Blue-winged Olives
Baetis
Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
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This seems to be a young larva of Limnephilus. Although not clear in the picture, several ventral abdominal segments have chloride epithelia.
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Identification: Key to Families of Mayfly Nymphs, Couplet 13
Identification: Key to Families of Mayfly Nymphs, Couplet 13
Error: Tried to access a key that isn't live.
Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
This couplet refers figures (usually designated 'sf') from this source.
You will need the source (make sure to get the same edition!) to see them.
This couplet refers figures (usually designated 'sf') from this source.
You will need the source (make sure to get the same edition!) to see them.
| Option 1 | Option 2 |
|---|---|
| Tarsal claws of forelegs differ in structure from those on middle and hind legs (source figs 13.26, 13.27) | Tarsal claws of all legs similar, usually sharply pointed, but can also be spatulate |
| Tarsal claws of middle and hind legs long and slender, about as long as tibiae | Tarsal claws variable in length; if tarsal claws of middle and hind legs long and slender, then tarsal claws usually shorter than tibiae; if middle and hind tarsal claws longer than respective tibiae, then legs wither bowed or labrum with a deep v-shaped notch |
| Remaining families: Ametropodidae and Metretopodidae | Remaining families: Acanthametropodidae, Ameletidae, Baetidae, Leptophlebiidae, and Siphlonuridae |
5 Example Specimens
This one's stunned or freshly dead in the pictures, because the water in my photographing tank was a bit too warm for him after being under the bright light for a while.
This nymph has double front tarsal claws and double gills on the first three abdominal segments.
This nymph has double front tarsal claws and double gills on the first three abdominal segments.
This nymph has double front tarsal claws and double gills on the first three abdominal segments. This one has a distinct dorsal stripe.
| 5 Example Specimens
I caught this Ameletus nymph with several others of the same kind. This was the most vivid example, but they all had quite a bit of striking and unusual red shading, especially on the last few abdominal segments.
I keyed it out under the microscope using Larvae and adults of Ameletus mayflies (Ephemeroptera: Ameletidae) from Alberta with slightly larger (10 mm), mature specimen with darkened wingpads. Microscope pictures are from that specimen. The characteristics in the key and most of the verification table point pretty clearly to Ameletus cooki, except that the coloration of the antennae more closely resembles Ameletus sparsatus. However, on other characteristics in which these species differ (spines on the dorsal surface of the front femora, which seem very short in this specimen; length of posterolateral spines on segments 8–9; length of spines on posterior edge of tergites 6–9), this is a better match for cooki, and that's probably the correct ID. 
A few of these nymphs turned up in my kick net sample, with this being the only one fully mature and ready to emerge. It keys pretty straightforwardly to Ameletus suffusus, particularly because of the triangular dark patch on the labrum. The abdominal markings are close but don't quite match other drawings, likely just individual variation.
This specimen was collected along with a female which was quite a bit larger, different in color, but otherwise seemed the same morphologically.
These nymphs were highly abundant in my early April kick net samples from the Yakima Canyon, and one of them emerged into a dun, which I photographed immediately. Similar-looking nymphs but with distinctly brighter color patterns were also abundant. I just photographed one. After extensive views under the microscope, it's clear the bright ones are males and the dull ones are females of the same species.
The most likely guess at the species is Baetis tricaudatus, which may be a complex of related species that haven't all been sorted out yet. It isn't a perfect fit to every key characteristic (and I never seem to find a Baetis that matches the expected pronotum color patterns, but that seems to be the closest. The microscope images here were taken with different specimens from the main photos (so I could dissect them while preserving that one intact), but clearly the same species. 
A nymph of the same species as this one emerged into a dun in my studio so I got photos of both stages.
