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Caddisfly Genus Dicosmoecus (October Caddisflies)

Taxonomic Navigation -?-
Species in DicosmoecusNumber of SpecimensNumber of Pictures
Dicosmoecus atripesOctober Caddis222
Dicosmoecus gilvipesOctober Caddis26

2 species aren't included.
Common Names
Pictures Below
This is a superhatch in the West. Gary LaFontaine had a self-proclaimed obsession with Dicosmoecus, and he devotes several pages of Caddisflies to stressing the importance of its larvae, pupae, and adults. He wrote:

The question for fly fishermen seeking big trout is: "Which insects provide the best opportunity for catching such fish?" My list would be: Giant Orange Sedge (Dicosmoecus sp.), Salmon Fly (Pteronarcys californica, a stonefly), and the Michigan Mayfly (Hexagenia limbata). Dicosmoecus is the most important -- and the contest is not even close.

His reasons for this judgement are five traits of Dicosmoecus:

  • Very large size: Adults 30mm long, and thick.

  • Activity concentrated within 2-3 weeks.

  • Emerges in the low, clear water of fall.

  • Active during afternoon and evening.

  • Abundant in rivers with sea-run trout.


I have never fished the West to experience Dicosmoecus, but I have enough experience with Hexagenia limbata to know that a greater insect must be truly amazing.
  

Where & When


Region: West

Time Of Year (?): Fall

See the species pages for details about distribution and emergence times.

Hatching Behavior


Time Of Day (?): Late afternoon to dark

Habitat: Shallow water

Emerging Dicosmoecus pupae may be the least important stage of the hatch. They will crawl out onto rocks to emerge when ideal rocks are nearby, but otherwise they emerge on the surface. Either behavior is likely to take place in water too shallow for trout to comfortably feed. The minority of the pupae emerging in deeper water are probably still prolific enough, at times, to excite the trout.

Egg-Laying Behavior


Time Of Day: Late afternoon through nightfall, peaking at sunset

The mating activity begins within two days of the first emergence. Ovipositing females fall and struggle on the surface to lay their eggs, making them perfect insects for the angler.

Larva & Pupa Biology


Diet: Detritus (Detritus: Small, loose pieces of decaying organic matter underwater.), algae, dead animals

Current Speed: Slow water early; faster water in later instars (Instar: Many invertebrates molt through dozens of progressively larger and better-developed stages as they grow. Each of these stages is known as an instar. Hard-bodied nymphs typically molt through more instars than soft-bodied larvae.)

Shelter Type: Plant matter early; gravel in the later instars (Instar: Many invertebrates molt through dozens of progressively larger and better-developed stages as they grow. Each of these stages is known as an instar. Hard-bodied nymphs typically molt through more instars than soft-bodied larvae.)
The larvae are unusually prone to behavioral drift (Behavioral drift: The nymphs and larvae of many aquatic insects sometimes release their grip on the bottom and drift downstream for a while with synchronized timing. This phenomenon increases their vulnerability to trout just like emergence, but it is invisible to the angler above the surface. In many species it occurs daily, most often just after dusk or just before dawn.) during the daytime in June and July, usually around 4:00 p.m. They may be in between cases when they do this, making them especially appealing and visible to hungry trout.

In mid- to late summer they enter diapause (Diapause: A state of complete dormancy deeper even than hibernation. While in diapause, an organism does not move around, eat, or even grow. Some caddisfly larvae enter diapause for a few weeks to several months. Some species of microscopic zooplankton can enter diapause for several hundred years.) until cooler fall temperatures trigger them to pupate in a synchronized way.

Pictures of 4 Caddisfly Specimens in the Genus Dicosmoecus:

Specimen Page:12
Female Dicosmoecus atripes (October Caddis) Caddisfly AdultFemale Dicosmoecus atripes (October Caddis) Caddisfly Adult View 6 PicturesThis specimen is 26 mm.
- Entoman
Collected September 27, 2013 from the St. Joe River in Idaho
Added to Troutnut.com by Entoman on September 29, 2013
Specimen Page:12

1 Underwater Picture of Dicosmoecus Caddisflies:

View Full SizeView Full Size (2.3X larger)
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These are probably 3rd instar larvae of Dicosmoecus gilvipes. The 4th instar larvae will be the round case made of sand grains.  In this picture: Caddisfly Species Dicosmoecus gilvipes (October Caddis). From the Touchet River in Washington.
These are probably 3rd instar larvae of Dicosmoecus gilvipes. The 4th instar larvae will be the round case made of sand grains.

In this picture: Caddisfly Species Dicosmoecus gilvipes (October Caddis).
Date TakenJun 23, 2011
Date AddedJun 27, 2011
AuthorBnewell
Camerau770SW,S770SW

Recent Discussions of Dicosmoecus

october caddis
Posted by Amosg on Oct 6, 2011
In Alberta they exist in very small numbers but are not important to fishermen--Amos
ReplyYou Western anglers - any experience with the Giant Orange Sedges? 7 Replies »
Posted by Troutnut on Jul 29, 2006
Last reply on Jan 7, 2009 by Dgracia
This seems to be a very important insect for which I have no experience and few sources. I want to be sure that my writeup is accurate and fairly complete. Do any of you who fish out west know any details I've left out?
ReplyOctober Caddis 19 Replies »
Posted by Taxon on Jul 29, 2006
Last reply on Oct 24, 2008 by Jack_k
Jason-

In the Pacific NW, Dicosmoecus are generally referred to as either October Caddis or Fall Caddis. The field guide I generally recommend for western fly fishers is Hatch Guide For Western Streams by Jim Schollmeyer. This is what he has to say about Dicosmoecus pupae, and I believe it clarifies any ambiguity of the passage you quoted from Caddisflies:

In the early or mid summer, the larvae reach maturity and move from the faster currents to the slower flows that are generally found along the margins of the stream. Then they attach their cases to the rocks, seal themselves inside, and begin pupation. This transformation takes about two months. When the pupae are ready to emerge anytime between late afternoon and dark, they chew open the front of their cases and swim or crawl to the surface. The ones that find exposed rocks cling to them close to or just above the waterline; their pupal shucks split open and the adults emerge. Larvae that took refuge and pupated behind unexposed mid-stream rocks pupate and emerge in the open water. Most pupae emerge from waters that are too shallow or too exposed for trout. Any pupae that emerge in deep or open waters are vulnerable as they swim to shore or the surface.


ReplyA good Dicosmoecus discussion 1 Reply »
Posted by Troutnut on Aug 22, 2006
Last reply on Aug 22, 2006 by Taxon
There's a good topic with people sharing their experiences with Dicosmoecus over at the Washington Fly Fishing Forum.
Reply

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