CH^STOTAXY. 45 



bescence should be restricted to short, recumbent, fine 

 hairs, while tomentum can only correctly be used as the 

 designation for flattened, scale-like or stubble-like, more 

 or less recumbent hairs, which gradually merge into dust 

 ox pollen, which is so generally present in diptera, and 

 upon which the determination of many species must 

 largely depend. 



CH^TOTAXY. 



Many years ago Rondani proposed the term macrochce- 

 t& for the large, differentiated bristles of flies, and sug- 

 gested their use in classification, though he made but 

 little use of them himself. L,oew also failed to appre- 

 ciate their full significance in the classification of diptera, 

 although he made considerable use of them, examples of 

 which may be found in his diagnostic characterization of 

 the Helomyzidae and Ortalidae. It was Osten Sacken 

 who proposed the term chcetotaxy to designate the science 

 of their arrangement, and who published an epoch-mak- 

 ing paper on the subject in 1881. Girschner, later, ex- 

 tended the system more widely for the calyptrate diptera. 

 At the present time a thorough knowledge of chaetotaxy 

 is indispensable for all who would deal with those fami- 

 lies, especially of the Cyclorrhapha, in which they are, 

 for the most part, so conspicuous, and upon which the 

 generic and specific distributions so much depend. Osten 

 Sacken, indeed, makes the arrangement of the bristles 

 almost fundamental as indices of relationships, and exag- 

 gerates their importance in some instances, especially so 

 when he would locate the Apioceridae with the Asilidae, 

 almost solely on their chaetophorous character. Hilari- 

 viorpha he refuses admission to the Empididae chiefly be- 

 cause of the absence of bristles, forgetting that some true 

 Empididae also absolutely lack bristles. However it 

 may be in particular instances, there can be no question 

 6 



