Hex Mayflies
Hexagenia limbata
The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
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As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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Identification: Key to Orders of Aquatic Insect Larvae and Wingless Adults
Identification: Key to Orders of Aquatic Insect Larvae and Wingless Adults
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Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
This first couplet rules out a different class (Collembola rather than Insecta), but it's included here because Collembola (Springtails) superficially look like insects and are often collected alongside aquatic insects.
| Option 1 | Option 2 |
|---|---|
| Abdomen with 6 or fewer segments and a ventral tube | Abdomen with more than 6 segments and without a ventral tube |
| Body tiny, 5 mm or less | Body size variable, often > 5 mm |
| Most with a forked structure (furcula) at tip of abdomen | |
| Remaining orders: Coleoptera, Diptera, Hemiptera, Hymenoptera, Lepidoptera, Megaloptera, Neuroptera, and Trichoptera | |
5 Example Specimens
This species of Brachycentrus was extremely common in mid-September kick net samples in the Yakima canyon.
With a bit of help from the microscope, this specimen keys clearly and unsurprisingly to Hydropsyche.
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
I already had one of these on the site, but these giant Hydropsychidae larvae are just too photogenic to pass up.
| |
| Collembola | Go to Couplet 2 |
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.
Couplet 1 (You are here)
Leads to Collembola:
- Abdomen with 6 or fewer segments and a ventral tube
- Body tiny, 5 mm or less
- Most with a forked structure (furcula) at tip of abdomen
Leads to Couplet 2:
- Abdomen with more than 6 segments and without a ventral tube
- Body size variable, often > 5 mm
Couplet 2
Leads to Couplet 3:
- Three pairs of jointed legs on thorax
Couplet 3
Leads to Couplet 8:
- True legs absent; fleshy, leg-like protuberances or prolegs may be present on thorax, but fewer than three pairs and not jointed
Couplet 8
Leads to Lepidoptera:
- Abdomen with at least 2 pairs (usually 5 pairs) of ventral, fleshy, leg-like protuberances tipped with tiny hooks (prolegs with crochets)
Leads to Couplet 5:
- Abdomen without paired, ventral, leg-like protuberances, or, if present, not tipped with tiny hooks
Couplet 5
Leads to Trichoptera:
- Last abdominal segment with lateral appendages bearing sclerotized, hook-like anal claws
- Antennae 1-segmented, inconspicuous
- Gills, if present, seldom confined to lateral margins of body
- Larvae frequently live in cases made of sand grains or bits of plant matter, but some are free-living
Leads to Couplet 6:
- Last abdominal segment without anal claws
- If anal claws present, antennae have more than 1 segment
- Gills inserted laterally
Couplet 6
Leads to Neuroptera:
- Mandibles and maxillae united at each side to form long, straight or slightly recurved, thread-like suctorial tubes
- Laterally inserted, segmented gills folded beneath abdomen
- Small, 10 mm or less Found in or on freshwater sponges
Neuroptera
Leads to Couplet 7:
- Mandibles not united with maxillae into long tubes; if mandibles suctorial, then strongly curved, sickle-shaped
- Gills seldom segmented and not folded beneath abdomen
Couplet 7
Leads to Megaloptera:
- Abdomen with 7-8 pairs of lateral filaments or gills, arranged 1 pair on each segment
- Paired terminal anal prolegs each with 2 claws or a single, medial, caudal filament
Leads to Coleoptera:
- Abdomen usually without lateral gills; if segmental gills present, then (a) only a single terminal anal proleg with 4 claws, otherwise anal hooks absent, or (b) segment 10 with 4 gills, or (c) caudal appendages paired or absent but never single
Leads to Couplet 9:
- Head capsule distinct, partly or entirely hardened, and usually pigmented, but may be deeply withdrawn in prothorax
Couplet 9
Leads to Couplet 10:
- Head capsule absent, not distinct, hardened, or pigmented, often consisting of a few pale rods
Couplet 10
Leads to Diptera:
- Posterior end of body with at least one or a combination of gills, hair brushes, a sucker, or breathing tube
Leads to Coleoptera:
- Posterior end of body simple or with small processes or isolated hairs, but without gills, brushes, suckers, or breathing tubes
Leads to Diptera:
- Body often 5 mm or larger, elongate, somewhat cylindrical, spindle-shaped or maggot-like
- Mouthparts may be reduced to a pair of retractile mouth hooks that move vertically
Leads to Hymenoptera:
- Body usually 5 mm or less
- Mouthparts may be reduced to a pair of opposable, acute mandibles that move horizontally
- Parasitoids on or inside insect hosts
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.
Orders excluded
These orders of Insecta are not included in this key: Ephemeroptera, Plecoptera, Odonata-Anisoptera, Odonata-Zygoptera, Orthoptera, Psocodea