8 



INTRODUCTION. 



extent. Often it is absent, and occasionally it is present when the 

 pulvilli themselves are wanting. 



The anterior legs are the four front legs taken together, the 

 posterior legs the four hind legs similarly considered. When 

 spoken of singly they are called the fore (front or first) pair ; 

 middle (median or second) pair; and the hind (or third) pair. 



Fig. 3. — Wing of Limnophila. 



The Wings. 



The Veins. — As a typical wing from which to illustrate the 

 venation,* having special reference to the Nematoceba, that of 

 Limnophila has been chosen. t 



The front margin of the wing is known as tlie casta, and is 



* The term neuration was employed by many of the older authors instead of 

 venation, but the latter is at present in almost universal use. 



t It must be understood that a diagrammatic wing to expound the whole 

 theory of venation throughout the Diptera is an impossibility, and the wing of 

 Limnophila is selected because it possesses the greatest number of cells and 

 veins in tiie Nematoceka. The relative lengths and positions of these are given 

 in the main as for Livinophila, but it must be understood that these propor- 

 tions vary in different groups and to a still greater extent in different families. 

 The student should be able, after mastering the terminology of one family, to 

 identify the corresponding veins in others, and to recognise which are present 

 and which are absent or modified. 



