Although not organic, it is an outer casing

C. J. Barnard, Animal Behaviour Ecology and Evolution, New York, John Wiley, 1983, p. 125

A different kind of example is the protective «  house » that caddis fly (Insecta : Trichoptera) larvae build around themselves. These « houses » are made up of sticks, stones and other debris which the insect gathers from the bottom of the stream or pond. Clearly the «  house » is the product of the insect’s behaviour and the behaviour, in turn a product of its genes. The «  house » can thus be viewed as an additional link in the embryological chain of events leading from genes through protein synthesis to the adult phenotype. Although not organic, it is an outer casing created by the caddis fly’s genes upon which the survival of those genes depends.