NOTE: I missed an important key characteristic the first time I tried to identify this one (robust setae on the abdominal sternites, which were harder to see than I expected but are clearly present), so I went on a bit of a wild goose chase and landed at a dead end. After spotting that characteristic, this one keys more straightforwardly to either Baetis tricaudatus or the Baetis piscatoris complex. It doesn't seem to be a perfect fit for either one in the key, but I'm going with tricaudatus based on range and abundance. It's not certain. However, I'm leaving the flawed analysis below with this disclaimer, because some aspects of how I approached that dead end might be informative in the future. ----Incorrect analysis below---- After spending a lot of time with this one under my shiny new microscope, I'm still not quite sure what it is. I botched my attempt to expose the mouth parts that might make the ID more definitive. Based on the key in Webb et al 2018's "Baetis Larvae of North America," here's my reasoning at each key couplet. Couplet 1. The pronotum lacks dark, submedian U-shaped markings. Also, if I were to follow through to couplet 2, there seem to be characteristics that rule out each of the options: the intercalaris complex is ruled out by the abdominal markings, and the caudal filaments have neither a dark median band (ruling out the flavistriga complex) nor uniform pale coloration (ruling out Baetis notos). This sends me with decent confidence to couplet 4. Couplet 4. I cannot find robust setae in my microscope on the scapes, pedicels, paraprocts, or sterna. I also do not see a pair of dark, bilobed markings on the pronotum. Unless I overlooked these characteristics, proceed to couplet 9. Couplet 9. Abdominal tergum 5 is a bit paler than adjacent terga, but "distinctly paler"? The figure for Baetis alius in the paper, as well as a very nice picture posted by Millcreek in the forum here, shows that Baetis alius would have darker tergites surrounding #5. So proceed to couplet 11. Couplet 11. The length of the gills is obviously less than 2X their width. This leads to the Baetis vernus complex, which could include that species or Baetis brunneicolor. This key doesn't say how to tell those species apart. Switching over to Burien et al 2018 as the source, the characteristics used to distinguish vernus from brunneicolor seem to rule out either one. Brunneicolor should have more uniformly brown abdominal tergites, whereas vernus should have a lack of visible tracheation in most of the gills. The fore femur length is about 3.8x its width. Also worth noting: In the genus ID, I thought I could see the villipore in my microscope, but I'm not sure. If I back out of Baetis altogether and assume there's no villipore, I end up at Fallceon, but this specimen doesn't seem to have the frontal keel on the head that's supposed to be present on Fallceon quilleri. So that seems like a dead end as well. |
| Go to Couplet 14 | Go to Couplet 15 |
Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Leads to Baetiscidae
(Baetisca):
(Baetisca):
- Mesonotum enlarged to form a shield extended to abdominal segment VI (in other words, they look like they're wearing a turtle shell)
- Abdominal gills enclosed beneath shield
Leads to Couplet 2:
- Mesonotum not enlarged as above
- At least some abdominal gills exposed
Couplet 2
Leads to Couplet 3:
- Gills on abdominal segments 2-7 forked, long, and pointy, with margins fringed creating a "feathery" appearance
- Usually with mandibular tusks projected forward and visible from above head Or, if tusks absent, head and thorax with pads of long spines (source fig 13.8)
Couplet 3
Leads to Couplet 7:
- Gills on segments 2-7 variable; if gills are forked, long, and pointy, then their margins not fringed (source fig 13.6)
- Mandibular tusks very rarely present
Couplet 7
Leads to Behningiidae
(Dolania americana):
(Dolania americana):
- Head and prothorax with dorsal pads of long spines (source fig 13.9)
- Mandibular tusks absent
- Abdominal gills ventral
Behningiidae
(Dolania americana)
(Dolania americana)
