18 MEDICAL ENTOMOLOGY 



same side, making with the termination of the central strand six equal 

 filaments. In Musca nebulo there are similar simple hairs on both sides 

 of the arista. In Glosslna the secondary hairs are confined to one side, 

 and each of them bears a set of five or six pairs of smaller hairs, giving 

 the organ a plumed appearance. The central strand of the arista is 

 termed the flagellum by some authors. 



In other families of Diptera conditions intermediate between these 

 are found. For instance, in the Asilidae, a family of predaceous flies 

 allied to the Culicidae and the Tabanidae, the antenna consists of three 

 joints like those of a Nematocerous fly, and a slender filament which 

 arises from the distal end of the last segment, and projects in line with 

 it ; this filament, the homologue of the arista, is termed the style. The 

 complex antenna of the Muscid flies has, in fact, been evolved from the 

 simple Nematocerous antenna by a concentration of the basal joints and 

 an elongation of the distal. The relation of this change in form to 

 the function of the parts has been discussed by Townsend. Accord- 

 ing to this authority, the sense pits in the terminal joint of the 

 scape are olfactory organs, and it is on account of the high degree 

 of development of the olfactory sense that the third antennal joint has 

 become so greatly enlarged, and has, so to speak, grown away from the 

 rest of the organ. The function of the arista is to act as a sensory and 

 tactile organ, for the protection of the very highly specialized third joint, 

 by giving warning to the fly when it approaches obstacles. He suggests 

 an interesting relation between the plumosity and the joints of the arista. 

 In those forms in which the arista is plumed only on one side, the 

 hairs are numerous and complex (cp. Glossina), and the small joint 

 between the arista and the scape is not well developed, the arista ap- 

 pearing to arise directly from the scape and to be capable of little or 

 no movement. On the other hand, in those forms in which the arista is 

 feathered on both sides the hairs are generally simple and comparatively 

 few in number, while the joint between the arista and the scape is well 

 developed, and is obviously capable of free movement. The range of 

 movement of the arista will thus counterbalance the paucity of the 

 sensory hairs, and a fly with a stationary arista, but with a complex 

 arrangement of sensory hairs, is as well equipped as one with few hairs, 

 but a wide range of movement. 



In the Pupipara the antennae are very much reduced, and are a little 

 difficult to recognize. The scape consists of a single joint, roughly oval 

 in shape, and enclosed in a deep chitinous pit on the surface of the head 

 capsule, from which it can be partially protruded by the action of 



