Larvae of predaceous flies that feed openly on plants can generally be distinguished by appendages and body size. The pair of breathing tubes at the rear (anal spiracles) of aphid flies (e.g., Leucopsis spp., Chamaemyiidae) are widely separated. Predaceous midges (Cecidomyiidae) have shorter tubes relative to their body length and they are closer together (e.g., aphid midges, Aphidoletes spp.) or touching (mite midges, Feltiella spp.). The breathing tubes of flower flies or syrphids (Syrphidae) are touching (fused), except for first instars of some species. The length of syrphids' anal spiracles varies by species and may project (Syrphus spp.) or not and appear as a discolored blotch (Allograpta spp.). 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. Appendages can differ for species that feed in protected locations or water. Credit: 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). https://books.google.com/books/download/The_Syrphidae_of_Ohio.pdf?id=MTcoAAAAYAAJ&output=pdf&sig=ACfU3U0i_aGC_z78oTtl6d1qFDcx6rtZEA.
