Crawling, rice-like things!?

On September 3, I saw this mini-mass of crawling things on my stone patio, moving more or less in a straight line. Each bug was about 1/2" long. They look like grains of wild rice and were crawling all over themselves. What might these things be?

Whoa, that’s a great photo!

It looks like you caught a glimpse of a swarm of fungus gnat larvae.

First, the hum-drum stuff:

Fungus gnats are insects in the family Sciaridae. As their name implies, their larvae dine on fungi, roots, and other organic material in the soil. They’re typically about the size of a grain of rice, and mostly translucent (apart from their “food channel” guts). They go through their egg-larva-pupa cycle in a few weeks, and emerge as adults that look very much like fruit flies.

Due to their fondness for damp soil and their speedy life cycle, fungus gnats are a common nuisance in houseplants and greenhouses.

Okay, now the amazing stuff:

Your photo caught the larvae performing a “rolling swarm”! This amazing behavior is employed by several kinds of caterpillars and other worm-like larvae around the world. The swarming larvae create a kind of super-organism that oozes along as though it were a single, giant worm.

But why?

As with most natural phenomena, there are probably multiple reasons. Here are my favorite three:

First, swarming is a proven survival strategy. From mayflies to cicadas, lots of insects have timed their life cycles to emerge in huge numbers for safety.

Second, forming a huge group is a great way to deter predators. By wriggling along as a snake-sized group, the larvae might scare off predators who’d otherwise eat an individual larvae. (After all, most humans are intimidated by these rolling swarms!)

Third, it’s an ingenious way to give yourself a speed boost.

Authors Dustin Sandlin and Aatish Bhatia published a great article proving that a caterpillar in a rolling swarm can out-pace a solo caterpillar. By climbing on the backs of their downstairs neighbors, the upstairs caterpillars can pick up some speed without having to crawl faster. It’s a bit like being a pedestrian on one of those moving walkways at the airport.

Don’t believe it? Luckily, Sandlin devised a terrific way to model rolling swarm behavior using Lego blocks (which Naturalist Trevor Wearstler and I couldn’t resist re-creating).

Pretend that these Lego bricks are caterpillars. Compare the solo blue brick with the blue brick that’s part of a swarm. Though every brick only advances one step at a time, the bricks in the swarm end up reaching the red finish line ahead of the solo brick.

Though these hypotheses would answer a lot, there are still plenty of fascinating questions, including:

• Why do people in the northern U.S. (Ohio, Minnesota, Maine, etc.) seem to primarily report these swarms in the fall?
• How does the swarm choose a direction? (After all, what’s the use in moving faster if you can’t decide where you’re going?)

Thanks for sharing your photo!

– Naturalists Chris Mentrek & Trevor Wearstler