EPHEMERIDAE
This is somewhat of an iconic mayfly, especially in the Midwest. Even a beer, Hexagenia IPA, (http://fallriverbrewing.com/beer/) is named after this mayfly. It is an IPA, unfortunately.
Only one genus in the family Ephemeridae has been documented in the literature from California. Cal EPA does list H. bilineata and Ephemera simulans from California, although these records have not been published in the primary literature. The family is quite widespread in the Midwest and eastern North America (see McCafferty 1975, Randolph and McCafferty 1998). The Hexagenia are famous for their massive hatches along the Upper Mississippi River and Great Lakes. These hatches are large enough to be picked up on Doppler Radar. The National Weather Service actually tracks these hatches on the La Crosse River, Wisconsin (https://www.weather.gov/arx/mayfly_tracking). No huge hatches as described from the Midwest have been documented in California (at least in recent history). Several fly-fishing web sites report decent hatches, but nothing approaching what one sees in the Midwest.
In California, previous to the work of Meyer and McCafferty (2008), this species was only reported from Fresno County and described as a new species, Hexagenia californica (Upholt 1937), which was later determined to actually be H. limbata. Spieth (1941) believed H. californica to be a subspecies of H. limbata, and Day (1956) followed this nomenclature in his book.
Hexagenia limbata (Serville)
Type Locale: North America (yes–vague but that is how these things used to be labeled)
California Distribution: Butte, Contra Costa, El Dorado, Fresno, Glenn, Kern, Kings, Lake, Lassen, Madera, Merced, Modoc, Placer, Sacramento, San Bernardino, Shasta, Siskiyou, Stanislaus, Sutter, Trinity, Tulare, Tuolumne, Yolo, Yuba
Adult: Day (1956) did include this species in his key. But a more complete adult key can be found in Spieth (1941). Even though only one species of Hexagenia is known from California, I would still use Spieth (1941). Spieth (1941) also provided very useful figures of the adult coloration of the different species of Hexagenia (see Plates 1-4)
Larva: McCafferty (1975) is the best key to the larvae (and larvae of all North American burrowing mayflies). Start with Couplet 19. One can almost use the color patterns depicted for adults in Spieth (1941) to identify larvae.
Identification Notes: In California there really is no problem in identifying this species. Other tusked mayflies (Anthopotamus, certain Paraleptophlebia) are not as large and lack the distinct color patterns.
Ecological notes: The biology of this species and H. bilineata have been studied extensively. A quick Google search will prove this. The larvae are generally known from lakes and large rivers, although I have seen reports of this species from irrigation ditches. If one is interested in detailed explanation on how this species burrows, see Keltner and McCafferty (1986). McCafferty and Pereira (1984) showed that abdominal coloration can be influenced by temperature, thus arguing why the subspecies based on color pattern are invalid.
References:
Day, W. C. 1956. Ephemeroptera. Pages 79-105 In Aquatic Insects of California. R. L. Usinger (ed). University of California Press, Berkley.
Keltner, J. and W. P. McCafferty. 1986. Functional morphology of burrowing in the mayflies Hexagenia limbata and Pentagenia vittigera. Zool. Journal of the Linnean Society 87: 139-162.
McCafferty, W. P. 1975. The burrowing mayflies (Ephemeroptera: Ephemeroidea) of the United States. Trans. Amer. Entomol. Soc. 101: 447-504.
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.
McCafferty, W. P. and C. Pereira. 1984. Effects of developmental thermal regime on two mayfly species and their taxonomic interpretation. Ann. Entomol. Soc. 77: 69-87.
Randolph, R. P. and W. P. McCafferty. 2008. Diversity and distribution of the mayflies (Ephemeroptera) of Illinois, Indiana, Kentucky, Michigan, Ohio, and Wisconsin. Ohio Biological Survey Bulletin NS 13 (1). 188pp