174 Maine Agricultural Experiment Station. 1917. 



small, situated close together above the jaws, of the usual form (see Fig 

 12, A). 



"The prothoracic spiracles are slightly elevated, blunt, short, horn- 

 shaped as seen from the side (Fig. 12, A, g), the semi-circular slit ap- 

 parently guarded by six, blunt teeth, one of the median ones emarginate or 

 imperfectly divided (Fig. 12, C). The posterior respiratory appendage 

 (Fig. 12, D, E) is" a little "longer than broad, testaceous brown, ringed 

 about mid-length, thence slightly constricted. The spiracles (a) moderate- 

 ly long, somewhat elevated above the surface ; the inter-spiracular spines 



(b) short" sharp ridge-like, "rather prominent. Dorsal spiracular spine 



(c) short compressed; its breadth about equal to diameter of the approxi- 

 mate circular plate (d). 



"These larvae were found, full-grown, at Cedar Point, July 7, 1911. 

 The larval stage continued indoors to July 11 and 12. 



"They were collected on the Poplar or American Aspen (Populus 

 tremuloides Mx.) in the well-knoAvn, characteristic galls on the ends of 

 the twigs, made by the aphid, Pemphigus" oestlundi Cockerell. 



The larvae taken from the Ash pseudo-galls, in correlation 

 with the g-reater freedom of motion afforded them all during de- 

 velopment, did not present such exceptional height and thickness. 

 While unusually corpulent, they retained the typical Syrphus 

 shape, as in torvus for example, and were less sluggish than those 

 described above. 



The length of the full grown larvae when extended exceeded 17 mm., 

 their greatest width fully 4 mm. and their height about 2.S mm. Cross- 

 section semi-circular. The ground color of these larvae (the body fluids 

 showing through the integument) is a dull ashy gray with areas of orange 

 and black where viscera of these colors showed faintly. The ashy gray is 

 overlaid extensively with ashy white adipose masses in the usual position ; 

 beginning at the anterior end with a narrow, mid-dorsal line which widens 

 wedge-shaped caudad until in the posterior half of the body it occupies 

 most of the width of the body. It is interrupted at each segment by a 

 prominent emargination at each side and by an elongate, pulsating area on 

 the middle line. The posterior respiratory organ is a warm brown in con- 

 trast with the whitish body. 



While the respiratory tube is not elongate, the stigmal plates are no- 

 ticeably more elevated than in Syrphus torvus, ribesii, etc. The respiratory 

 organ in knahi is at least as long as its width at the tip; in that of torvus 

 the length does not exceed two-thirds the width. The length of the tube 

 is from 0.53 to 0.61 mm., its width from 0.49 to 0.57 mm., with an average 

 of 0.525 mm. and its height 0.31 to 0.335 mm., average 0.325 mm. The 

 transverse diameter of the circular plate is about 0.06 mm. its dorso-ven- 

 tral diameter about 0.09 mm. ; the inner margins of the two about 0.129 

 mm. distant. The median spiracle measures about 0.16 mm. in a straight 

 line, its width about 0.012 mm. 



The surface of the respiratory tube is only indistinctly and irregularly 

 papillose, polished; with shallow grooves along the median line, dorsad 



