The mosaic darners of the genus Aeshna (aka blue darners) are large, strong-flying dragonflies with late-season flight periods. Most are similar to each other in size and general coloration, so in-hand examination is usually necessary to identify them. The most important distinguishing characteristics for this genus are the shapes and colors of the pale stripes on the sides of the thorax, especially the first or anterior stripes (here referred to as anterior thoracic side stripes [ATSS]), and the shapes of the cerci (upper pair of claspers) at the tip of the abdomen (whether paddle type or wedge type). Other marks that are often helpful include the presence/absence of a black line across the face, and the sizes of the pale spots on top of the abdominal segments (S), including the presence/absence of a spot on S10. Refer to the images of Aeshna species on the species pages of this website to compare shapes of thoracic side stripes and consult any dragonfly field guide for illustrations of the claspers (some guides are listed in the Resources Section).
The anterior thoracic side stripes (ATSS) of both genders are unmistakably thin and zigzagged (hence its common name). The abdomen is unusually heavily spotted, especially in side view; pale blue on males, yellow or white on most females (some are male-like). Further, this is a small species (total body length averages less than 2.5 inches; most mosaic darners are more than 2.5 inches long). The face has a thin black cross-line. Males have paddle-type claspers. Flight is low, with infrequent hovering, and it often perches horizontally on the ground – all unusual characteristics for a mosaic darner in Wisconsin.
This boreal species occurs widely throughout northern Canada, but in the upper Midwest is found only in the northern-most parts of Michigan, Minnesota, and Wisconsin. The preferred habitat is small, shallow, cold-water pools (typically 10 square meters or less in size) in bogs and sedge fens, including pools that may dry in summer. This species is rare in Wisconsin, documented here for the first time in 2004 in Douglas and Ashland counties, with a known breeding population in a Stockton Island fen.
Throughout its range, the flight season is from early June to late September.