DIPTERA _ 289 



terior lobe (Fig. 499, 1). In certain families there is a membrane beneath the 

 base of the wing and above the halter or rudimentary hind wing ; this is the 

 alula or alulet. The alulae are well developed in the common housefly. 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 



R< /?j+i 



2d A+Cu, 



O* 



Fig. 499. — Wing of Conops. 



called the tegulce by many writers on Diptera; but the term tegula 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 re- 

 stricted to that use. The terms alula and alulet are also often misap- 

 plied, being used to designate the posterior lobe of the wing. 



R> R,+> 



id A 



FlG. 500. — Wing of Anisopus. 



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

 study of the wing of Anisopus (Fig. 500), which is very generalized in 

 structure, except that vein R is only three-branched, which in certain 

 still more generalized forms is five-branched (e.g., the primite wing of 

 Psychoda, Fig. 506). In the figures of wings in this chapter both the 

 veins and the cells are lettered. The letters 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 letter as the vein that forms its front mar- 

 gin when the wings are spread. When a cell is divided by a cross-vein 

 the two parts are numbered 1st and 2d. Thus in Anisopus, cell M 2 is 

 divided, and the parts are designated as the 1st cell M 2 and the 2d cell 

 M 2 (Fig. 500, 1st M 2 , 2d M 2 ). A cross-vein is marked by an arrow. 



