SYRPHIDAE OF MAINE. 225 



on Elm agree with Williston's description of (pistica=) 

 radicum. In these species the third joint measures, — maxi- 

 mum width, .32 mm. by maximum length about .5 mm., the 

 ratio being roughly 1 :i.6. 



However, the specimens reared in Maine are much less 

 evidently pilose, and have the third antennal joint noticeably 

 smaller and more elongate ; measuring, maximum width, 

 .25 mm., maximum length .42 mm., the ratio being roughly 

 1:1.7. 



Another difference between the above groups of specimens 

 (which may or may not hold true for a larger series) is in 

 the length of the posterior, or medio-cubital, cross-vein at the 

 outer end of the discal cell. In the Maine specimens 

 (pisticoides) this vein is about .45 mm. long, one-fourth shorter 

 than in the Ohio specimens {radicum) where it measures nearly 

 .6 mm. 



The above specimens are therefore referred to Pipiza pisti- 

 coides. 



Larva. (Fig 29-7) Dimensions, average of eight, length, 6.75 mm., 

 maximum width, 2.3 mm., height 1.8 mm. A wrinkly, moderately spiny, 

 pale-yellowish or flesh-colored larva, usually much obscured by the 

 woolly excrescence of the aphids among which it is found. Segments 

 5 to 11 inclusive of nearly equal width, the fourth and twelfth a little 

 more than half as wide. The remaining anterior segments retracted 

 except when the larva is active, then strongly accuminate to the terminal 

 mouth-parts. Mouth-parts of the aphidophagous type with slender 

 accessory thorns and a pair of prominent, short and heavy lateral mouth- 

 hooks. The anterior larval spiracles slender, inconspicuous. 



Each segment with one prominent, and two less prominent, transverse 

 wrinkles, the latter sometimes sub-divided. The produced edges of the 

 segments form two, zig-zag, lateral carinse one above the other, com- 

 posed chiefly of the fleshy, conical elevations which bear the dorso- 

 lateral and lateral segmental spines. The entire dorsum covered with 

 minute, close-set pale integumental spines, .015 to .03 mm. long, with 

 broad bases and acute tips. The segmental spines likewise pale, about 

 .1 to .15 mm. long, of two sub-equal parts, the terminal part slender, 

 accuminate, the basal part more than twice as broad. 



The posterior respiratory process with a moderate constriction about 

 mid-length ; the basal half sub-conical fleshy and papillose, the distal 

 half polished sub-cylindrical, a little flattened dorso-ventrad, at the tip 

 evidently of two tubes fused mesad. The entire process is elevated to 

 a length of about .5 or .6 mm., the polished distal part measuring about 

 .25 mm. in length ; maximum width about .25 mm., maximum heigth 

 about .15 mm. The spiracular slits prominent, usually raised on radiating 



