DITTERA. 



421 



hind wing; this is the alula (al'u-la) or alulet (al'u-let). The alulae 

 are well developed in the common House-fly. Each alula, in those 

 species where the alulae are well developed, consists of two lobes 

 which fold over each other when the wings are closed. The alulae 

 are called the tegula by many writers on Diptera; but the term tegula 



TII4+S 



(V, 



VII i 



' VII2+ IX " 



FIG. 496. Wing of Conops: ae, axillary excision ; /, posterior lobe. 



was first used in insect anatomy for the cup-like scale which covers 

 the base of the wing in certain insects, as most Hymenoptera, and 

 should be restricted to that use. The terms alula and alulet are also 

 often misapplied, being used to designate the posterior lobe of the 

 wing. 



The plan of the venation of the wings can be easily learned by a 

 study of the wing of Rhyphus (Fig. 497), which is very generalized in 

 structure, except that vein III is only three-branched, while in cer- 

 tain still more generalized forms it is five branched (e.g., Protoplasa, 

 Fig. 504; and Psychoda, Fig. 500). In the figures of wings in this 

 chapter both the veins and the cells are numbered. The numbers 

 outside of the margin of the wing refer to the veins ; those within, to 

 the cells, except when otherwise indicated by a dotted line or by an 

 arrow. It should be remembered that each cell bears the same num- 

 ber as the vein that forms its front margin when the wings are spread. 

 When a cell is divided by a cross-vein the two parts are numbered 

 ist and 2d. Thus in Rhyphus, cell V 2 is divided, and the parts are 

 designated as the ist cell V 2 and the 2d cell V 2 (Fig. 497, ist V 2 , 2d 

 V 2 ). A cross-vein is marked cv. 



In the Diptera veins IV and VI are not developed. Vein I ex- 

 tends along the costal margin of the wing ; it usually ends somewhere 

 near the apex of the wing; in Rhyphus it ends at the tip of vein IIL+s 

 (Fig. 497). In some families it extends entirely around the wing; it 

 is then called the ambient vein. Vein II is simple. Vein III is typi- 

 cally five-branched; but the number of branches is usually reduced to 



