DTJBHAM DIPTEEA. 1 7 



(C.II.2.) The vein above it is V.3, the one below it is 

 generally V.4. The main stem of the upper 

 system is V.I., that of the lower is V.5. The vein 

 between V.I. and V.3., if there be one, is V.2. If 

 V.3. springs from V.I., then V.2. is absent. The 

 vein below V.5. is V.6. Sometimes there is 

 another vein V.7., which rises independently in 

 the base of the wing. 



(3) Regions. — The wing is divided longitudinaly into a 



front and hind region by an imaginary line drawn 

 from the base to the tip, through the cross-vein 

 (X.4.) ; and transversely into a basal and apical 

 region by a line across the wing, from the middle 

 of the fore-margin and at right angles to it; or 

 into four independent regions, the fore and hind- 

 basal regions, and the. fore and hind-apical regions. 



(4) The Longitudinal Veins. — The vein which bounds 



the wing at the margin is called the costa (C). It 

 sometimes stops suddenly near the tip of the 

 wing, sometimes it is attenuated round the hind- 

 margin. It generally bears short bristles — 

 ciliated — with sometimes a row of strong thorns 

 among them — spined. The setula is the strong 

 spine on the costa at the end of V.I .a, Each 

 division of the costa is numbered according to 

 the number of the cell which it bounds. C.I. a. 

 is the costa from X.I. a. to the end of V.I .a., &c. 

 V.I. The first vein below the costa, always present 

 and strong. It often consists of two long 

 branches. 

 V.I .a. The upper branch of V.I. usually weak and 



often absent. 

 V.l.b. The lower main branch of V.I. when V.I .a. is 



present. 

 V.2. It springs from the lower side of V.I., and is 

 often absent. 



B 



