The "conchicolous habit"--hermit crabs have it, some bees and wasps have it, but a few spider species lift shell dwelling to new heights: they use silk to hoist their prefab homes into the air. A BBC crew working in Madagascar has finally caught the efforts of one of these species, Olios coenobitus, on film.
O. coenobitus is in the family Sparassidae, the huntsman spiders. At least one other species, Pellenes nigrociliatus, a jumping spider (family Salticidae), has also been found living in suspended snail shells, in Poland. Neither of these spiders can produce the stretchy silk used in the catching spirals of orb weavers and their descendants. In fact, few spiders in either of these families make any kind of web. Their hoisting ropes consist of major ampullate dragline silk, the super-strong silk araneomorph spiders use to rappel. In the lab, these spiders have been observed to lift shells more than 20 times their own body weight.
It might seem inconceivable that the same strange behavior would evolve in two species of spider that aren't terribly closely related. But it looks strange only to us humans. An empty shell isn't so different from the burrows spiders dig themselves or the crevices in bark they often occupy. And spiders frequently use silk to suspend killed prey and egg sacs. They pull on silk threads all the time to bring them under tension. We'll probably never know exactly how this feat evolved, but many spiders already had the tools and behaviors needed to accomplish it. Looked at that way, it's surprising this behavior hasn't evolved more often. But then again, even these spiders' closest relatives get along fine without it. In evolution, "good enough" is good enough.
Read how the film was made here.