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.
Originally published May 9, 2005 on Fly Anglers Online by Bert.