The long axis of the house

Michael H. Hansell, « A progress report on some approaches to the study of larval house building with particular reference to Lepidostoma hirtum », in Hans Malicky (Edit.) Proceedings of the first International symposium on Trichoptera, 1974, La Hague, Dr W. Junk b.v. Publishers, 1976, p. 181-184.

Early in the 3rd instar the larva of Lepidostoma hirtum change from buiding a sand grain house of round section to building a house of vegetation panels which is square in section. The panels are almost rectangular and one row covers each of the four sides of the house. Panels cut from dicot leaves such as those of deciduous trees are usually placed on the house with their upper leaf surface facibg outwards. Panels cut from monocot leaves, such as grass orreeds are usually oriented with the leaf veins across the long axis of the house. These and other features illustrate the variety of behaviour information contained in the houses of Lepidostoma larvae in particular and caddis larvae in general.

Ethology can broadly ask four kinds of questions ; ones of phylogeny, ontogeny, causation and functions. Phylogeny : The rich behaviour repetoire of caddis larval house building suggests that comparative behavioural studies would yuield information on phylogenetic relationships. Fossil caddis houses offer a rare opportunity to study the fossil record of behaviour yet this field has not attracted attention. Ontogeny : A number of measures of the houses building performance pf Lepidostoma change over time, some of these changes could be due to internal changes with age ; change over time, some of thses changes could be due to internal changes with age ; however, bearing in mind the plasticity of some aspects of house building behaviour, the possibility of the learning of skills of caddis larvae should be seriously investisgated. Causation : This has been the most investigated aspect of house building behaviour. In Lepidostoma it has been shown that hairs at the anterior and posterior of the body regulate many aspects of house building, (Hansell, 1973, 74). It has been shown that dark/light cycle influences quality of building but no effects of hormone administration on house building have yet been demonstrated. A general feature of caddis house building just starting to be investigated is how a larva whose particle selection apparatus (head and legs) grows in a stepwise manner, manages to construct a house which grows in a curvilinear manner.The preliminary hypothesis is that during any instar the distention of the abdomen is monitored and this informations adjusts the calibration of the selection and building apparatus resulting in a gradual increase in house width and particle size. Function : The mechanical properties of the panel house of Lepidostoma show that firstly it is stronger than as equivalent sand grains houses for most types of external loading and that it is very elastic. The elasticity is not due to the properties of the silk but of the vegetation panels ; this could be the reason for the preferred orientation of monocot and dicot leaf panels. This house strength and elasticity might serve to protect Lepidostopma larvae from predation by small fish incapable of swallowing whole houses. The house of this and other species may have other functions, such as to protect larvae from wave or current action or conceal them from predators.