272 BULLETIN 31, UNITED STATES NATIONAL MUSEUM. 



Volucella. The larvae are parasitic upon liumblebees, living in tbeir 

 nests. The very great resemblance of the adult of some species to the 

 bees makes this fact the more interesting. 



STRUCTURAL CHARACTERS. 



Structural characters in the Syrphidai are derived from all parts of 

 the body, though those from the thorax are unimportant. The most 

 useful are found in the wing, but a much larger number exist in the head, 

 and are, intrinsically, scarcely less valuable. Next in importance come 

 the various ornamental and functional characters of the legs; while the 

 abdominal ones are superior only to those of the thorax. The follow- 

 ing remarks, it will be understood, are based chiefly ui^on my autoptic 

 knowledge, and such reliable observations and descriptions by other 

 authors as have been accessible to me; they may, in some cases, require 

 modification to express the requirements of exotic genera. 



Most excellent structural characters are presented by the antennae. 

 With proper caution, they are often serviceable in generic distribution, 

 though by far the greater number will only permit of specific emjiloy- 

 ment; some minor variations are sexual, or even individual. Formerly 

 much greater reliance was placed upon the structure of these organs, for 

 generic distinguishment, than our present knowledge will confirm. The 

 basal joint is seldom elongate, and when so, the elongation is rarely 

 pronounced. It may be seen in Jlicrodon, Ghrysogaster, Griorhina, Mera- 

 pioMus, Spilomyia, Ceria, etc., and has only subordinate value, other- 

 wise related species differing materially in this respect. The second joint 

 is more rarely lengthened, extraordinarily in Spheconujia, moderately in 

 species of Ghrysogaster, Pipizella, Lepromyia, Ghrysotoxum,2{icrodon, and 

 Ceria. The third joint offers the greatest diversity in form, and the most 

 useful differential antennal characters. In the, Melanostomini, Syrphini, 

 Spheginini, and Brachyopini it may be oval, but in none of our genera is 

 it distinctly elongate or noticeably widened. In the Uristalini, Serico- 

 myini, and Xylotini it is predominantly dilated, rarely longitudinally 

 oval, never at all elongate. In the Microdonini and Volucellini it is 

 lengthened ; I know of no exception, though in some species of the lat- 

 ter, as V. obesa, the elongation is only slight; yet even here the joint 

 is not oval, but pointed. Among the Ghilosini we find the greatest 

 variableness; in Fipiza, Paragus, and Lepromyia, more or less slender; 

 in Ghrysogaster^ with intermediate forms between marked elongation and 

 orbicular; in the other genera, oval, orbicular, or subquadrate. In the 

 Milesini it is always short; in Sphecomyia, sp.,i)ointed; in the other gen- 

 era, transverse. So far as my knowledge reaches, species with elongated 

 antennae are not very active in their habits, and frequently have pilose 

 eyes. 



There seems to be a definite relation existing between the neuration 

 and elongation of the antennae. Wherever the last section of the fourth 

 vein and the posterior cross-vein show a tendency to a rectangular po- 



