In North America there is only one genus in this family and one species known from California. Thus identification is straight-forward, at least as far as the California species is concerned. The best key to use for Isonychia is by Kondratieff and Voshell (1984), which includes all North American species. In Day’s Aquatic Insects of California (Day 1956), the species can be found under what was at the time considered the subfamily Siphlonurinae. Locale data below are from Meyers and McCafferty (2008).

Isonychia velma Needham

Type Locale: Yolo

Note: Needham (1932) listed the type locale as “Putah Creek near Maticello.” The actual location is likely the town of Monticello, which is under water now due to the construction of Monticello Dam on Putah Creek. Work on the dam was completed in 1957, creating Lake Berryessa [Kondratieff and Voshell (1984) noticed this misspelling]. In essence, the type local is no longer there.

Putah Creek below the dam is now popular for trout fishing, and has been extensively collected by the Putah Creek Council as part of a New Zealand mud snail research project, but this species has not been found in Putah Creek below the dam in recent years (this is my local stream, and I have collected extensively there).

California Distribution: Amador, Butte, El Dorado, Humboldt, Mariposa, Mendocino, Nevada, Placer, Plumas, Shasta, Sierra, Siskiyou, Sonoma, Tehama, Trinity, Yuba.

Adult Description: Needham (1932) described the adult (repeated in Traver 1935), providing a figure of the male genitalia [see Figs. A and B in Needham (1932)]. Kondratieff and Voshell (1984) provided a more extensive description and additional figures, including the genitalia, abdominal coloration, and wings (see Figs. 4, 26, 32). The species is included in Day’s (1956) key, but Kondratieff and Voshell’s (1984) work is more complete.

Larval description: Day (1952) provided the first larval description of this species, and figured the distinctive foreleg (Plate VIII, Figs 2 and 3) and repeated that figure in Day (1956) (Fig. 3:11f). Kondratieff and Voshell (1984) provide a more detailed description, including a figure of the mouthparts (Fig. 34).

Identification Notes: In California the larvae are easy to identify in that: 1) it is the only species of the genus known to occur in the state; and 2) the distinctive elongated setae on the foreleg, which is used by the larvae for filter feeding.

Ecological notes: Day (1952) described the species as occurring in shallow riffles in larger rivers, commonly found on wood. Kondratieff and Voshell (1984) just repeat Day’s (1952) description. Although based on the locale data from Meyers and McCafferty (2008), I would include medium sized streams (I wouldn’t put Putah Creek in the same size category as the Russian or the American Rivers).

In the Bohart Museum of Entomology at UC Davis I identified species in their collection collected from the American River in Sacramento (Sacramento Co) and Putah Cr (Lake County). The Lake County location might serve as in informal replacement for the type locale.

References:

Day, W.C. 1952. New species and notes on California mayflies. Pan-Pacific Entomol. 28: 17-39.

Day, W. C. 1956. Ephemeroptera. Pages 79-105 In Aquatic Insects of California. R. L. Usinger (ed). University of California Press, Berkley.

Kondratieff, B. C. and J. R. Voshell, Jr. 1984. The North and Central American species of Isonychia (Ephemeroptera: Oligoneuriidae). Trans. Amer. Entomol. Soc. 110: 129-244.

Meyers, M. and W. P. McCafferty. 2008. Mayflies (Ephemeroptera) of the far western United States. Part 3: California. Trans. Amer. Entomol. Soc. 134: 337-430.

Needham, J. 1932. Three new American mayflies (Ephemeroptera) Can. Entomol. 64: 273-276.

Traver, J. R. 1935. Part II. Systematic. Pp. 237-739 in The Biology of Mayflies by Needham, J.G., J. R. Traver, and Y.-C. Hsu. Comstock Publishing Company, Inc. Ithaca.