One of a child’s first natural history lessons is usually that caterpillars grow up to become butterflies or moths. Eager to witness this transformation, we seek out caterpillars in yard and field to shepard through this remarkable metamorphosis. We soon learn the importance of raising the larvae on the same species of plant on which they were found. For many of us, this is our introduction into the interdependence of plants and animals, and the complex life cycles of even common organisms around us.
A couple of years ago I was doing an insect survey on a property along the Detroit River. I noted a pretty little moth which was quite common; many appeared freshly emerged, which made me curious about what the caterpillars fed on. From my photograph (above) I identified the moth as a Chestnut-marked Pondweed Moth. “Pondweed” is a pretty generic term, but I’ve generally heard it refer to submerged plants in the genus Potamogeton. Sure enough, Potamogeton is the host plant for this species…and the larvae are aquatic. I consider myself pretty well-versed in various butterfly and moth species and their host plants, but aquatic caterpillars were new to me.
This moth is not unique. In this same genus are species whose larvae feed on waterlilies, watermilfoil, and other water plants. Most feed on submerged parts. Other moths in the same family feed on algae scraped from rocks or diatoms trapped in silken sheets spun by the caterpillar. Many have gills for all or part of their larval stage. Females of some species may submerge themselves in an air pocket to lay eggs up to four meters underwater! Members of a number of other moth families are also known to have aquatic larvae.
When I think of flying insects that have an aquatic larval stage, my first thought is always dragonflies. Then many species of flies, as well as beetles, caddisflies, and some true bugs. Now I can add moths to this list, something I never imagined when I raised my first sphinx moth from a “tomato worm” as a bright-eyed child.