The baetids are a group of mayflies common in California streams and very well known by fly fishers. A google search of Baetis and California will yield numerous web sites focused on fishing applications, especially of the Blue-Winged Olive.
This is one of my favorite groups to work with, but historically this has been a hard group to identify to species, especially the adults. This is due, in part, to their small size. The male genitalia in the adults are not sclerotized, thus limiting their usefullness in species identification. And so adult identification can be a bit of an art. The larvae in most groups are quite readily identifiable, and so if you can collect larvae, that can help focus your efforts on adult identification. Learning how to use the mouthparts in the larval stage is the key for this group.
Morihara and McCafferty (1979) really is the starting point for larval identfication. Historically the genus Baetis was a dumping ground for small maflies with two large forewings and really tiny hind wings. The work by Morihara ana McCafferty (1979) provided the first real useful tool for identifying these larvae. Many of the species in this work have been moved to other genera, but the key and descriptions are still very useful, but with the understanding you will need to update the name (Mayfly Central is a good place to start for updated nomenclature).
For the adults the keys in Traver (1935) and Day (1956) are still the ones to have on hand, in addition to a handful of papers with descriptions of more recent species and updated keys. The adults are tough, and some of the couplets in Traver’s (1935) key are hard to use unless you have seen lots of adults. One frustrating example reads “Eyes large; eyes small”. No figures are provided, either. Good luck with that one.
When identifying larvae, if you can learn what the labial palp for each genus looks like, you can speed up the identification process significantly. That is the first thing I look at when identifying the larvae as the palps are pretty distinct for each genus.
Acentrella
A relatively recent key to the North American larvae can be found in Jacobus and McCafferty (2006) in a paper describing a new species from the Great Smoky Mountains National Park (I actually helped collect this one). A key to the adult North American Acentrella can be found in Burian and Myers (2011).
Acentrella insignificans (McDunnough)
Type Locale: British Columbia
California Distribution: Alpine, Butte Colusa, Del Norte, Fresno, Glenn, Humbot, Inyo, Kern, Los Angeles, Mendocino, Merced, Modoc, Monterey, Placer, Sacramento, San Benito, San Joaquin, Siskiyou, Sonoma, Stanislaus, Tehama, Trinity, Yolo.
Larva: As mentioned above, the most recent key for this group can be found in Jacobus and McCafferty (2006). I have not used this key myself, but instead have used Morihara and McCafferty (1979) extensively. But the Jacobus and McCafferty (2006) key will get you to this species within two couplets, and so I see no reason to plow through the Morihara and McCafferty (1979) key. They key difference between this species and A. turbida (McDunnough) is the presence of hindwingpads on the A. insignificans, which are absent on A. turbida [although the hind wings can look like threadlike structures–somewhat similar to Fig. 4 in Weirsema (2000)].
Adults: Burian and Myers (2011) provided the most recent key. I wish there were more figures in their key to make life easier for the non-specialist, but in California there are only two species known, with the big difference being A. insignificans have hind wings whereas A. turbida do not. Easy. There is a figure of the male genitalia in McDunnough (1926). See Figure 7.
Acentrella turbida McDunnough
Type Locale: Alberta
California Distribution: Butte, Colusa, El Dorado, Fresno, Humboldt, Kern, Mariposa, Merced, NEvada, Sacramento, Santa Barbara, Shasta, Sisiyou, Sonoma, Stanislaus, Sutter, Tehama, Trinity, Yolo, Yuba.
Larva: The most recent key for this group can be found in Jacobus and McCafferty (2006). Larvae of A. turbida are not included in the Morihara and McCafferty (1979) because at that time the species was still considered a species within the Pseudocloeon. McCafferty el al. (1994) provide a useful description of the larvae, including a key to the larvae and adults. They major difference between this species and A. insignificans (McDunnough) is the absence of hindwingpads. Keep in mind the hind wings can look like threadlike structures–somewhat similar to Fig. 4 in Weirsema (2000)]. You do have to look closely to see these.
Adults: Burian and Myers (2011) provided the most recent key. In California there are only two species known, with the big difference being A. insignificans have hind wings whereas A. turbida do not. McCafferty et al. (1994) did provided figures of larave and photographs of the larvae and adults.
Apobaetis etowah (Traver)
California Distribution: Lassen, Stanislaus.
Larvae: Day (1955) described A. indeprensus Day from specimens collected in the Tuolumne River 6 miles outside of Modesto. Gets one to think if that location is now in the city proper? Meyer and McCafferty (2003) later discovered Day’s species to be the same species as A. etowah (Traver). A relatively recent description of the larvae with new figures, including a key to the Apobaetis, can be found in Cruz (2020). In addition a key to the Apobaetis was published by de Lima et al (2024). This paper describes new species from Brazil, and so the key can be used with California species, but it is not necessary since there is only one. There is only one species of Apobaetis in California, and so larval identification is easy. The species has very distinct labial palps [See Fig. 6 in Day (1955)] that look like no other mayfly in California. Day (1955) described the habiat where he found this species as follows: “warm water species showing remarkable ability to survive under marginal conditions. The lower Tuolomne River has been dredged, diverted, and dammed, and has suffered pollution from irrigation run-off, crop-dusting, sewage, and industry.” In addition, according to Day, it seems this species can survive relatively high water temperatures-up to 82 degrees F. If the Tuolomne River has been cleaned up, I wonder if this species is still there??