Leads to Couplet 4:
- Head and prothorax without pads of long spines
- Mandibular tusks present
- Abdominal gills lateral or dorsal
Couplet 4
Leads to Couplet 6:
- Foretibiae not adapted for burrowing (source fig 13.8)
- Abdominal gills lateral
Couplet 6
Leads to Couplet 5:
- Foretibiae modified for burrowing, either broad or with tubercles (source fig 13.7)
- Abdominal gills dorsal
Couplet 5
Leads to Euthyplociidae
(Euthyplocia hecuba):
(Euthyplocia hecuba):
- Mandibular tusks with many long setae (source fig 13.4)
Euthyplociidae
(Euthyplocia hecuba)
(Euthyplocia hecuba)
Leads to Potamanthidae
(Anthopotamus):
(Anthopotamus):
- Mandibular tusks without many long setae (source fig 13.8)
Leads to Couplet 19:
- Tips of mandibular tusks curved upward when viewed laterally (source fig 13.10)
- Ventral apex of hind tibiae developed as a distinct acute point (source fig 13.12)
Couplet 19
Leads to Polymitarcyidae:
- Tips of mandibular tusks not curved upward when viewed laterally (source fig 13.10)
- Ventral apex of hind tibiae rounded (source fig 13.13)
Leads to Couplet 20:
- Two rows of long hair-like setae present on inner margins of tibiae and femora of forelegs
Couplet 20
Leads to Couplet 8:
- Long hair-like setae absent from forelegs, or not arranged as above
Couplet 8
Leads to Couplet 9:
- Gills on abdominal segment II operculate or semioperculate
Couplet 9
Leads to Couplet 11:
- Gills on adbominal segment II neither operculate nor semioperculate, either similar to those on succeeding segments or absent
Couplet 11
Leads to Leptohyphidae:
- Gills on abdominal segment 2 triangular, subtriangular, or oval and not meeting medially (source fig 13.15)
- Gill lamellae on segments 3-6 simple or bilobed, without fringed margins
Leads to Couplet 10:
- Gills on abdominal segment 2 quadrate, meeting or almost meeting medially (source fig 13.16a)
- Gill lamellae on segments 3-6 with fringed margins (source fig 13.16b)
Couplet 10
Leads to Caenidae:
- Mesonotum without anterolateral lobes (source fig 13.18)
- Operculate gills on abdominal segment 2 not fused medially
- Hind wing pads absent
Leads to Neoephemeridae
(Neoephemera):
(Neoephemera):
- Mesonotum with distinct rounded lobe on anterolateral corners (source fig 13.17)
- Operculate gills on abdominal segment 2 fused medially
- Hind wing pads present
Leads to Ephemerellidae:
- Gills absent on abdominal segment 2, rudimentary or absent on segment 1, and present or absent on segment 3 (source figs 13.19, 13.44); abdominal gills when present on segments 3-7 or 4-7 consist of oval anterior / dorsal lamellae and posterior / ventral lamellae with numerous lobes
- Paired tubercles or spines often present on abdominal terga, ranging in size from very prominent to subtle, but absent in some common species
Leads to Couplet 12:
- Gills present on abdominal segments 1-5, 1-7, or 2-7
- Abdominal terga usually lacking paired tubercles
Couplet 12
Leads to Couplet 21:
- Head and body distinctly flattened, with eyes and antennae on dorsal surface, and mandibles not visible from dorsal view
Couplet 21
Leads to Couplet 13:
- Body either not flattened (source figs 13.33, 13.24), or flattened but with mandibles visible dorsally and forming an integral part of the flattened head surface (source fig 13.25)
Couplet 13
Couplet 13 (You are here)
Leads to Couplet 14:
- Tarsal claws of forelegs differ in structure from those on middle and hind legs (source figs 13.26, 13.27)
- Tarsal claws of middle and hind legs long and slender, about as long as tibiae
Couplet 14
Leads to Couplet 15:
- Tarsal claws of all legs similar, usually sharply pointed, but can also be spatulate
- Tarsal claws variable in length; if tarsal claws of middle and hind legs long and slender, then tarsal claws usually shorter than tibiae; if middle and hind tarsal claws longer than respective tibiae, then legs wither bowed or labrum with a deep v-shaped notch
Couplet 15
Leads to Ametropodidae
(Ametropus):
(Ametropus):
- Tarsal claws of forelegs simple, with long slender denticles (source fig 13.26a)
- Spinous pad present on forecoxae
