• Confocal laser scanning microscope (CLSM);
  • construction behaviour;
  • material choice;
  • phenotypic plasticity;
  • sediment geology;
  • surface roughness;
  • Trichoptera

1. The mineralogical/petrological composition of the substratum influences aquatic organisms in several ways. However, the actual mechanisms are often unclear. Some caddisfly larvae actively concentrate smooth quartz particles in their portable cases thus producing a smooth inner surface of the case wall.

2. The particle surface-roughness preference of Perissoneura paradoxa (Odontoceridae) McLachlan inhabiting granite areas in relation to the mineral composition and surface roughness of sediment particles was examined.

3. Field surveys revealed that quartz was consistently smoother than other minerals but that it became rougher in larger size fractions, and the relative abundance of quartz decreased in larger size fractions. Consequently, smooth particles were less abundant in larger size fractions of the sediment. When larval cases were compared with sediment, quartz was actively concentrated in smaller cases of early instar larvae but was gradually less abundant in larger cases of well-grown larvae. Because larvae use larger particles as they grow, late instar larvae develop an unselective choice of mineral types.

4. Subsequently, we experimentally forced the larvae to choose from a mixture of equal amounts of two artificial particles that had different textures (rough and smooth). The proportion of smooth particles chosen by larvae gradually decreased as they grew larger.

5. These results indicate that the larvae varied in their degree of preference according to particle availability in the surrounding sediment, which is governed by mineral composition and weatherability. We suggest that case-bearing caddisfly can adapt to the local sediment environments by varying their standard and/or criteria for material choice. In this study, the possible mechanism for the variation is discussed.