Let me begin by admitting that when I worked in Hawai`i, I didn’t pay much attention to the tiny moths that I sometimes scared out of the moss. So this is a post about what I missed.
Hawai`i, being remote and geologically active, is famous for its endemic, explosive evolutionary radiations: a single founding population, finding itself far from both its natural food sources and its natural predators, diversifies into a flock of functionally diverse new species in a relatively short time. The honeycreepers, descended from a finchlike bird, are well-known for this; the Hawaiian picture-wing Drosophila flies are another oft-cited example.
There may be more species in the endemic Hawaiian moth genus Hyposmocoma than in the Hawaiian section of Drosophila, and I personally think these moths are way cooler. Consider the many decidedly non-mothlike things these guys do:
- The caterpillars make cases for themselves out of silk and bits of vegetation, pebbles, and other detritus. Silk-spinning is not unusual for a moth (viz. silkworms), but it’s usually reserved for building a coccoon to protect a pupa. Hyposmocoma caterpillars carry their silk homes on their backs much like a caddisfly larva. This fascinates me because caddisflies are the sister group to butterflies and moths. Is Hyposmocoma case-making an example of reversion to an ancestral state?
- The cases come in a wide variety of shapes—researchers studying them classify them into such categories as purse-, bugle-, cone-, and burrito-shaped. (Some of them look like oyster shells to me.) You can see some examples of these cases and of the adult moths here. Both moths and cases are quite pretty, but I expect they would be highly cryptic in their natural habitats.
- Four known species in the genus eat snails; they are the only lepidopterans to do so. I’ll let the researchers who discovered this behaviour describe it:
When [the caterpillars] encounter a resting snail of the native genus Tornatellides, they immediately begin to spin silk webbing attaching the snail shell to the leaf on which it rests, apparently to prevent the snail from sealing itself against the leaf or dropping to the ground once the larva attacks the soft tissue of the living snail. The larva then wedges its case next to or inside the snail shell and stretches much of its body out of its silk case, pursuing the retreating snail to the end of the shell from which there is no escape.
- Several species have amphibious caterpillars—that is, they can develop successfully either completely submerged in water or on dry land. While many insects, including caddisflies, dragonflies, and stoneflies, have aquatic young that become terrestrial adults, their young are obligately aquatic—they can’t develop out of water. The amphibious Hyposmocoma species are thus unique among insects. This ability has evolved several times independently within the genus. When underwater, the larvae can anchor themselves to the substrate with silk, preventing them from being swept away by strong currents. Scientists suggest that this amphibious lifestyle may be an adaptation to frequent floods in the rainforests in which these species live. Additionally, the limited diversity of insects with aquatic young in Hawai`i compared to such habitats on the mainland may have opened up a niche for these moths to occupy.
So let this be a lesson to me and to all of us who are focused on charismatic macrofauna that we should pay attention to invertebrates once in a while. You never know what they’re up to.
Rubinoff D, & Haines WP (2005). Web-spinning caterpillar stalks snails. Science (New York, N.Y.), 309 (5734) PMID: 16040699
Rubinoff D, & Schmitz P (2010). Multiple aquatic invasions by an endemic, terrestrial Hawaiian moth radiation. Proceedings of the National Academy of Sciences of the United States of America, 107 (13), 5903-6 PMID: 20308549