l8 DIPTERA [CH. 



Counting forwards from the third longitudinal, which is 

 forked, occur the second and first longitudinals, these three 

 veins arising from a common stem and radiating to the costal 

 vein, which runs round the edge of the wing. Between the 

 costal vein and first longitudinal, is present an auxiliary, or 

 subcostal vein, which runs parallel with the costal for a short 

 distance and then curves into it. Near the base of the wing 

 the two latter veins are connected by the short humeral cross- 

 vein, which is very constant. The subcostal, and the first 

 three longitudinals support the anterior half of the wing. The 

 fourth and fifth longitiidinals arise together and diverge towards 

 the margin ; each of them forks before reaching the edge of the 

 wing. The sixth longitudinal arises more or less independently, 

 and runs towards the hind margin of the wing. The three 

 latter veins together support the posterior half of the wing. 

 The fourth and fifth longitudinals are connected by two cross- 

 veins, the anterior basal near the axil and the posterior cross- 

 vein in the outer half of the wing. Between the fifth and sixth 

 longitudinals is a posterior basal cross-vein. It will be seen 

 that the wing is divided by its longitudinal nervures into two 

 fields separated by an interval which is traversed only by the 

 anterior cross-vein. 



The various spaces or cells included between the veins are 

 distinguished by special names ; their arrangement may be 

 understood by referring to the figure (Fig. 5). 



In addition to this system of nomenclature, that proposed 

 by Schiner is used by a certain number of writers. One great 

 source of confusion between this and the older system is the 

 different meaning of the subcostal ; Schiner employs the term 

 mediastinal for the subcostal, and subcostal for the first longi- 

 tudinal ; moreover, the corresponding cells are similarly named. 



The surface of the body in flies differs considerably in the 

 nature of its vestiture. Sometimes the integument is almost 

 naked, but generally it is covered with hairs, or bristles, or 

 sometimes scales. Those flies which are provided with an 

 armature of bristles, or macrochcBtcB, may be termed chceto- 

 phorous ; and where there is no definite arrangement of 

 bristles, the fly is said to be eremochcEtous. Osten Sacken 



