TAXONOMY OF MUSCOIDEAN FLIES — TOWNSEND 33 



It has been conclusively proved by the experiments of Hauser and 

 others that the sense of olfaction is located exclusively in the an- 

 tennae in Sarcophaga, Callipkora, and Cyuomyia, and not at all in 

 the palpi. This has also been demonstrated in many Hymenoptera, 

 Lepidoptera, Orthoptera, and Staphylinidae ; but in certain Hemip- 

 tera experimented with it was found that the loss of their antennae 

 did not affect in any way their sense of smell. Certain Coleoptera 

 were only partially affected by the excision of their antennae. 



The olfactory organs of the Muscoidea consist of (i) a thick 

 nerve trunk arising from the brain and passing into the antennae; 

 (2) a sensitive apparatus at the end, consisting of rod-like modified 

 hypodermis cells, connecting with the nerve-fibre terminations; (3) 

 a supporting and accessory structure consisting entirely of pits. The 

 same is true of the other Diptera, the Lepidoptera, Orthoptera, and 

 probably the Hemiptera ; but in the Neuropteroid orders, the Coleop- 

 tera, and the Hymenoptera, the accessory structure consists of peg- 

 like projecting epidermal invaginations filled with a serous fluid. 

 Both pegs and pits occur, however, in the Coleoptera and Hymen- 

 optera, while only tactile hairs were found by Hauser in Pyrrhocoris 

 of the Hemiptera, though Lespes has recorded the presence of pits 

 in that order. 



It should be mentioned here that another sense, capable of distin- 

 guishing between various degrees of atmospheric pressure, is be- 

 lieved to reside in certain sensory structures, like the sensillum placo- 

 deum, found in the antennal joints of bees and wasps. It is evident 

 that insects have some means of perception, through certain sense- 

 organs, of approaching changes in meteorologic conditions. 



Arista. — The arista is the persistent rudiment in the Cyclorrhapha 

 of the terminal antennal joints still to be found in many of the lower 

 groups of Orthorrhapha. In the development of the third antennal 

 joint of the Muscoidea as a special olfactory sense organ, the arista 

 has become dorsal or basal, being left to occupy a position to one side 

 during the extraordinary development of the joint away from it. It 

 is invariably situated close to the base of the front edge of the joint. 

 Its persistent retention in this position indicates that it is to some 

 extent functional. 



It is a rule in nature, which carries no exception, that there is a 

 reason for everything that exists. Therefore there is some cogent 

 reason for the pubescence, plumosity, and nudity of the arista, as 

 well as for its presence. The arista has become subordinated to the 

 third joint, but retained as an accessory. It therefore must be func- 

 tional. The point is to discover its function, which must be the key 

 to the explanation of its varying degrees of pubescence and plumos- 



