Mayfly Species Stenonema ithaca (Light Cahills)

A reddish brown species; penes distinctly L-shaped; no spiracular dots; tails pale, joinings darker.

Head red-brown; purplish band across middle area of carin; black rings at bases of antennae. Thorax deep red-brown; pleura somewhat paler than notum and sternum. Median area and posterior margin of pronotum blackish. A purplish black streak laterally on prothorax extending down across fore coxa; similar dark streak on pleura anterior to wing roots. Dark purplish or reddish brown markings on pleura below root of each wing. Coxae deep red-brown, apical and lateral margins and a small apical spot reddish black. Posterior portion of mesosternum darker than remainder of sternum. Fore femur light reddish brown, with median and apical dark purplish red bands; tibia yellowish red, blackish at apex; tarsus yellowish with smoky tinge, joinings narrowly darker. Basal joint of fore tarsus somewhat less than 1/2 the length of the second. Middle and hind legs yellowish, sometimes with faint reddish tinge; femoral bands as on fore leg; tips of tibiae and tarsal joinings reddish brown. Wings hyaline; longitudinal veins yellowish to reddish brown, cross veins dark red-brown; veins of costal margin slightly thickened, cross veins not dark-margined. Only a very slight crowding of cross veins at bulla; usually 1 in costal space, 2 or 3 in each of two following spaces. Costo-apical region somewhat opaque, may be faintly brown-tinged; a faint reddish or brownish stain occasionally present in stigmatic area, typically without such a stain. Veins of hind wing paler and finer.

Abdominal tergites 1-8 largely occupied by wide reddish to purplish brown bands on the posterior margin, from which deep shading of same color extends forward almost to the anterior margins. A mid-dorsal streak of same dark color extends the length of the abdomen, sometimes divided by a very narrow pale median line. Near each spiracle is a pale irregularly oval space surrounded by purplish grey markings. No dark spiracular dots. A pale yellowish area remains on each side of each tergite on anterior margin; pleural fold usually narrowly pale. Sternites greyish, more or less tinged with pale reddish or purplish; posterior margins narrowly purplish grey, so that abdomen appears somewhat annulate. Traces of brownish triangles in posterolateral areas may be present. Segments 9 and 10, often the posterior portion of 8, opaque, bright reddish with distinct yellow tinge; tergites with usual dark markings. Penes distinctly L-shaped; of rubromaculatum (now a synonym of Stenonema modestum) type. Tails pale greyish or yellowish white; joinings narrowly deep purplish brown, alternate joinings wider.

This species is darker red-brown than S. fuscum (now a synonym of Stenonema vicarium) or rivulicolum (now a synonym of Stenonema vicarium), which it somewhat resembles; abdominal sternites dark-tinged and with darker posterior margins; the penes are also more sharply L-shaped.