Considerable degree of versability

Donald Redfield Griffin, Animal Minds. Beyond Cognition to consciouness, Chicago, University of Chicago Press, 2001, p. 84-85.

Caddis Fly Cases

One of the more impressive examples of complex structures built by animals we usually consider to be very simple are the cases and nets built by the larvae of caddies flies. These are abundant animals in freshwater streams and ponds, and many of them construct shelters of various types attached to vegetation or to the bottom of the body of water they occupy. They develop from eggs laid in the water by winged female caddis flies, which emerge and mate after a long period as aquatic larvae. The larvae of North america caddis flies and the cases they construct havr been described in detail by Wiggins (1977), who illustrates numerous variations in case and net construction characteristic of the numerous genera and species The larvae themselves are rather nondescript and difficult to identity, but the shelters are so characteristic that they are often used by systematic entomologists for species identification. The cases built by caddis fly larvae may employ bits of leaves, particles of sand, or other avilable materilals including the empty shells of very small snails. They are cemented together by silk secreted from glands on the larva’s head. Sometimes caddis fly larvae crawl out their cases, and they may fight over cases, or one may evict the occupant of a case and take it over (Otto 1987a, 1987b ; Englund and Otto 1991).

Like other insects, these larvae grow through successive stages in which the exoskeleton is shed. In early stages the Small larva uses minute and ordinarily homogeneous particles to form a roughly cylindrical case around its body. These cases may increase the flow of water over the gills (William, Tavares, and Bryant 1987) and also protect the otherwise vulnérable soft-bodied larva from small fishes or other predators such as immature dragonflies. The case is not totally impervious ; it has an opening at the posterior end to allow fèces to pass out, and water circulates freely from an anterior opening to the posterior one so that the gills can extract oxygen from the water. In many species the case is portable and is carried about as the animal moves by pushing its head and thoracic segments carrying the six legs out through the front opening. The cases held close to the body by hooklike projections from the abdominal segments. In constructing their cases, caddis fly larvae are somewhat selective about the materials used. Some species cut pieces from the leaves of aquatic plants, and others construct fine Meshed nets that serve to strain minute animals and plants from the flowing water.

The case-building behavior of a few species of caddis fly larvae has been studied carefully in the laboratory by Hansell (1984, 1968a, 1968b, 1968c). He conducted detailed observations and experiments with Silo pallipes, which begins its larva life by constructing a simple cylindrical tube of sand grains cemented together. This species passes through five instars, between which the animal sheds its external skeleton and grows a larger one. In its first instar the Silo larva occupies a roughly cylindrical case composed of particles about one-half millimeter in diameter. But toward the end of this instar it adds two larger sand grains at the sides of the front end. During the second instar it adds two additional, still larger particles, and through the three succeeding instars it selects at each molt larger grains of sand for the anterior opening. At the end of the fifth instar the case is about ten millimeters long, and the larger anterior grains are two five milimeters in diameter. Throughout this growth the larva enlarges its cylindrical case by adding more small particles.

Hansell’s observation showed that the larvae reach out of their cases and feel particles in the immediate vicinity with their anterior appendage and reject many that are either too large or too small. Having felt one of appropriate size they move it into position and secret silk to fasten it there. On a very small size scale this behaviour is flexible and adapted to the available particles and the stage of case construction.

Hansell (1972, 1974) also studied another species of caddis fly larva, Lepidostoma hirtum, which cuts panels from bits of leaves to form a floor, roof and two sides. All of the panels are approximately rectangular pieces, one or two millimeters in size, held together at the edges by secrete silk. The resulting case is strengthened by a staggered arrangement of the pieces. Each joint between two side plates intersects with the middle and not the edge of a roof plate. When Hansell cut away the front of end of a case to form a continuous, smooth front edge, the larva cut leaves of different shapes from those normally used and glued them into place so as to restore the staggered arrangement. These simple insect larva thus exibit a considerable degree of versability not only in the initial construction of their cases but in repairing them. Despite our customary assumption that insect larva exhibit only stereotyped behavior, they have highly organized central nervous systems with hundreds of neurons and synapses that are quite capable of organizing relatively complex and flexible behavior.

In yet another species of caddis fly, Macronema transversum, the larvae construct a more elaborate case, which provides not only a shelter but a food-gathering mechanism, as describe in détail by Wallace ans Sherberger (1975). This case is a chamber considerably larger than the animal’s body and roughly oval in shape. From the upstream end rises a tubular extension with a roughly ninety-degree bend and an opening facing the direction from which water is flowing. From the other end of the oval chamber rise a shorter outlet tube. The entire chamber is only two or three centimeters in length, but the flow of water by the surrounding stream serves to ventilate it fairly well owing to its construction. Opening into the side of the main chamber is a separate tubular structure corresponding to the normal caddis fly case. But both ends of this open into the larger chamber. The end where the larva places its head opens into the upstream side of the main chamber, and water flows through this side chamber past the larva’s gills and out a posterior opening into the down stream part of the main chamber. Finally the larva constructs a fine mesh net across the middle of the large chamber. Small animals and plants are caught on this net and serve as food. The entire structure is roughly comparable in complexity to the nests of many birds.