22 OHIO BIOLOGICAL SURVEY 



pearance. It is possible also that the structures (c), are comparable to the 

 upper jaw, or they ma.v be equivalent to lateral booklets ("mandibles" of 

 J. B. Smith ) . In front of the mouth- parts on the ventral side of the body 

 is a pair of structures which I believe are antennae. They would com- 

 pare with the antennae in the other forms with the two apical segments 

 much elongated and tapering at the tip, the basal part not prominent. 

 I have not been able to find anterior spiracles in the larvae I have exam- 

 ined. There are no signs of pro-legs. 



The posterior respiratory appendage resembles in superficial appear- 

 ance that of aphidophagous species, consisting of two, fused, short, rigid 

 tubes. The essential structure however is quite different. In place of 

 three pairs of slit-like spiracles we have an arborescent pattern such as is 

 figured in PI. IX, Fig. 1S4. The colors of these larvae are usually a 

 dirty white or drab tinged with yellow or brown. 



CHARACTERS AVAILABLE FOR CLASSIFICATION 



Though distinct as a family, the larvae of Syrphidae do not present 

 easy specific characters. It is often much easier to recognize the species 

 by a tout-enscmbh\ than to define the characteristics. In the aphidopha- 

 gous forms the color and vestiture are helpful. The mouth-parts and an- 

 terior spiracles doubtless present constant differences, but these are small. 

 On this account and because of the habit of the larvae of retracting their 

 head segments, the.se parts are exceedingly hard to see clearly. Hence 

 thev are almost worthless for the ready distinction of species. The 

 number of mouth-hooklets seems to vary somewhat, and their size, shape, 

 and location, as also the length and shape of the jaws, is constant for the 

 species, but varies somew^hat among the different genera and species. Of 

 these characters probably the only one easily enough made out to be of 

 service is the presence or absence of a pair of large booklets, remote from 

 the side of the jaws, which I have called the outer pair of iiioiith-hooks. 

 These for example are wanting in Dideafasciata, and present in the three 

 species of Svrphiis and the two of Paragus which I have examined (Figs. 

 3; II, c; 23, a; 47; 67, c; 81, d; and 91, a;). The ornamentation of the 

 anterior spiracles (Figs. 12, 48, 83, 92,) seems to differ. However, after 

 spending considerable time, with strong illumination and high micro- 

 scopic powers, in determining the actual nature of this character in a 

 single specimen, I concluded that it would be of little use for the 

 practical separation of the species. 



We have come to the conclusion that the best available, and perhaps 

 the only absolutely dependable, basis of separation for aphidophagous 