Adult: This species is identified with having only two wings and a distinct penes cover (although “distinct” is a relative term). Meyer and McCAfferty (2003) discuss the variablity of the penes cover, and so one should review that paper when delaing with the adults. Day (1955) provided a description of the adult including the genitalia (Fig. 13), which was redrawn in Edmunds et al. (1976 ) (Fig. 359).
Genus Baetis
First, how to pronounce this genus. I learned it with a long “e” sound, so Beetus, similar to how we say Caesar, with the long “e”. But many say it with the long “a” sound, Bate-us. It does not really matter but I will keep my pronunciation.
The first truely useful key to the larvae was produced by Morihara and McCafferty (1979). This key revolutioned the ability to identfy these larvae, in part with the extensive us of mouthpart morphology. This is a go-to paper one must have. Relatively recently, Webb et al. (2018) created a key with the intent to provide a tool for those who are doing biassessments. In this paper the authors recognize some groups are poorly known and others may be part of a species complex, which makes species identification difficult. I would have both keys in front of me when looking looking at species of Baetis. My go-to would still be the Morihara and McCafferty as it is easier to use (the couplets are shorter), and then supplement that with the Webb at al. document. The adults are still a bit if an enigma.
Baetis adonis Traver
Type Locale: San Gabriel Mountains in Southern California
California Distribution: Colusa, Contra Costa, Humboldt, Inyo, Los Angeles, Mariposa, Napa, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara, Sonoma, Stanislaus, Ventura.
Larvae: McCafferty et al. (2008) described the complicated taxonomic history of this species, but for identification purposes, the key by Morihara and McCafferty (1979) is the one to use. In this work the species will key to Baetis sp. B.
Adults: Traver (1935) originally described this species, and included a desription of the adult. And it is included in her key to the Baetis adults. Day (1955) also included this species in his key, but McCaefferty and Silldorff (1998) provide a better description, including a figure of the male genitalia.
Baetis alius Day
Type Locale: Russia River near Geyserville, Sonoma County.
California Distribution: El Dorado, Mariposa, Mendocino, Sonoma, Yolo.
Larvae: Day (1954) did not provide a description of the larvae as he did not know it at the time , but Morihara and McCafferty (1979) did describe the larvae and included it in their key and description of North American species of Baetis. I find this species to be distinct because of the convex nature of the segment 2 of the labial palp being convex, where the more typical condition of for that section to be convex (compare Fig. 33c in Morihara and McCafferty with Fig. 21b, for example). The figures do not do it jsutice–once you see this shape, it is very distinct. McCafferty et al. (2008) justifies synonomizing B. moqui Wiersema et al. (2004) as a synonym of B. alius. The Wiersema paper provided nice figures of this species, but one must keep in mind this is now considered a synomym of B. alius. What is notable, in addition to the labial palp, is gill 1 can be highly reduced in size or absent altogether.
Adults: Day (1954) described the adults, with the key to seperating this species from all others due to the absence of a tubercle on the basal segment of the genitalia (see Fig. 6 in Day 1954). I have not seen these adults myself, and so do not know how distinct this really is. The key in Day (1955) is not satisfying.
Baetis bicaudatus Dodds
Type Locale: Colorado
California Distribution: Alpine, El Dorado, Fresno, Humbolt, Inyo, Los Angeles, Mono, Nevada, Sacramento, Santa Barbara, San Bernardino, Shasta, Siskiyou, Trinity, Tulare
Larvae: This is one of the species where the name is actually useful in identifying it–the larvae have two tails. This contrasts with most Baetis in that most have three tails. In addition B. bicaudatus likes cold water, at least in my experience. And generally higher elevations. Webb et al. (2018) described some complications with identifying the larvae based on the fact the medial caudal fililaent (the middle tail) has been described as variable in length by some workers. But based on genetic data, they argued that in order for a species to be B. caudatus, the middle tail should only be 1-2 two segments in length. The larvae are easily keyed out in Morihara and McCafferty (1979). A key is also in the Webb et al. (2018) paper, but it is a bit busier than the Morihara and McCafferty key.
Adults: Day (1955) does include this species as well at Traver (1935). The identifying features are the hindwings with only two veins and with a costal process that is not curved. In Fig 25a in Day (1955) it does show a third vein, but it is really small. That is what is meant by only two longitudinal veins being present. Annoying, isn’t it?
Baetis diablus Day
Type Locale: Mt. Diablo, Contra Costa Co.
