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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
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.

Dorsal view of a Skwala (Perlodidae) (Large Springfly) Stonefly Nymph from the Yakima River in Washington
This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Identification: Key to Families of Stonefly Nymphs, Couplet 3

Identification: Key to Families of Stonefly Nymphs, Couplet 3

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.
These mouthparts are typically easy enough to see under a microscope, but the need for optics (except on the largest specimens) makes this an unfortunately difficult couplet for anglers so early in the key. It is often possible to skip this couplet by recognizing the general distinctive shapes and colors of the families, as Perlidae and Perlodidae look very different (with various distinctive yellow and dark brown/black color patterns) from Taeniopterygidae and Nemouridae (with wingpads angled out to create a triangular appearance, and typically duller, more uniform brownish colors). The relatively difficult task for which mouth parts might be required here (although it's still possible at a glance, with experience) is distinguishing between the skinny little nymphs of Chloroperlidae and those of Capniidae and Leuctridae.
Option 1Option 2
Paraglossae and glossae extend forward about the same distance (sf 16.18) (be careful not to confuse them with the labial palps, which might be large and partially obscuring these parts, as in the illustration)
Mouthparts of a Zapada cintipes (Nemouridae) nymph.
Mouthparts of a Zapada cintipes (Nemouridae) nymph.
Paraglossae much longer than glossae
The paraglossae (red box) extend far beyond the glossae (green box) in this Perlidae nymph.
The paraglossae (red box) extend far beyond the glossae (green box) in this Perlidae nymph. Figure from this Hesperoperla pacifica nymph.
Paraglossae (narrow red box) and glossae (wide green box) of a typical Chloroperlidae nymph
Paraglossae (narrow red box) and glossae (wide green box) of a typical Chloroperlidae nymph. Figure from this Sweltsa borealis nymph.
Remaining families: Capniidae, Leuctridae, Nemouridae, and TaeniopterygidaeRemaining families: Chloroperlidae, Perlidae, and Perlodidae
5 Example Specimens
5 Example Specimens
Go to Couplet 4 Go to Couplet 7
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Leads to Pteronarcyidae:
  • Ventral gill tufts present on thorax and abdominal segments 1-2 or 1-3
Leads to Couplet 2:
  • Ventral gill tufts absent from abdominal segments 1-2 or 1-3
Couplet 2
Leads to Peltoperlidae:
  • Single, double, or forked conical gills present at least behind coxae of second and third leg
  • Thoracic sterna posteriorly produced, overlapping succeeding segment
  • Some genera appear roach-like
Leads to Couplet 3:
  • Gills, if present, not conical
  • Thoracic sterna not overlapping
Couplet 3
Couplet 3

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Couplet 3 (You are here)
Leads to Couplet 4:
  • Paraglossae and glossae extend forward about the same distance (sf 16.18) (be careful not to confuse them with the labial palps, which might be large and partially obscuring these parts, as in the illustration)
Couplet 4
Leads to Couplet 7:
  • Paraglossae much longer than glossae
Couplet 7
Leads to Taeniopterygidae:
  • First and second tarsal segments near equal in length
  • Midline of wing pads strongly divergent from body axis
  • One genus often with each coxa bearing a single, segmented, ventrally directed, finger-like gill (sf 16.20) or abdomen terminating in a large, ventral, shield-like plate (sf 16.53)
Leads to Couplet 5:
  • Second tarsal segment much shorter than first
  • Midline of wing pads parallel or divergent
  • No coxal finger-like gills nor large ventral, shield-like plate
Couplet 5
Leads to Nemouridae:
  • Midline of metathoracic wing pads strongly divergent from body axis
  • Usually robust and squat body, such that extended hind legs reach about to the tip of the abdomen
  • Cervical gills present (sf 16.22-16.25) or absent
Leads to Couplet 6:
  • Midline of metathoracic wingpads, if present, nearly parallel with body axis (sf 16.11-16.12)
  • Elongate body, such that extended hind legs reach far short of the tip of the abdomen
  • No cervical gills
Couplet 6
Leads to Capniidae:
  • Postmentum small, its anterior portion not covering bases of maxillae (sf 16.26)
  • Terga and sterna of abdominal segments 1-9 separated by membraneous pleural fold (sf 16.28)
  • Abdominal segments widest posteriorly
  • Hind wingpads about as broad as long (sf 16.30-16.31), reduced, or absent
  • Abdominal terga always with a posterior setal fringe (sf 16.32)
Leads to Leuctridae:
  • Postmentum large, with its anterior portion partially covering basis of maxillae (sf 16.27)
  • At most, abdominal segments 1-7 separated by membraneous pleural fold (sf 16.29)
  • Abdominal segments uniformly cylindrical
  • Hind wing pads usually longer than wide (sf 16.12)
  • Abdominal terga of some genera (Despaxia, Perlomyia, Paraleuctra, and Zealuctra) without a posterior setal fringe
Leads to Perlidae:
  • Highly branched filamentous gills on sides and venter of thorax
Leads to Couplet 8:
  • Branched, filamentous gills absent from thorax although sometimes it has a single thumb-like gill (sf 16.34) or rarely forked gills (sf 16.35)
Couplet 8
Leads to Chloroperlidae:
  • Apical maxillary palpal segment small, asymmetrically set on penultimate segment
  • Tails 3/4 or less the length of the abdomen
  • Head and thoracic terga usually without a distinct pattern of dark and light pigments
Leads to Perlodidae:
  • Apical maxillary palpal segment subequal in length to penultimate segment and symmetrically attached
  • Tails more than 3/4 the length of the abdomen
  • Head and thoracic terga usually with a distinct pattern of dark and light pigments

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