The Stream Doctor

May 9th, 2005

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.


Q. I have been fishing the caddis hatch on the Arkansas River near Salida for the past 15 years or so and have been duly impressed with the numbers of these creatures. During that time I have noticed that something very curious has happened to the cases.

When I first started fishing this hatch, the cases that I saw all had a square cross section. Every reference that I have seen refers to it as Brachycentralis occidentalis, and it is described as having a case with a square cross section. A few years ago I noticed that some of the cases were round and some were square. A couple years ago I noticed that a square case was very rare. Last year and this year I saw no cases that were square, all are round. All of these cases I refer to contained pupae, so they are not something left over from another hatch late last year, for example.

The hatch comes off on schedule; the adults appear to my untutored eye to be the same as always; so what's going on?

Two other questions regarding insects on the Arkansas at this time of year:

This year for the first time I saw cases of another caddis that I do not remember seeing before. The case is coarse sand grains and is about 1.5-2.0 cm long and about 4-5 mm diameter. I broke one open, and it contained a pupa that was reddish brown. Do you have a name for this one?

Another common bug that seems to hatch from the water resembles slightly a small crane fly. It has two wings that lie flat but are angled at 45 deg. from the abdomen. The length is about 1 cm or so. The abdomen is slender but is enlarged some at the end. The whole thing is gray.

Thank you for any help. Regards, Bob Pearson

A. Thanks for the questions; I hope I can do them justice.

First, the caddis case question. According to the book An Illustrated Guide to the Mountain Stream Insects of Colorado, 2nd ed. there are two species of Brachycentrus in Colorado B. americanus and B. occidentalis (there is no caddis genus named Brachycentralis). Both occur in the Arkansas River drainage. Then going to the definitive work on immature caddisflies, "Larvae of the North American Caddisfly Genera (Trichoptera)," we find that the typical case for all species (13) in the family Brachycentridae is the four-sided "log cabin" that you described. It further states that some or all of the case is occasionally made of silken secretions, but some are constructed entirely of small rock fragments.

There are four other genera in the family Brachycentridae; they are Eobrachycentrus, Adicrophleps, Amiocentrus, and Micrasema. The first two do not occur in Colorado, the third is not found in the Arkansas River drainage, but Micrasema does occur in this drainage. The case of Micrasema is described as round, straight or curved, constructed of sand or of ribbon-like pieces of plant materials wound around the circumference, or largely of silk alone.

Well, how does this fit with your observations? Frankly, it would be quite surprising and unexpected to find that the larvae in the round cases were a different genus (Micrasema ?) and it was replacing Brachycentrus by outcompeting it for existing resources. If, in fact, the population is still dominated by Brachycentrus, as would be expected, then we'd need to find an explanation of why they've changed their case building habits from square to round. This, too, is awfully strange. Thus, I'm going to beg off of trying to give you a definitive answer to your observations because neither of these suggestions is plausible. I think we'll only be able to know for sure if somebody can take some samples and make some positive identifications and remember, this needs to be done from adults. Also, there would have to be some extensive spatial sampling to be sure that the presence/absence of round and square cases that you observed was widespread and not a local occurrence.

Second, the question about the "new" cases. There's no way that I could identify the insect you've described on this brief description, and I'd be foolish to even try. There are many caddisfly genera that build their cases of sand grains and have the dimensions that you mention. If someone made me take a guess, I would probably guess a member of the family Limnephilidae because they are widespread, have cases as you've described, and there are 14 different genera in this family found in the Arkansas River drainage.

Third, the "common bug." I'm not even going to make a guess on this one. An entomologist might find something distinctive in your description, but it's beyond my expertise. Sorry.

Well, I hope this is of interest. If I can help further, please get in touch. ~ Bert

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.


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