SYRPHIDAE OF MAINE. 159 



measuring 10 to 11 mm. in length by 5 to 5^ mm. in maximum width, and 

 only about \ l /s mm. in height. 



Very different in shape from the usual aphidophagous larva, suggest- 

 ing somewhat the larva of Microdon and indeed forming a most interest- 

 ing intermediate step, structurally, between the typical aphidophagous 

 larva, such as Syphus americana, S. torvus, Sphaerophoria cylindrica, and 

 others and the extremely specialized larvae which live in the nests of ants. 



When the larva is resting its shape from above is broadly ovate, some- 

 what narrower and less truncate at the anterior end. The entire lateral 

 and anterior margins are deeply serrate, there being three large and sub- 

 equal and one much smaller serration to each of the seven principal seg- 

 ments (Fig. 8, F & H). The three larger serrations bear at their tips the 

 lateral, posterior ventro-lateral and anterior ventro-lateral segmental 

 spines; the median, dorsal and dorso-lateral ones retaining their normal 

 positions on the dorsal surface of the somites. The fourth and smaller 

 serration is nude of bristles. Each of these principle serrations, or lat- 

 eral, cone-shaped processes, is covered with papillae similar to those over 

 the entire dorsum of the larva, so that in outline they appear secondarily 

 serrated with ten or a dozen papillae on each side. The segmental spines 

 are pale, colorless, of the usual two segments, the basal one 0.035 to 0.04 

 mm. in length and of about equal width, the distal one peg-like, 0.055 to 

 0.075 mm. in length, by 0.015 to 0.0185 mm. in diameter, accuminate toward 

 the tip. The median, dorsal, and dorso-lateral spines are considerably 

 smaller, measuring not more than 0.05 mm. in length, and exceedingly 

 inconspicuous, it being almost impossible to detect them in the dorsal 

 view, though visible at good magnifications in profile (See Fig 8, D). 



The anterior segments, retracted under segment four when the larva 

 rests, are, when it is active, protruded as a blunt, cone-shaped projection 

 bearing dorsally the anterior spiracles and, terminally, the retractile an- 

 tennae and mouth parts. As seen in caudal aspect (Fig. 8, D) or in cross- 

 section the larva is very broadly sub-triangular, the apex of the triangle 

 being at the mid-dorsal line and the lateral margins bearing the fringe of 

 serrations already described. 



The length of the posterior respiratory organ of the larvae measured, 

 (Fig. 8, E, G) varied from 0.8625 mm. to 0.9525 mm., with an average of 

 practically 0.9 mm. ; the width at the tip varied from 0.4875 mm. to 0.5025 

 mm., averaging 0.494 mm. ; the height at the tip ranged from 0.2625 mm. 

 to 0.285 mm., averaging 0.27 mm. The median, slit-like spiracle ranged 

 in length from 0.1275 mm. to 0.1387 mm. (averaging 0.134 mm.) by 0.015 

 mm. in width. The diameter of the circular plate varied from 0.0675 to 

 0.075 mm. averaging 0.0712 mm. ; and these are situated with their median 

 margins from 0.08 to 0.09 mm. apart. The color of the respiratory tube 

 is brown, its surface rugose, with a shallow impression on the median line 

 and a very moderate incision between the stigmal plates. The dorsal 

 spiracular spine is very inconspicuous but the other interspiracular spaces 

 bear broad rugose carinae, somewhat more elevated than the slit-like spir- 

 acles. 