California Distribution: Contra Costa, Mono. The Mono County record reported from Convict Creek (See McCafferty et al. 2008) is interesting and I think worth a visit to the site. Unfortunately the locale labels for each of these specimens are vague (Mt Diablo and Convict Cr).
Larvae: Unknown.
Adult: Day (1954) provided a figure of the genitalia (Fig. 3) and the hind wing (Fig. 4), arguing this species was unique in the genus by “having a forked second hindwing vein…The fork of the second hind wing vein is unlike that of B. parvus or B. devinctus, as it branches much closer to the distal margin of the hind wing.” Note: B. parvus is considered a synonym of Diphetor hageni (Eatoni)–see details on this species in the Diphetor section.
McCafferty et al. (2008) provided an analysis of the adult, comparing it with both B. tricaudatus Dodds and B. magnus McCafferty & Waltz, cautioning the fork in the hindwing can be variable. McCafferty et al. (2008) floated the idea that this species might just be a variant of B. tricaudatus, but more dat were needed to make this determination.
Baetis flavistriga McDunnough
This is one of the first species I learned to identify.
Baetis magnus McCafferty & Waltz
The name is appropriate, this is a big mayfly, at least for those in the genus Baetis.
Baetis notos Allen & Murvosh
Type Locale:
Larvae:
Adults:
Baetis palisadi Mayo
Type Locale:
Larvae:
Adults:
Baetis piscatoris Traver
Type Locale:
Larvae:
Adults:
Baetis tricaudatus Dodds
Type Locale:
Larvae: Ok–this one is complicated It all over the place, and some argue there are actually multiple species that make up this complex.
Adults:
Burian, S.K. and L. W. Myers. 2011. A new species of Acentrella Bengtsson (Ephemeroptera: Baetidae) from New York and New England (USA), redescription of the nymph of A. parvula (McDunnough), and key to known adults males of Nearctic Acentrella. Aquatic Insects 33: 305-334
Cruz, P. V. 2020. Redescription of the three species of Apobaetis Day, 1955 (Ephemeropters: Baetidae). Zootaxa 4808: 317-330.
Day, W. C. 1954. New species of California mayflies in the genus Baetis. The Pan-Pacific Entomol. 30: 29-34.
Day, W. C. 1955. New genera of mayflies from California (Ephemeroptera) The Pan-Pacific Entomologist 31: 121-137.
Day, W. C. 1956. Ephemeroptera in Aqautic Insects of California with keys to North American genera and California Speci. pp. 79-105. R. L. Usinger, ed. Univerdsity of California Press.
de Lima, C. R., P.V. Cruz, and N. Hamada. 2024. Five new species of Apobaetis Day (Epjemroptera: Baetidae) and updated key to nymphs. Zootaxa 5397: 91-115 https://doi.org/10.11646/zootaxa.5397.1.5
Jacobus, L. M. and W. P. McCafferty 2006. A new species of Acentrella (Bengtsson (Ephemeroptera: baetidae) from Great Smoky Mountain National Park, USA. Aquatic Insects 28: 101-111
McCafferty, w. P. and E. L. Silldorf 1998 Reared associations and equivanlency of Baetis adonis and B. caelestis (Ephemeroptera: Baetidae) Entomol. News 109: 261-265.
McCafferty, W. P.,M. J. Wigle, and R. D. Waltz. 1994. Systematics and biology of Acentrella turbida (McDunnough) (Ephemeroptera: Baetidae). Pan-Pacific Entomologist 70: 301-308.
McCafferty, W. P., M. D. Meyer, R. P. Randolph, and J. M. Webb. 2008. Evaluation of mayfly species originally described as Baetis Leach (Ephemeroptera: Baetidae) from California. Proc. Entomol. Soc. Wash
McDunnough, J. 1926. New Canadian Ephemeridae with notes IV. The Canadian Entomologist 58: 296-303.
Meyer, M. D. and W. P. McCafferty. 2003. New synonym of Apobaetis etowah (Traver) (Ephemerotpera: Baetidae). Pan-Pacific Entomologist 79: 249.
Morihara, D. K. and W. P. McCafferty. 1979. The Baetis larve of North America (Ephemeroptera: Baetidae) Trans. Am. Entomol. Soc 105: 139-221.
Wiersema, N. A. 2000. A new combination for two North American small minnow mayflies (Ephemeroptera: Baetidae). Entomological News 111: 140-142.
Wiersama, N. A., C. R. Nelson, and K.F. Kuhnl. 2004. A new small minnow mayfly (Ephemeroptera: Baetidae) from Utah, U.S.A. Entomol. News 115: 139-145.
Webb, J. M., L. M. Jacobus, and S. P. Sullivan. 2018. The state of systematics of North American Baetis Leach, 1815 (Ephemeroptera: Baetidae), with recommendations for identification of larvae. Zootaxa 4394: 105-127. https://www.mapress.com/zt/article/view/zootaxa.4394.1.6/14047