Polymecus alnicola Ashmead, Bull. 45 U. S. Nat. Mus., 1893, p. 283.—Brues, Bull. 22, Conn. Geol. Nat. Hist. Surv., 1916 (1917), p. 536.
Female.—Length 2 mm. Head shaped as in pallipes Ashmead; face not elevated in middle; occiput, vertex, most of cheeks, interocellar area, and face around the ocelli strongly shagreened, much as in pallipes; rest of face finely shagreened, without well marked transverse striae above antennal sockets; antennal process truncate anteriorly, the sides only slightly ridged.
Antennae moderately slender, the joints, except the scape, combined about equal in length to the thorax; scape thickened subapically, little shorter than the five succeeding joints united, pedicel slightly over twice as long as wide, about as long as the two following joints united, a little wider than the fourth; third joint as wide as long, narrowed basally, as wide as the fourth, half as long as the fourth; fifth and sixth about as long as the fourth, a little thicker, seventh nearly as long as and considerably thicker than the second; eighth and ninth distinctly but not much longer than wide, slightly shorter than, but as wide as, the seventh; tenth joint oblong-oval, as long as the second, twice as long as wide; a little narrower than the ninth, obtuse at tip.
Thorax twice as long as wide; pronotum shagreened on the sides of the median area; notauli complete; median lobe of mesonotum rather strongly shagreened, the two lines on anterior third narrow and depressed, not very distinct; lateral lobes shagreened anteriorly along their inner margins, and along their outer margin to apical half, otherwise polished; scutellum transverse, highly convex, margined laterally, shagreened and with a few short white hairs on the sides; metapleurae, propodeum, first tergite laterally, and second tergite at extreme base in the foveae, sparsely covered with short white hair.
Abdomen a little longer than the head and thorax united, as wide as the thorax, shaped somewhat as in pallipes but shorter than in that species; segments one and two united as long as the thorax, median area on first tergite quadrate, the dorsolateral ridges sharp and rather prominent; second tergite about as long as wide apically, somewhat narrowed basally, the sides feebly curved; basal fovea broad and deep, covered with strong striae which radiate fan shape nearly two-thirds the length of the segment from the base; area between the foveae with a few short striae laterally; third tergite four times wider at base than long, three-fifths as long as the fourth, the sides oblique; fourth tergite twice as wide at base as long, the sides oblique, straight, continuing the sides of the third; fifth as wide at base as long, the sides not quite as oblique as those of the two preceding segments; last segment as long as the fourth, one and one-half times as long as wide, acutely pointed apically.
Front wings extending to the apex of the fifth segment, hyaline, covered thickly with short pubessence. Black; antennae and last joint of each tarsus uniformly dark brown; legs reddish brown, the front tibiae, middle and hind tibiae at base, and tarsi, tinged with yellow.
Male.—Length 1.70 mm. Pedicel twice as long as wide, narrowed proximally; third joint one-third as long as the second and much narrower, closely jointed to the fourth; fourth a little shorter than, but conspicuously thicker than the second, not as thick as in pallipes, indistinctly emarginate proximally; joints four to nine, inclusive, seen laterally, slightly but distinctly longer than wide; ten conical, pointed, distinctly longer than the second.
Abdomen slightly shorter than the head and thorax united, about as wide as the thorax, twice as long as wide, broadly rounded apically, not much narrowed basally; front legs a little lighter than in the female, yellowish, the femora medially brownish; antennae dark brown.
Type locality.—District of Columbia.
Type.—Cat. No. 24602, U.S.N.M.
Redescribed from the types in the United States National Museum. They were reared April 30 and May 1, 1884, from a Cecidomyid gall, Cecidomyia serrulata Osten Sacken, found on alder. This species with its moderately short abdomen in the female is intermediate between such forms as vancouverensis Ashmead and herrickii Packard.