In California the only caenids reported are from the genus Caenis*. There are other genera in this Family (Amercaenis, Brachycercus, Cercobrachys, Latineosus, Sparbarus, and Susperatus) but as of right now they have not been documented within California. The most complete key to the genera can be found in Sun and McCafferty (2008). Lu Sun did some incredible SEM and artwork in that paper.
The only paper you need for California species is by Arwin Provonsha (1990), who I worked with while at Purdue University. Arwin did a great job in providing keys, detailed descriptions and figures, and making identification of this small in stature species manageable. This was the first group I learned on. I took Arwin’s key, sat down with thousands of specimens, turned the radio on, and began my mayfly journey in the basement of Entomology Hall.
Day (1956) didn’t treat this group in any detail. Older keys to the species can be found in Traver (1935) but Provonsha (1990) is the one to use.
*Note: To pronounce Caenis correctly, keep in mind the C is pronounced with like an S, and you don’t pronounce the “a” but rather say “ae” with a long “e” sound, just as you would in Caesar.
Caenis amica Hagen
Type Locale: Baltimore, Maryland [Provonsha (1990) discovered there is some confusion over this–he tells the story in his paper]
California Distribution: Lassen, Mendocino, Modoc, Placer, Plumas, Riverside, San Diego, Sierra, Siskiyou, Trinity, Ventura
Adult Description: Provonsha’s (1990) key is the best one available. He provided figures to dorsal and ventral color patterns (Figs. 69 and 70).
Larval description: As with the adults, Arwin illustrated the dorsal color pattern of the larvae (see Fig. 2). Couplet 7 in Arwin’s key asks you to look at the posterior margin of sternum 7 to see if it is rounded or truncate (more blunt on the end). Compare Figs. 40 and 41. I found it easier to see this trait by removing the 10th (or last) abdominal segment, the one that holds the tail. Once I started doing that, separating this species from C. latipennis became much easier.
Identification Notes: In identifying species of Caenis, color pattern is quite useful, and the color patterns seem to hold up in preserved specimens. I like to compare the figures Arwin drew to the specimen in front of me. That works pretty well.
Ecological notes: When collecting adults, look in the spider webs near bridges in the morning. This is where I have found species of Caenis. For the larvae, rwin noted the species can be found areas with little to no current–ponds and slower areas of rivers.
Caenis bajaensis Allen & Murvosh
Type Locale: Baja California Sur (See Allen and Murvosh 1987)
California Distribution: Colusa, Glenn, Fresno, Los Angeles, Monterey, Napa, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara, Santa Clara, Stanislaus, Tehama, Tulare, Ventura.
Adult Description: The adult color patternhelps makes this species easy to identify [see Fig. 80 in Provonsha (1990)]. The head is all gray as is the abdomen. Allen and Murvosh (1987) did not describe the adult.
Larval description: This species was originally described from larvae only (See Allen and Murvosh (1987)], and Provonsha (1990) expanded considerably on that description, providing excellent Figures (Fig. 7, for example). Some C. latipennis may also have this notch, but because of the gray coloration of C. bajaensis, this species will key out quite quickly in Arwin’s key.
Identification Notes: The larvae have a distinct notch in the 9th sternite. As with C. amica, it can be easier to see the notch by removing the last abdominal segment.
Ecological notes: Of the known USA species, this is most represented in the Southwest into Mexico. Provonsha (1990) noted larvae can be found in sandy-bottomed streams.
Caenis latipennis Banks
Type Locale: Washington
California Distribution: Butte, Calaveras, Imperial, Kern, Lake, Madera, Monterey, Napa, Placer, Sacramento, Stanislaus, Tehama, Trinity, Yolo
Adult Description: Provonsha’s (1990) key is the best one available. He provided figures to dorsal color patterns (Fig. 73).
Larval description: Again, Arwin illustrated the whole larvae (Fig 3). As with C. amica, Couplet 7 in Arwin’s key asks you to look at the posterior margin of sternum 7 to see if it is rounded or truncate (more blunt on the end). For C. latipennis it is truncate, but it can also be somewhat notched. I asked Arwin about this, and he had noticed this, too, but he did not include this variation in his key.
Identification Notes: For the larvae, one does have to also look at spines on the tarsus (the segment the claws are attached to). I also found this hard initially without taking the leg off. In addition Arwin noticed a latitudinal gradient in color pattern for this species. How this manifests itself in California, I do not know. It would be interesting to see if there is an elevational gradient although I don’t see any high elevation records of this species in California.
Ecological notes: Arwin noted this species seems to occur everywhere, but prefers slower waters and can be found in ponds. He also noted some specimens have been collected from sewage lagoons.
Caenis punctata McDunnough
Type Locale: Quebec, Canada
California Distribution: Riverside
Adult Description: Provonsha’s (1990) key is the best one available. He provided figures to dorsal color patterns (Fig. 74).
Larval description: See Provonsha (1990) for the best description and figures.He didn’t do a whole larval drawing, but he did illustrate the diagnostic characters.
Identification Notes: Again, identifying the larva requires looking at the shape of sternite 9. Take it off. That’s the easiest way to earn to see it.
Ecological notes: First reported by McCafferty and Meyer (2007). They reported this species was not expected to be found in California based on the known distribution of this species at the time. Arwin reported this species was collected from sand or fine gravel substrates in still water.
Caenis youngi Roemhild
Type Locale: Montana
California Distribution: Kern, Mono
Adult Description: Roemhild (1984) provided a pretty good description of the species, but Provonsha (1990) puts it in context with other Caenis, and so Provonsha’s (1990) paper would be the one to use. See Fig. 75 for a full illustration of the adult.
Larval description: Roemhild (1984) described larvae believed to be this species, but it had not been reared at that time–it was an educated guess. Use Provonsha (1990) for larval identification.
Identification Notes: Elevation may be a helpful character separating this species from others. However, this species is quite distinct in having a dorsal projection on the larvae and adults [see Figs 33 and 61 in Provonsha (1990)].
Ecological notes: Roemhild (1984) noted this species and other Caenis in Montana occur below 1000m and prefer warmer waters. The Mono county locale does not fit this pattern, however. A Google Earth search shows Glacier lodge at 7800ft (almost 2400 m). Provonsha (1990) noted other location data for this species range from 1900-2100m–so this may be a high elevation species of Caenis.
References:
Allen, R. K. and C. M. Murvosh. 1987. Taxonomy and zoogeography of the mayflies (Ephemeroptera: Insecta) of Baja California. Ann. Entomol. Soc. Amer. 76: 425-433.
Day, W. C. 1956. Ephemeroptera. Pages 79-105 In Aquatic Insects of California. R. L. Usinger (ed). University of California Press, Berkley.
McCafferty. W. P. and M. Meyers. 2007. Insecta, Ephemeroptera: Transcontinental range extension in western North America. Check List 3: 51-54.
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.
Roemhild, G. 1984. A new species of Caenis (Ephemeroptera: Caenidae) from Montana, USA. Aquatic Insects 6: 7-11.
Sun L; McCafferty WP. 2008. Cladistics, classification and identification of the brachycercine mayflies (Insecta: Ephemeroptera: Caenidae). Zootaxa 1801:1-239.
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.