Friday, August 12, 2011

Aquatic Caterpillars?

While wading around in knee-deep water, trying out my new plastic shoes, I spotted a green plant that somehow looked out of place. It looked just like a flat Christmas Cactus leaf, except for the fact that this cactus was under 10 inches of water. Once plucked, I discovered that it was anchored with silken threads to the roots of another plant.

Hmmm. The mystery deepens. Carefully tearing the leaf-looking structure in half revealed the pudgy pro-legs, lateral protrusions, and hardened head of a homely-looking yellow caterpillar. What in the world is a caterpillar doing here, living underwater?

A few photos and emails later, and Dave (the Natural Heritage Program's "invertebrate guru") pointed me towards the Petrophilia moths, a little-known genus of aquatic moth. Wait a minute, did you say aquatic moth?  As it turns out, there are a number of terrestrial moths, beetles and flies that have an aquatic larval stage as part of their life cycle.

There are 16 known species of Petrophilia moths in the U.S. and southern Canada. The adults look like your typical brownish-white small nocturnal moth, flying between June and September. When the female moth is ready to lay eggs, she crawls underwater with a thin layer of air (called a "plastron") surrounding her body, held in place by hydrophobic hairs and serving as a kind of gill.

Ventral view showing pudgy prolegs and protusions
After her eggs hatch, each larval moth spins a silken case (usually a flat silken sheet on a submerged rock) and proceeds to its job of eating algae and diatoms. The one I found had made a green case by folding over an unidentified aquatic leaf, which it then anchored to another plant.

When the larvae is ready to pupate into an adult moth, it does so in an underwater, air-filled cocoon. Presumably, it splits the cocoon open, inflates a plastron of air, climbs out of the water, and spreads its wings to dry and harden before flying away.

Mystery solved - for now. Now I want to splash around in the dark to see if I can find and photograph the nocturnal adult moths.