30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



of the specially developed senses, should afford the most important 

 characters for taxonomic use. 



Here, and practically here alone in the Muscoidean anatomy, are to 

 be found certain useful atavic characters pertaining to organs not of 

 any functional importance in the economy at present, but possessing 

 phylogenetic significance as indicating origin and relationships. 

 These are of especial value for the separation of families and sub- 

 families. It has long been recognized that rudimentary organs in 

 recent forms bear a significant relation to those of their allied prede- 

 cessors. Such are physiologically non-functional now. and appear in 

 more or less developed condition only in the embryo, but were func- 

 tional throughout life in the early fossil forms. They have been lost 

 through disuse, involving a process of degeneration or retrogressive 

 development. If, then, these organs present sufficient variation, their 

 rudimentary presence is of much importance to us in the prepara- 

 tion of a natural taxonomic system. Atavic characters, to be of use, 

 must be exhibited by parts which vary sufficiently to offer con- 

 veniently distinguishing marks. To be of use in the separation of 

 higher, or family, divisions, the parts must present just enough 

 variation to offer distinctive characters that will hold throughout 

 considerable aggregations of forms. 



Such are the characters afforded by the facial plate in its lower 

 extent, and by the facialia, vibrissa! angles, and peristomalia. The 

 parts in question present sufficient variation to afford distinguishing 

 characters. These are all atavic, and possess in consequence a high 

 phylogenetic significance. They are connected with the portions of 

 the head whose development in the nymph is not influenced by the 

 coincident development of functional parts. While the development 

 of the highly sensory third antennal joint affects in a degree the 

 upper portion of the facial plate and determines the character of the 

 fovese, its influence does not extend below the -vibrissal angles. 



Atavic characters are afforded by the wing veins in a remarkable 

 degree, but the general plan of venation is too uniform to afford us 

 good family characters. They can be used in higher and lower 

 divisions. It may further be noted that, since the wings are so 

 highly functional in a mechanical (not sensory) way, the characters 

 derived from lesser variations in venation would in any event be sec- 

 ondary in importance to the head characters just mentioned. It 

 must be borne in mind that the wings are of great functional im- 

 portance, and the veins bear the mechanical strain incident upon 

 their use, while the special head characters above pointed out, whose 

 importance as affording distinctions for higher divisions has been 

 dwelt upon, are in nowise connected with any present function, 