Leads to Metretopodidae:
- Tarsal claws of forelegs bifid, giving the appearance of two claws
- No spinous pad on forecoxae
Leads to Leptophlebiidae:
- Gills on abdominal segments 2-7 either forked , in tufts (source fig 13.28), have all margins fringed (source fig 13.29), or have double lamellae that terminate with a single filament at multiple points
- Apicolateral margin of maxillae with dense brush of long setae (source fig 13.33)
Leads to Couplet 16:
- Abdominal gills not as described; gills either approximately oval or heart-shaped; gill lamellae either single-, double-, or triple-folded (source figs 13.35-13.38); gill lamellae with posterior surface membrane just covering individual tracheal branches in one rare genus (source fig 13.39); abdominal gills never terminate as a sharp filament or with multiple points
- Apical margin of maxillae variable, but never with dense brush of long setae (source figs 13.34, 13.43)
Couplet 16
Leads to Baetidae:
- Labrum with median notch on anterior margin (except in uncommon genus Apobaetis, which lacks well-developed posterolateral projections on abdominal segments 6-9)
- Maxillae without pectinate spines
- Middle tail variable, ranging from the same as the outer tails to a barely-visible stub less than the length of tergite 10
- Antennae length variable
Leads to Couplet 17:
- Labrum without median notch on anterior margin
- Maxillae have pectinate spines if antennae longer than twice the width of the head
- Middle tail length approximately equal to outer tails
- Either antennae shorter than twice the width of the head (source fig 13.41) or labrum with notched anterior margin and maxillae with pectinate spines (source fig 13.43)
Couplet 17
Leads to Acanthametropodidae:
- Tibiae and tarsi bowed
- Tarsal claws very long and slender, with those on hind legs about as long as hind tarsi (source fig 13.26)
- Rare and found in large rivers
Acanthametropodidae
Leads to Couplet 18:
- Tibiae and tarsi not bowed
- Tarsal claws usually not long and slender
Couplet 18
Leads to Ameletidae
(Ameletus):
(Ameletus):
- Maxillae with crown of pectinate spines
- Abdominal gills roughly oval single lamellae, with a distinct sclerotized band along lateral margin and usually with a similar sclerotized band on or near the mesal margin
Leads to Siphlonuridae:
- Maxillae without pectinate spines
- Abdominal gills variable, but without such sclerotized bands or edges, and often with two lamellae per segment; usually much more restricted and weakly sclerotized, appearing nearly transparent
Leads to Palingeniidae:
- Mandibular tusks with a distinct lateral keel that is more or less toothed and with some multi-point spurs (source fig 13.101)
Leads to Ephemeridae:
- Mandibular tusks without a toothed lateral keel (source fig 13.10)
Leads to Isonychiidae
(Isonychia):
(Isonychia):
- Gills on abdominal segment 1 dorsolateral, similar in position and structure to gills on segments 2-7
- Gill fibrils shorter than gill lamellae
Leads to Oligoneuriidae:
- Gills on abdominal segment 1 ventral, whereas gills on segments 2-7 dorsal (source fig 13.14)
- Gill fibrils longer than gill lamellae, or gill lamellae absent
Oligoneuriidae
Leads to Pseudironidae
(Pseudiron centralis):
(Pseudiron centralis):
- Tarsal claws as long as or longer than tarsi
- Tibiae and tarsi bowed
- Rare
- Found in large, sandy rivers
Leads to Couplet 22:
- Tarsal claws much shorter than tarsi
- Tibiae and tarsi straight
- Common
Couplet 22
Leads to Arthropleidae
(Arthroplea bipunctata):
(Arthroplea bipunctata):
- Segment 2 of maxillary palpi longer than width of head and usually held conspicuously at or behind the side of the head (source fig 13.61)
- Uncommon and found at the edges of stream pools, pond margins, swamps, and bog swales
Leads to Heptageniidae:
- Segment 2 of maxillary palpi not greatly elongated, inconspicuous at side of thead
Start a Discussion of this Couplet
References
- Merritt R.W., Cummins, K.W., and Berg, M.B. 2019. An Introduction to the Aquatic Insects of North America (Fifth Edition). Kendall/Hunt Publishing Company.