One of Earth’s most ancient groups of insects, mayflies (Order Ephemeroptera) are famously short-lived as adults—some living less than four hours before dying.

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So you might imagine that immature mayflies try their best to live a long and happy life.

Mayflies hatch in streams and rivers, spending their immature life underwater as wingless nymphs, crawling around rocks to scrape and nibble on algae and bacteria.

Streams are dangerous places for mayfly nymphs, with hungry fish patrolling the currents and predatory insects prowling the bottom. To reach new feeding areas or to escape attacks by other insects, mayfly nymphs push themselves off the stream bottom and wiggle into the current, drifting downstream before settling on another rock.

But it turns out that mayfly nymphs are pretty smart about when NOT to drift. Experiments on Baetis mayfly nymphs in California compared their drift behavior in streams containing both predatory insects and trout against streams containing only predatory insects.

Trout are visual predators that hunt during the day. Predatory insects like stonefly nymphs hunt during the night.

In streams with trout, Baetis nymphs actively drift at night to find new patches of food and to escape stonefly attacks, but they completely stop drifting during the day to avoid fish attacks.

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In streams without trout, Baetis nymphs still drift at night. But they also will drift to new food patches during the day since there are no fish to attack them.

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How Baetis nymphs detect the presence of fish versus insect predators is still a mystery—it might be differences in scent—but what’s for sure is that these insects aren’t just drifting randomly in streams. It’s another fascinating peek at the fly-eat-fly world out there!

Check our facts:

Bruce G. Hammock and William C. Wetzel. 2013. The relative importance of drift causes for stream insect herbivores across a canopy gradient. Oikos. doi: 10.1111/j.1600-0706.2013.00319.x

Bruce G. Hammock, Nickilou Y. Krigbaum, Michael L. Johnson. 2012. Incorporating invertebrate predators into theory regarding the timing of invertebrate drift. Aquatic Ecology. Doi: 10.1007/s10452-012-9388-x

Richard W. Merritt, Kenneth W. Cummins, Marty B. Berg, editors. 2008. An Introduction to the Aquatic Insects of North America. 4th ed. Dubuque (IA): Kendall/Hunt Publishing Co.

Today’s special guest post was created by Ben Young Landis and ecologist Bruce G. Hammock. 

Ben Young Landis (who created the art and Vine, as well as helped write the post), is a writer, creative designer, and piscivore. Follow him on Twitter at @younglandis.

Bruce G. Hammock (who helped write the post) is an aquatic ecologist at the University of California, Davis. His PhD research on mayfly ecology involved months of backcountry camping in Sequoia and Kings Canyon National Parks. When he’s not studying underwater critters, he’s dangling off a cliff or a climbing wall somewhere, or being an extra in his brother’s movies.