$6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



compound eve structures is incorrect. However this may be, it 

 seems certain that insects possess a clear and perfect vision. 



Mouth parts. — Kraepelin has recorded taste-pits, with hairs or 

 pegs arising from them, on the proboscis of Musca (Zeitsch. f. 

 Wissens. Zool., xxxix, 1883). 



The palpi are probably not generally gustatory in function in the 

 superfamily. In certain of the forms they are with little doubt prac- 

 tically non-functional, and some forms have in consequence more or 

 less completely lost them. In others their very considerable, some- 

 times extreme development, indicates some function, which may be 

 either gustatory or tactile. In certain insects they are olfactory in 

 function, but probably not in the Muscoidea. They furnish charac- 

 ters of not more than generic value. 



Wings. — The venational characters are in the main quite constant. 

 The wings themselves are highly functional, but this does not neces- 

 sarily imply that the style of venation is functional. However, as 

 already pointed out, the plan of venation is so comparatively uniform 

 in the superfamily that it yields no characters for separation of fam- 

 ilies. The venational characters are of very great importance in 

 separating this superfamily from the Anthomyioidea, but do not be- 

 come again available for taxonomic use in the Muscoidea until we 

 descend to tribes and genera. 



It is reasonable to attach high importance to the main features of 

 the dipterous wing venation, since the wing system of Diptera is a 

 very highly specialized type. The hind pair has undergone atrophy, 

 its rudiments being diverted to another function, and the entire 

 flight function, at least so far as propelling power goes, has been 

 concentrated in the front pair. As a natural consequence of this 

 high wing specialization, the venation is a practically non-functional 

 system of long standing, extending over a sufficient period of time to 

 allow its systemic features to become well fixed and quite constant. 

 There are a few minor venational characters that can not be relied 

 upon in certain restricted groups. The last section of fourth vein 

 (or apical crossvein) may vary in degree of curvature, but not in 

 kind. The hind crossvein may vary in strength of sinuosity, but the 

 double curve is never entirely lost in the same form. 



There is some ambiguity involved in the term "apical crossvein." 

 as it has been used in the past. In certain genera it is impossible to 

 decide its true limits. The use of the term should therefore be re- 

 stricted to those cases in which its entire course is exactly defined. 

 The apical crossvein lias resulted from a bifurcation of the fourth 

 vein at its point of flexure. In those genera showing what has been 

 called a "stump, or a wrinkle, at bend of fourth vein," the point of 



