34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



itw The joint is mainly olfactory, and certainly highly sensory. As 

 such it is highly important to the insect. The arista is directed for- 

 ward, outward, and downward from its insertion on the anterior 

 basal edge of the joint. This would indicate that it is primarily 

 functional as a tactile sensory organ for the protection of the highly 

 functional third joint. Such an indispensable organ in the economy 

 of the insect as the third antenna! joint would naturally demand the 

 presence of some tactile sense organ extended before its exposed sur- 

 faces, to serve as a warning against contact with foreign objects. 

 In other words, the arista has taken to itself the original function of 

 the antenna, on account of the latter being practically turned into an 

 olfactory sense organ. The bristles of the facial and frontal areas 

 protect the other parts of the head from injurious contacts. 



What light does this function of the arista throw on the question 

 of its nudity, pubescence, or plumosity? Simply that the separate 

 hairs have a tactile function, pointing in all directions from which 

 danger may come. It is to be noted that the plumosity is always 

 stronger on the upper or outer than on the under or inner side. 

 Those forms which have the basal joints of the arista elongated lack 

 the plumosity. This elongation of the basal joints indicates an in- 

 creased freedom of movement of the arista. When bare of plumos- 

 ity the arista either is long and tapering, indicating a somewhat 

 restricted movement in the comparatively short basal joints, or it is 

 short, stout, and geniculate, with greatly elongated basal joints, indi- 

 cating much freedom of movement. The nudity of the arista may 

 be generally taken to indicate greater freedom of movement in its 

 basal joints, and its shortening", when combined with geniculation, 

 still further increase of movement. In any case, the function of the 

 organ is seen to lie a tactile one, intended to guard the highly sen- 

 olfactory pits and nerve-ends located in the third antennal joint. 



Those forms which have the arista more or less atrophied doubt- 

 less have the third antennal joint less highly olfactory and more 

 tactile in function. 



From this functional nature of the arista we can only conclude, 

 in accordance with the general and almost invariable rule, that it 

 possesses little value for the definition of subfamilies and higher 

 groups, but that its characters may well be employed in the separa- 

 tion of tribes, genera, and species. 



Eyes. — The organs of vision are with little doubt more highly 

 developed in the Muscoidea than in any other superfamily of non- 

 aerial insects. These flies possess, on the whole, a distinctively 

 terrestrial life-habit, in contradistinction to an aerial one. The rela- 

 tively small percentage of ach?etophorous and subachretophorous 



