Rock Loving Moths Along a Rocky River

While exploring the shores of the Hoosic River last week, I encountered clouds of tiny dust-colored moths mobbing the riverside flowers. I cannot be sure of the moths' species, but I have learned that they belong to a group of moths called Petrophila, which means "rock lover." This is a very appropriate name for moths that frequent the truly rocky shores and shallows of this rushing river, for these moths must have both underwater rocks and rushing water to breed.  The female, clutching an air-bubble against her ventral surface to allow her to breathe, climbs down the surface of a submerged rock and deposits her eggs there. The larvae hatch, feed, and pupate underwater and only resurface as adults. 

Low water this week reveals just how rocky the Hoosic River bottom is, which makes it the perfect habitat for this group of moths whose very name, Petrophila, means they must love rocky sites.

Clouds of the Petrophila moths mob the riverside flowers in late summer, feeding on the pollen and nectar as they prepare to mate and lay eggs on the river's underwater rocks. Dozens of moths nearly overwhelmed this cluster of goldenrod flowers. 

There are probably several species of Petrophila moths that gather along the Hoosic's rocky riverbanks this time of year. These two feeding on Sneezeweed flowers are possibly the Two-banded Petrophila (Petrophila bifascialis). The hind wings of this species have a row of black/metallic spots that make one spider enthusiast theorize that they’re Jumping Spider mimics (viewed from the rear, the spots are supposed to resemble the flat face and eyes of a predatory jumping spider). But as another photo I took reveals, not all rival spiders are intimidated by that resemblance.

This good-sized spider was lurking amid the goldenrod blooms that mobs of tiny Petrophila moths were feeding on. The size, brown color, tapering abdomen, and broad scalloped stripe down its middle suggest to me it could be a Nursery Web Spider, a species that does lie in wait for prey instead of weaving an entrapping web. I have to assume that the spider was also hoping to dine at this site, and not on vegetarian fare! 

To learn lots more fascinating details about this interesting group of moths, be sure to visit this site from the University of Wisconsin.