I came upon this creature today during my usual after lunch walk and luckily I had my camera with me. It was suspended from a young sycamore tree, blowing back and forth in the wind attached to an almost invisible silk thread.
It is the caterpillar of a bagworm moth (Thyridopteryx ephemeraeformis). After one of these caterpillars hatches in the spring, it builds itself a case from bits and pieces of leaves and twigs attached together with the silk it produces. It then carries its case with it as it moves around feeding on tree leaves. It eventually pupates inside the case. While the males are small winged moths, the females are wingless, legless and wormlike.
They are quite fascinating creatures. Disappointingly, however, almost all the information I could locate about them on the Internet was more or less about how destructive the caterpillars are to trees. Is that all the entomologists care about?
Even assuming that a given species is a major consumer of an important crop, can we be sure it is worth controlling, both in terms of dollars and in terms of insecticidal pollution of the environment? What price for an apple perfect in every detail, a back yard wholly free of mosquitos? ... I am as much in favor of alleviating hunger and sickness as anyone else. But I do think we are much too ready to tack the label "pest" on almost any insect that comes along.Howard Ensign Evans, Life on a Little-Known Planet, 1968.
There are more pictures of bagworms at BugGuide.Net.