Larvae of predaceous flies that feed openly on plants can generally be distinguished by appendages and body size. The pair of rear breathing tubes (anal spiracles) of aphid flies are widely separated. Predaceous midges have shorter tubes relative to their body length and they are closer together (aphid midges) or touching (mite midges). The breathing tubes of flower flies or syrphids are touching (fused), except for first instars of some species. The length of syrphids' anal spiracles varies by species and may project or not and appear as a discolored blotch. If anal spiracles of the predaceous fly larva are touching and the body is 1/8 inch (3 mm) or longer it is a syrphid; predaceous midges are 1/12 inch (2 mm) or less in length.
Photo by Peterson A. 1960. Larvae of Insects. Part II. Edwards Brothers. Ann Arbor, MI. Aphid fly and midges used with permission of H. Peterson. Syrphids from Metcalf CL. 1913. Syrphidae of Ohio. Ohio State Univ. Bull. 1(1).