The effects of particle surface texture on silk secretion by the caddisfly Goera japonica during case construction

Caddisfly larvae secrete silk to line the inner walls of their portable cases to create smooth surfaces that enhance their respiratory efficiency. In this study, Goera japonica Banks larvae were allowed to repair their portable cases using artificial mineral particles with different surface textures to compare the conformation and quantity of silk secreted on the inner case surface. Three types of particles with different surface roughness were used in the experiments; surface roughness was determined by confocal laser scanning microscopy. Particle selection experiments were conducted for every possible pairing of the three particle types. Thick silk layers were secreted on cases built using rough particles, hardly any silk layers on cases built using smooth particles and brindled thin silk layers were deposited over intermediate particles. The mean silk secretion rate was much higher (244%) with rough particles than with smooth ones, while the rate was somewhat higher (60%) with intermediate particles. In addition, smooth particles were strongly preferred to rough particles, while smooth particles were slightly more preferable than intermediate particles. These results suggest that G. japonica larvae preferred particles with smoother surfaces as case-building materials, because silk production can be decreased as surface texture becomes smoother. Also, there may be a trade-off between the costs of material selection and silk secretion for case construction by caddisfly larvae; saving energy by secreting less silk on smoother particles allows more energy to be spent on selecting smoother particles or on other activities.