CHAPTER III 



SECTION 2 



THE ORDER DIPTERA : ORTHORRAPHA-BRACHYCERA 



THE characteristic feature of this division of the Orthorrapha is the 

 short type of antenna. Usually there are two or three simple joints, 

 with or without a terminal arista, which in itself may consist of one or 

 two joints with a terminal style. There is, however, a good deal of 

 variation in the form of the antenna in the different families. In the 

 Phoridae, for instance, the antenna consists of two or three simple joints, 

 with a dorsally placed arista, thus approximating to the type seen in the 

 Muscoidean flies, while in the Asilidae there are three joints and a 

 terminal style. In the Tabanidae the antenna is specially modified and 

 of an unusual shape. The wing venation is always more complex than 

 that in the nematocerous flies ; a discal cell is always present, and the 

 second long vein is never forked. 



Of the families contained in this division the Tabanidae are by far the 

 most important, both because of their numerical preponderance and of 

 the occurrence of the blood-sucking habit in so large a proportion of the 

 species. In the Leptidae there are three or four species which are said 

 to suck blood, but the habit is the exception rather than the rule in that 

 family. The Asilidae, or ' robber flies ', are well known predaceous in- 

 sects ; certain African species are said to attack man occasionally, though 

 it is doubtful whether they do so with the intent to suck blood. 



FAMILY TABANIDAE 



Large, small, or medium-sized flies, with robust thick-set bodies and 

 large heads, the surface of which is almost entirely occupied by the 

 compound eyes. Antennae porrect and variable in structure, and always 

 three-jointed; the third joint is composed of from four to eight segments 

 more or less distinctly separated from one another. In most of the genera 

 the antennae are shorter than the head ; in Chrysops they are longer. 

 The third joint often ends in a short or long blunt-pointed process. The 

 eyes may be pubescent or bare, contiguous or nearly so in the male, with 

 variously coloured bands in which the facets are large ; in the female the 



