This sawfly larva (which is common in central Illinois) feeds externally on leaves of Orbexilum (formerly Psoralea) onobrychis (Fabaceae). Larvae of the first generation of the year mature in early to mid-May (those in the present image were photographed on 5 May). The larva pupates inside a round, fuzzy, yellowish cocoon that is attached to the host plant. Larvae can be seen much later in the year than May; evidently, there is more than one generation per year.

Based on adult characters (see comments on the images linked to this one), this sawfly keys to Sphacophilus apios; however, in the original 1933 description of that species, Ross (Proceedings of the Entomological Society of Washington 35, p. 15) states that the color of the larval head of S. apios is "gray yellow orange with gray orange mottling." The larval head of the Orbexilum feeder, on the other hand, always is uniformly blackish, as seen in this photo.