SYNOPSIS OF NORTH AMERICAN SYRPHIDiE. 275 



with the hind femora elongated and thickened. In the MiUsini the great- 

 est elongation of the legs occurs, in the Chrysogaster-\ike forms the most 

 shortening. 



SECONDARY SEXUAL CHARACTERS. 



Secondary sexual characters occur with great frequency in this fam- 

 ily, and will offer many interesting subjects for future study. They are 

 found in most parts of the body, but, as one would expect, are met with 

 in the greatest diversity and number in the legs 



In the head a difference, constant throughout the family, is found in 

 the eyes. In nearly all the males they are contiguous for a longer or 

 shorter distance between the ocelli and antennae (holoptic) ; in a few 

 genera (as Microdon, Chalcomyia, Crioprora, Triodonta, Mallota, Helo- 

 philus, etc.) they are more or less widely separated by the front (di- 

 choptic); yet in every case, to the i)racticed eye, the male can be dis- 

 tinguished with tolerable certainty by the front alone; even when the 

 female is unknown there is never any excuse for confounding the sexes. 

 The male eyes when dichoptic will always present an angularity that is 

 wanting in the other sex, and there is always some narrowing of the 

 front. The female front, though sometimes considerably narrowed, 

 always has the margins straight, without angulation. In a few genera, 

 all holoptic, the males show areas of enlarged lacets near the upper 

 portion of the eyes. This enlargement is most noticeable in AUograpta 

 and Catabo7nha, but is also indicated in the forms related to Xylota. 

 Although I do not know the habits of Catahomha, one can safely say 

 that those with this enlargement are disposed to hover motionlessly in 

 the air, and are especially well gifted with rapid flight. The male eye 

 frequently has a greater pilosity than the female, as it is also often 

 larger. 



In a number of genera there is a greater development of the lower 

 part of the front in the male, forming a protuberance upon which the 

 antennae are located. This is most apparent in the genera allied to 

 Milesia. There is not much difference in the antennce. In a number of 

 genera (e. g., Chrysotoxum, CMlosia, Brachyoim, Volucella, Sericomyia, 

 etc.) the third joint is larger and broader in the female, and in some 

 the arista is less plumose. In the structure of the face the most impor- 

 tant variations will be found in Myiolepta, where the male is provided 

 with a small, but prominent, tubercle near the middle, which is wanting 

 in tlie female. Similar differences will be seen in some other genera, 

 as in species of GhUosia, Chrysogastey, and Pterallastes. The other differ- 

 ences between the sexes have already been adverted to. In the mouth 

 organs I have observed but slight sexual differences ; in some few gen- 

 era, as in CriorJiina spp., the female proboscis is longer than that of the 

 males. 



In the thorax I know of no structural sexual differences; nor, so far 

 as I am aware, do any such exist in the wings. 



In the abdomen, in addition to the i)rimary characters, no differences 

 of importance j)resent themselves. The shape in the male is frequently 



