionally this ptilinum remains inflated in the perfect insect, 

 as I have seen several specimens of Syrphus with it still blown 

 out even in the dried insects. 



In many Cyclorrhaphic Diptera, especially among the higher 

 Muscids, there is a well defined band, varying from a very narrow 

 space to one of considerable width, on the inner or frontal side of 

 the eyes, often slightly different in colour from the rest of the 

 frons, not infrequently appearing brilliantly shining white when 

 viewed from above or below. This is known as the frontal ocular 

 orbit, or inner orbit ; other margins of the eyes are spoken of as 

 the facial (below the antennas) and posterior orbits respectively. 

 These orbits are rarely present in the NEHATOCEBA. 



Below the antennae, as far as the mouth, is the face, and 

 posterior to the face below the eyes are the cheeks or gence. 



The epistome, or epistoma, is a slightly vague term, but strictly 

 speaking it means the mouth-opening, and an indefinite space 

 immediately contiguous thereto. It is probable that a good many 

 authors have used the term to include the whole of the lower 

 part of the head when not dealing with any particular part ; in 

 fact, I have used the term myself in this general sense when a 

 very brief description sufficed for the whole of the underside of 

 the head. 



Many groups of Diptera possess a system of bristles about the 

 head as well as on the thorax, but since this does not occur in the 

 NEMATOCERA, the subject is not entered upon here.* 



The Thorax. 



The thorax in the Diptera is seldom divisible into the component 

 parts prothorax, mesothorax, and metathorax but forms a uni- 

 form oval, oblong, elongate, or sub-quadrate central piece in the 

 front of which the prothorax is sometimes discernible, though in the 

 NEMATOCERA, especially in the TIPULID^E, it is often obvious in the 

 form of a circular flattened disc, termed by Osten Sacken the collare. 



The front part of the thorax when elongated to any appreciable 

 extent is termed the neck, but in some families the head is too 



* See Glossary, Chtetotaxy. 



