Aquatic Insects of Michigan

by Ethan Bright, Museum of Zoology Insect Division and School of Natural Resources and Environment
University of Michigan

Aquatic Isopoda (Sowbugs) of Michigan - Identification

Richard Snider (1991) published a review of the Isopoda of Michigan, and little work since then has been made to update this work. The great majority of the estimated 10,000 described species of isopods (sowbugs, pillbugs) are terrestrial or marine, but about 130 species of freshwater forms occur in North America north of Mexico. Williams (1972) provide a general survey of the freshwater fauna for North America, the majority of which in Eastern and Southeastern parts of the United States. Of the freshwater fauna, almost all of them are associated with some sort of lotic habitat, including springs, streams, and even subterranian habitats. Isopoda is quite depauperate in Michigan, with only 4 known freshwater species recorded from the state. The freshwater diversity of isopoda increases markedly in more eastern and southern states, particularly in regions of karst topography and Appalachian river systems.

Brown (1975) synonomyzed the species of Asellus with Caecidotea, although this may not be universally accepted.

The key below is modified from Williams (1972) and Pennak (1989).

1a	a. Anterior margin of the cephalothorax with a low but distinct carina	Lirceus lineatus (Say)
	b. Suture bewteen the exopod segments of pleopod 3 running from the median posterior angle very obliquely toward the lateral margin
	Also: Uropods cylindrical
1b	a. Anterior margin of the cephalothorax without a carina	Caecidotea, 2
	b. Suture bewteen the exopod segments of pleopod 3 running from the median posterior angle much less obliquely toward the lateral margin
	Also: Uropods cylindrical
2a(1b)	Medial process absent	Caecidotea intermedius (Forbes)
	also: Caudal process present, usually with an acutely pointed apex; cannula (tube) short and wide
2b	Medial process present	3
3a(2b)	Cannula long and narrow	Caecidotea racovitzai (Williams)
	Also: Pleopods 1-2 about same length
3b	Cannula short and wide	Caecidotea forbesi (Williams)
	Also: Ant2 shorter than body; medial process long and narrow

References

Snider RJ. 1991. The Michigan isopoda fauna. Michigan Academician 226:195-200.
Williams WD. 1972. Freshwater isopods (Asellidae) of North America. Water Pollution Control Research Series 180-50 ELDO/72, United States Environmental Protection Agency:1-45.