Under certain circumstances

D. Wallace, B. Wallace & G. N. Philipson, A Key to the Case-Bearing Caddis Larvae of Britain and Ireland, Cumbria, Frehswater Biological Association, 1990, p.5

A « caddis » is popularly considered to be an aquatic insect living in a portable tube or case which it has made from various materials. There are, however, many caddis-fles whose larvae do not make cases and this difference in habit has resulted in a convenient division of Trichoptera larvae into «  case-bearing » and «  caseless ». Unfortunately, the division is not as precise as one might have hoped. Some « case-bearing » larvae are caseless until ther reach final instar (Hydroptilidae) ; others may abandon their cases under certain circumstances and drift without cases in the stream flow (Glossosomatidae) ; a few, at least in captivity, readily leave and re-enter their cases (Phryganeidae). Conversely, «  caseless » larvae construct fixed shelters of various materials in which they live or pupate ; such shelters, dislodged when sampling, may well be found to contain a larva which the unwary might consider to be « case-bearing ». For such reasons, the family grouped as «  cased » or « caseless » must be carefully defined on grounds of larval morphology.