32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



located in the third joint of the antennae, which contains numerous 

 olfactory pits communicating with the main nerve trunk by means of 

 minute nerve-ends. 



According - to Gustav 1 fauser (Zeitschr. f. Wissens. Zool., xxxiv, 

 pp. 307-403. 1880), who studied over sixty species of Diptera in this 

 connection, the Muscoidea and other cyclorrhaphous Diptera. and 

 also the Brachycera. have the olfactory pits without exception con- 

 fined to the third antennal joint. Their number varies greatly in 

 different forms of Cyclorrhapha. Certain syrphids, as Helophilus 

 ftorens, have only one pit on each disk of the third joint, while 

 Bchinomyia grossa has two hundred. In certain forms the pits are 

 compound, containing from ten to one hundred olfactory hairs aris- 

 ing from the coalescence of the several original pits. Xo compound 

 pits occur in the Tipulidae, but only simple ones with a single olfac- 

 tory hair, such as are found in the brachycerou- | s. str. 1 forms only. 

 The latter have also compound pits, containing from two to ten 

 nerve-terminations. 



The olfactory pits are sac-like invaginations of the external chit- 

 inous integument, and are of various shapes in different fornix of 

 diptera. They are always open externally, and never closed by a 

 membrane. In the Cyclorrhapha, and the Muscoidea especially, the 

 pits differ hut little in the various forms. Ilauser (1. c.) figures and 

 describes at length those of Muscina stabulans as generally typical 

 of not only the Cyclorrhapha, but the Brachycera s. str. as well. He 

 gives a figure of the third antennal joint in longitudinal section 

 showing simple and compound pits, the pits themselves being shown 

 in both transverse and longitudinal section and from above. The 

 main nerve trunk, accompanied by the much smaller tracheal trunk, 

 passes through the second antennal joint entire and without division. 

 but on entering the third joint gives off a very small branch to the 

 arista, to which also runs a small branch of the trachea. The bulk 

 of the nerve trunk continues undivided and undiminished into the 

 mass of the third antennal joint, where it branches in all direction-., 

 but especially apically and inferiorly (opposite the edge bearing in- 

 sertion of arista), the main trachea following it with less branching. 

 This centralization of nerve-branches, nerve-ends, and olfactory pits 

 in the apical and ventral tracts of the third antennal joint — that is to 

 say. outside the aristal area — bears out the conclusion that the arista 

 was originally terminal and that the highly functional extra-aristal 

 area of the joint has simply grown away from it as fast as more 

 space was required by the advancing development of the olfactory 

 sense. 



