ILLINOIS BIOLOGICAL MONOGRAPHS orly attenuated, twelve-segmented, subcylindrical larva. The cephalopharyngeal skeleton consists of the three typical sclerites of cyclorrhaphous larvae. The vertically-moving mandibular sclerites are supplemented by one or several pairs of small chitinized lateral hooks which work horizontally to seize the prey. Above the mouth opening is a pair of soft, fleshy antennal tubercles, each bearing apically a pair of minute processes. The segments of the head are retractile, making their number difficult to distinguish. The short, rigid, heavily chitinized, anterior spiracular processes are generally considered by authors to be prothoracic, and the segment on which they are located appears to be the third. Behind this are nine segments exhibiting considerable secondary segmentation. Each segment bears twelve two-segmented, but simple spines, more or less elevated on rounded tubercles. On the anterior segments, they are in a nearly transverse row, but on the main body segments the various pairs occupy successively more posterior annulets, always in a definite position. The six segmental spines of each side have been named by Metcalf (1913b), beginning at the mid-dorsal line and progressing outward, as illustrated in Fig. 2: median, dorsal, dorso-lateral, lateral and two pairs of ventrolaterals, posterior ventro-lateral, and anterior ventro-lateral. The integument is tough, but transparent and delicate in appearance. It is indistinctly divided into segments, each with four or five annulets. On the ventral surface are numerous lump-like expansions, and the whole surface, more or less, is brought into play as a creeping sole, but there are never any proleg-like structures as in the saprophytic forms. Two anterior respiratory processes and a single posterior respiratory process are usually present. The posterior spiracles in this group are always three pairs of slits borne on the tip of two rigid tubes more or less completely fused into a posterior respiratory process which often protrudes considerably from the surface of the body. The surface on which the spiracles are borne has been designated the posterior spiracular plate. In addition to the spiracles, the posterior spiracular plate bears the scar of a former spiracle, a circular, rather weakly chitinized area, the circular plate, always located dorsad of the center of the posterior spiracular plate. Median to the circular plate there may be present a spine or ridge, elevated above the surface, the dorsal spur, and between the spiracles are, variously, nodules, setae, or frilled lamellae, but, in aphidophagous syrphids, never plumose hairs. These structures are referred to as interspiracular nodules, interspiracidar setae, and interspiracular lamellae. The rectal gills are structures which seem to afford, in different genera, interesting characters. They are rarely found extruded in aphidophagous forms, although the writer has seen some score of larvae of this type with the rectal gills fully everted in preserved larvae. No infallible me...