DIP TER A. 
421 
hind wing; this is the. alula (al'u-la) or alulet (al'u-let). The alulse 
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 alulse 
are called the tegulcz by many writers on Diptera; but the term tegula 
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 Rhypkus (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 
1st and 2d. Thus in Rhyphus , cell V 3 is divided, and the parts are 
designated as the 1st cell V 2 and the 2d cell V 2 (Fig. 497, 1st V a , 2d 
V a ). 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 III 4+5 
(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 
