SYNOPSIS OF NORTH AMERICAN SYRPHID^. 277 



ill Microdon {Ubrisfes), but is also seeu iii many of the ChUosini and 

 Melanostomini ; rarely if at all in the Eristalini and following gronps. 



In coloration and pilosity by far the greater number of speeies offer 

 distinct differences. The most striking examjjle of this is seen in Trio- 

 duiita curvipes, where the male abdomen is red and the female black; 

 singularly the hind legs in this species offer more sexual peculiarities 

 than in any other. In some species with banded abdomen the yellow 

 markings show a greater tendency to confluence in the female. In 

 many the markings of the abdomen are more conspicuous in the male, 

 or obsolete in the female; species of Eristalis and Helophilus will be 

 recalled as examples. Again, in the male there is sometimes a ten- 

 dency to melanism, in the antennae (e. g., Chilosia), the legs {Syrphus), 

 face (ibid.), coloration of the abdomen {Chilosia, Sphegina), and thorax 

 {Helophilus). As a general rule the male coloration is more strongly 

 indicated ; such we know is in conformity with the general zoological 

 law, tliat the male shows the greater number of ornamentations, both 

 in color and structure. In the male the pile throughout is frequently 

 longer, more abundant, and sometimes darker in color. 



None of the foregoing sexual differences appear to be characteristic 

 of the family. Thus, holoptic males are predominant throughout the 

 order ; differences in the size of the antennae may be met with in the 

 Muscidce, etc.; similar or allied structural variations in the legs, in the 

 TacJiinidce, Dolichopodidw, Midasidce, Asilidw, etc.; of the abdomen in 

 Stratiomyidw, etc. On the other hand, sexual differences in the wings, as 

 in the Asilidcc, and certain ornamentations of the legs, as in Asilidce, 

 Tachinidct', J)olichopodidcc, Empidie, etc., do not occur here. 



What is the purpose of these sexual peculiarities ? This is a question 

 easily asked, but far less readily answered. ^ uiy this may be said: 

 their use in general does not appear to be commensurate with their 

 oftentimes striking development. Species, otherwise closely allied, 

 sometimes show reniarkable differences in tuis respect. Thus, Helophi- 

 lus distinctus and H. chrysostomus are so closely related that they aj)- 

 parently have been hitherto confounded, yet in the former the male hind 

 €oxa3 bear a strong spur, wanting in the latter. Very closely allied 

 species of Xylota show the same peculiarity. Tropldla mamillata, not- 

 withstanding the well-developed basal femoral process, does not differ 

 much from T. quadrata. The same may be said of T. calcarata, with 

 its coxal spur. Mcsograpta (jemiiiata and 2L. parruhi are closely allied, 

 yet in the former the male bind femora ; '^ thickened and arcuated. 

 The terminal tibial spur in Helophilus., Xylota, etc., is very uncertain in 

 its appearance, while the internal median spur iu MuUota cimbiciformis 

 appears only to be a dimorphic })eculiarity. Similar dimorphism appar- 

 ently exists in such species as Sepedon annatus and S. imsillus, for 

 instance, in the Sciomyzidw. There (;ertaiuly, however, seems to be a 

 functional purpose for these structural developments; the only thing 

 surprising is that their possession so often appears to have no effect 

 upon other characters. We can readily imagine that the processes and 



