Once upon a summer, sometime ago, two biology students, happened, on their camping trip, to pitch up beside a tributary of the Gave de Gavarnie in the Hautes-Pyrenees. They were in search of a research project, so it was with much excitement that they spotted in the little mountain stream beside their tent, vast numbers of caddis fly larvae of the species, Allogamus auricollis. (They weren't to know they were A. auricollis at this time though - even the Natural History Museum in London, wouldn't know this then. It took a French trichopteran expert, Monsieur H Decamps who had spent years studying them, to subsequently identify the little encased creatures, drowned in formalin, and sent to his lab.)

The Caddis Fly

Caddis fly larvae live in fresh water. Many species build tubular shaped 'cases' which enclose their abdomens, made by spinning silk to which fragments of stone, twigs, or even shells are stuck¹.
They rhythmically move their abdomens creating microcurrents inside the case, aiding respiration via 'gill filaments'. Hook-like anal claspers grip the inside of the cases to hold the larvae in place.

Species identification is aided by the fact that architectural design, and use of materials in the cases, is specific to each species.
This US website shows some designs of caddis cases found in the Upper Delaware river. A. auricollis make theirs out of the tiny fragments of stone found in mountain streams.

Caddis flies are closely related to butterflies and moths. Adults emerge around late September from pupated larvae, which have undergone complete metamorphosis inside their cases. They live for around a week; mating occurs and eggs are laid in jelly-like masses close to fresh water.

The Adventure

The students observed the creatures fanned out across the stream bed, row upon row of them, holding their legs out against the main current of the stream. Research went ahead with the aid of nets, quadrates, (1 metre square metal structure), test tubes, formalin, field microscope, dissection kit, indicator paper, and a bit of help from some young 'enfants' in the vicinity, camping with their parents. Much fun was had by all. Data was recorded with pencil and paper, (pencil is always preferred 'in the field' because of problems with inclement weather). Everything was then packed away and the two students headed off to the summit of the High Pyrenees and over the border down into Spain.

The exciting denouement was that this 'waving of A. auricollis' legs in the current' thing was new to science. For sure many other species were known to do this, such as the brachycentrids Oligoplectrum and Brachycentrus, and several Asian limnocentropodids, but not apparently A. auricollis.

They were filter feeding. The bristled middle and rear legs are held in the water current in order to collect various species of water borne diatoms, which are then combed off by the front legs. There are two structures which form the mouth parts. The mandibles have brushes of hairs on the dorsal edge, and there are combs of hairs on the labrum - so the mouth parts are able to remove the algae from the bristles on the front legs. It is a beautiful elegant design of a transference mechanism from back to front via various structures.

But the students also saw some A. auricollis in the areas of slow eddying water, which were feeding directly on the algae living on the boulders.

It always struck them as quite funny that Monsieur Decamps had spent years studying Allogamus auricollis but had only ever seen the plodders on the rocks... never having seen them filter feeding, and yet they had, after five minutes of pitching their tent.

Post Script

Dr PC Barnard of the Natural History Museum, London, requested specimens of Allogamus auricollis for the museum's collection, which were gladly donated. Hopefully they are still buried somewhere in its vaults.