Phoresy by aquatic dipterans has been widely reported among many freshwater aquatic organisms including insects, snails, bivalves, and fish. This attachment is practiced by dipterans among the Chironomidae, Simuliidae and Ceratopogonidae. A phoretic relationship is thought to provide Chironomid midges with food resources, substrate and a mechanism of dispersal.  In a few instances, this relationship has been reevaluated and judged to be parasitic. During a crayfish survey of the Susquehanna River in Pennsylvania using 100 baited wire traps at 11 locations, we collected specimens of the rusty crayfish (Orconectes rusticus) at one location that had cases of an aquatic dipteran attached to them.

Results/Conclusions

Three O. rusticus specimens were collected that had two, four and 10 dipteran cases, respectively, attached to their exoskeletons.  All of the larvae inside the cases were identified as Rheotanytarsus sp. Eight of the chironomid cases occurred within the cervical ridge on the dorsal and lateral surface of the carapace.  Two cases were on the side of the carapace above the third and fourth sets of walking legs; four cases were on the ventral surface of the merus or the dorsal surface of the carpus of the chelae. One case each was located along the postorbital ridge or the rostrum.  All of the chironomid cases were positioned in locations where they would escape grooming by crayfish. Not all of the cases contained chironomid larvae, therefore we cannot be certain if different larvae built each one. Sediments suspended in the water column near this sampling location likely facilitated the phoretic relationship at this site by providing larvae on crayfish the materials needed to build cases. This is the first record of a phoretic relationship between an aquatic dipteran and crayfish.