Analysis of modes of pupation in Trichoptera reveals two fundamental types of pupal enclosures and concomitant systems for water circulation. In one (Rhyacophilidae, Hydrobiosidae, Glossosomatidae, Hydroptiladae-infraorder Spicipalpia Weaver), pupating larvae construct a closed cocoon of parchment-like silk, usually discrete from the pupal enclosure of small stones; water currents bathe the external surface of the cocoon during metamorphosis. In the other type (most families in the suborders Annulipalpia Martynov s.s and Integripalpia Martynov s.s.), larvae construct a pupal cell with open meshes or holes at each end, permitting water currents to bathe the surface of the pupa directly during metamorphosis. Exceptions in the Philopotamidae, Stenopsychidae, Ecnomidae, and Phryganopsychidae are considered. The function of trichopteran cocoons during metamorphosis is considered, indicating that osmotic relations in closed cocoons of parchment-like silk in the Rhyacophilidae and allied families impede the efficiency of respiration mediated solely by diffusion of oxygen across the semipermeable wall of the cocoon. Because ovoid, closed cocoons of parchment-like silk also occur in primitive Lepidoptera, the sister group of Trichoptera, cocoons of this type are proposed as part of the groundplan of Trichoptera. Consequently, the open pupal cells of Annulipalpia (retreat-makers) and Integripalpia (case-makers) are interpreted as derived. It follows that the habitat common to the families constructing closed cocoons-cool, flowing waters-would likely have been the habitat in which Trichoptera originated. The hypothesis proposed for the phylogeny of pupation in Trichoptera is that the closed cocoon of semipermeable silk in the ordinal groundplan required Trichoptera to become aquatic in cool, lotic waters; and that evolutionary innovation through subsequent derivation of cocoons of permeable silk with ventilatory openings enhanced the efficiency of respiration, enabling Trichoptera to invade warmer waters of reduced current, and opening the way for radiation of the major lineages now extant-the retreat-making (Annulipalpia s.s.) and the case-making (Integripalpia s.s.) families. An alternative form of the higher classification of Trichoptera is proposed and discussed, elevating the Spicipalpia, termed the cocoon-making Trichoptera, to the rank of a third suborder, co-ordinate with the Annulipalpia and Integripalpia.