TAXONOMY OF MUSCOID^AN FLIES — TOWNSEND 31 



either mechanical or sensory, in the economy of the adult insect. 

 The type of venation furnishes atavic characters of value in sepa- 

 rating higher divisions. The bristles or hairs of certain thoracic 

 plates likewise furnish atavic characters of high value here. 



Atavic characters also occur to a limited extent in the abdomen, 

 chiefly in the atrophied basal segment, which can be clearly made out 

 only by detaching the parts. These are also too uniform to be of 

 use for the separation of the larger divisions, so far as we yet know. 

 But their comparative study offers promising results. 



Practically all the other portions of the Muscoidean anatomy are 

 preeminently functional, even including the halteres, tegukne, etc., 

 and the parts of the head other than those enumerated above. The 

 frontalia and lunula may he practically non-functional, but they like- 

 wise do not present sufficient variation to offer any useful characters 

 for family separation. The second antennal joint is probably not 

 functional, although in the Nemocera it is the seat of the so-called 

 "Johnston's organ," whose function is supposed to be auditory. This 

 organ does not appear to be developed in the Cyclorrhapha. Prac- 

 tically the only character afforded by the second antennal joint, 

 however, is that of relative length compared with the first joint, and 

 this is at best available only for subfamily and generic separation. 

 The arista is doubtless functional. A consideration of certain char- 

 acters of functional parts, and especially of the physiological func- 

 tions of certain of these parts whose characters have in the past been 

 largely used in taxonomy, is now taken up. 



Antennas proper. — The first and second antennal joints are prac- 

 tically non-functional. The third joint is highly functional, and 

 hence does not afford reliable taxonomic characters for higher 

 divisions than species, and within certain limits for genera. The 

 relative length of third joint to second affords no valid character, 

 and especially gives a wrong impression in those forms having the 

 second joint elongate. The first joint is almost universally short, 

 but the second is often more or less elongated, and in some cases 

 strongly so. The relative length of second joint to first affords a 

 good generic character. The third joint affords excellent specific 

 characters, so far as its relative length and size go, with proper 

 recognition of sexual variations. Its shape may furnish characters 

 of generic, or even of tribal, value. 



The olfactory sense is very highly developed in the Muscoidea. 

 Blow flies will come for miles to decaying, and even to fresh, meat 

 shortly after its exposure to the air. Most other members of the 

 superfamily possess this high olfactory sense, though in some it is 

 developed in a varying degree. The sense of smell in these flies is 



