Rammassage des étuis

Heather Angel & Pat WolseleyThe Family Water Naturalist, Londres, Bloomsbury Books, 1992, p. 76-77.

Of all the freshwater animal architects, the cased caddis must be among the most familiar. The British freshwater biologist Dr Norman Hickin, who has made a life long study of caddis, has suggested that these insects may be named after  « caddis-men » – the travelling vendors or ribbons, cloths and braids (caddis), which they pinned to their coats.

Caddis larvae use one of two main building methods to construct their cases. The majority begin by spinning a silken cylinder around their body, the threads being produced from the salivary glands in the mouth of the larva. To this cylinder, the building blocks- stones, sand grains, empty snail shells, twigs, leaves or stones- are cemented. A pair of hooks attached to the last abdominal segment latches into the walls of the cylinder, which is then dragged along as the larva moves around. A few caddis use a different method of case construction : the larva walks over a sandy section of the stream bed spinning silk threads over it. When a carpet of threads has been produced with the sand grains incorporated, the larva abandons its old case and quickly rolls itself up in the new one, the overlapping seam of which is « sewn » up !

As well as protecting the soft bodies of the larva, the cases camouflage them. Heavy cases help caddis-larvae to maintain their position in fast-flowing streams, whereas cases made from the leaves of Canadian pondweed aid buoyancy for some pond-dwelling caddis. On warm summer days, they can be seen grazing the algae off the surface of other pondweed plants, trapping the bubbles of oxygen given off by them in the entrance of their cases. Once sufficient oxygen has been collected, a caddis floats up to the next strand of pondweed and starts feeding again. If it floats to the surface, the larva simply pushes out the bubbles and sinks down to the pond floor.

Caddis larvae are usually very conservatrive in their choice of materials for case construction. However, some species of Limnephilus may start their cases using plant remains, then suddenly shift to other materials, such as small stones or snail shells, and then back to plant remains at a later date.

Projects

  1. Collect 20 cased caddis larvae from a pond. Examine their cases, drawing different types and recording what proportion are constructed from live plants, dead leaves, twigs, sand grains or shells. Return the caddis to the pond and repeat with 20 caddis larva collected from a stream. Make sure to return them to the stream. Compare the results. Which type of case blends in best with its natural surroundings ? Is this the most abundant type ?
  2. The cased caddis Anabolia is easily recognized by the very long twigs attached to the back of its case. Collect a number and drop them into a slow-flowing stretch of Stream. How do the cases align themselves to the current ? How is this an advantage to the animals when they feed ?
  3.  Collect some duckweed and search for cases of the moth Cataclysta lemna. How does the case construction differ from caddis cases made from living plant material ? Examine the undersides of waterlily pads for larger cases of Nymphula nympheata larvae. What proportion of the pads are used by this larva as a shelter ?