THE ANTENNAE 17 



number of joints, set one behind the other. Their appearance differs 

 greatly in the different genera of Diptera, but in general there are 

 two forms, one met with in the Orthorrapha and the other in the 

 Cyclorrapha. The distinction between the two forms was used by 

 the older systematists to divide the Diptera into the two primary divi- 

 sions known as the Nematocera and the Brachycera, which correspond 

 roughly with the two divisions named above (see Chapter III). In the 

 Nematocera all the joints of the antenna are similar to one another, 

 except one or two at the base, which are usually larger. There may be 

 from four to sixteen joints, each cylindrical or sub-cylindrical in shape, 

 and generally tapering a little from base to apex. The joints may be 

 adorned in various ways, and the adornment may differ in the two sexes, 

 thus offering a ready means of distinguishing them. They may be plu- 

 mose in both sexes, as in Phlebotomus, or plumose in the male and not in 

 the female, as in Culex. The whorls of hairs are attached to the base of 

 each segment. In the Brachycerous flies the antenna consists of a small 

 number of dissimilar joints, of which the basal ones are the most modified. 

 In the Tabanidae, which are Brachycerous Orthorrapha, the apical joint 

 is much stouter than the rest, and may be provided with a hook-like pro- 

 jection ; the distal part may be ' ringed,' or incompletely divided into a 

 number of segments. In the Cyclorrapha, which includes all the blood- 

 sucking Muscidae, such as Philaematomyia and Glossina, the form of 

 the antenna is highly modified from the original shape ; it is composed 

 of two distinct portions, the scape and the arista. The scape represents 

 three basal joints. The first two of these are small and inconspicuous, 

 but the third is very much enlarged and forms a conspicuous oval mass, 

 which hangs down on the front of the head. It may be adorned in 

 various ways, with short downy hairs or with short stout bristles, and it 

 always shows on its anterior surface several 'sense pits', which, when 

 seen in section, are found to be areas where the chitin is thin, and where 

 there is a large accumulation of nervous tissue. In cleared preparations 

 they appear as small circular clear spaces. The arista is derived from the 

 terminal segments of the antenna. It is generally described as consisting 

 of a single stout forwardly directed bristle, arising from the proximal end 

 of the terminal segment of the scape, but a closer examination shows 

 that at the junction of the arista with the scape there is interpolated 

 another small joint. The arista may bear different kinds of smaller 

 hairs, arranged in various ways, and its appearance is an important 

 factor in classification. In Lyperosia minuta (in which three joints can be 

 distinguished in the arista), there are five simple hairs arising from the 

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