The Stream Doctor

November 21st, 2005

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

Q. Your features are very well received and missed with each issue of FAOL.

I read that during insects molting the exoskeleton forms as a soft, wrinkled layer underneath the hard parts (exocuticle plus epicuticle) of the old exoskeleton. Once the new exoskeleton has formed, the insect is ready to shed what's left of its old exoskeleton. As the ecdysis process continues the old exoskeleton splits apart and is shed leaving the new exoskeleton that hardens within a short time. My question is "during the molting does the aquatic insect release to the water column fluids that alert trout to their presents?"

Thanks for your help in better understanding stream ecology. Best regards, Mike Harvell

A. Thanks for the kind words.

During molting, portions of the old cuticle are digested and absorbed, while a new cuticle is deposited beneath, much as you described above. The outer part of the old cuticle is loosened and finally split, beginning at the top of the head and thorax. This resorption of portions of the old cuticle would suggest that no fluids are released to the water column. I then read through five pages on molting in Encyclopedia of Insects (Resh and Cardé, Academic Press) that covered much detail on the subject in terms of hormonal control, etc. The important point that would seem to directly apply to your questions is that during the molting process, molting fluid is resorbed into the hemolymph for use in the next molting cycle. Further, a waxy, waterproof layer is secreted over the new epicuticle to prevent dessication. I would suspect this last statement would apply more to terrestrial insects, though this wasn't specified in the text, but if the same thing happens in aquatic insects, this layer would probably prevent external loss of any fluids prior to their resorption into the hemolymph. I then called a couple of colleagues to get their thoughts on the matter. One said that the waterproof layer alluded to above may even be more important in aquatic insects because of their continual battle to prevent excess sorption of water; the second said that he was unaware of the release of molting fluids.

I'll add another personal observation concerning "clues" released by aquatic insects; it doesn't have anything to do with molting, but it's interesting. A friend and I were observing a colony of black fly larvae on a large stone in the East Fork of the Salmon River. As is customary, the black flies were scattered along the upper lip of the rock where the current broke over. As we watched, a large, perlid stonefly larvae began moving horizontally across the rock about two or three inches below the black flies. As soon as it reached a point directly "downstream" of a black fly, it turned, moved up the stone, and grabbed the black fly in its mandibles. It then dropped down a bit, resumed crossing the rock, and repeated the performance as soon as it was directly below a black fly. How did the stonefly detect the black fly? Our guess was that it was receiving chemical clues from the black fly, but without further proof, I guess other possibilities could have been working, e.g., sight, detection of differences in current from the body of the black fly. Anyway, it held our attention for the better part of half an hour.

Best wishes, ~ 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

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

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