The Beetle That Doubles as a Mass Transit System


Werner Rose via Wikimedia Commons

In the tropical forests of Central and South America, a fantastic looking beetle begins its life in the drabbest of places. The harlequin beetle (Acrocinus longimanus) is named for the eye-catching splashes of black and red (or yellow) that cover its forewings. While seemingly conspicuous (some of the patterns look like something Guy Fieri would have on a bowling shirt), the designs actually help the beetles blend in with the mottled trunks and branches of trees, and it’s in the dull brown wood of fallen, decaying tree limbs that a mother harlequin beetle lays her eggs. She gnaws into the bark and deposits her eggs, usually in a batch of 15 to 20. The larvae dig deeper into the wood when they hatch, and again when they’re ready to pupate. Then they spend the next few months developing and growing into their adult form. 

Meanwhile, on those same rotting branches, the pseudoscorpion Cordylochernes scorpioides scuttles about eating small insects. Lacking the tails and poisonous stingers of their namesake, the true scorpions, these little teardrop-shaped arachnids instead take down their prey with venom secreted through their pinching claws. When a branch becomes too decayed to make a good home, or there are too many pseudoscorpions and not enough food to go around, some of these guys will have to find another dead limb to live on. But they’re tiny (often just a few millimeters long) and wingless, and the next branch might be far away. How do they get from Point A to Point B?

The answer is right underneath them. When the the fully-grown harlequin beetles chew their way back out from the branches, one of their first orders of business is to find a branch to eat bark and fungus from. Since both species are looking for the same thing, the pseudoscorpions hitch a ride on the beetles. 

After a beetle bores its way out of the wood, pseudoscorpions swarm around it and pinch its belly with their claws. Annoyed at the less than warm welcome to the outside world, the beetle flexes its abdomen and flicks its forewings, exposing its back and giving the pseudoscorpions space to climb aboard.

As they shuffle onto the beetle, the male pseudoscorpions will make room for females, but shove each other around as they jockey for the best spots and try to keep other males from climbing on. When the beetle takes off, the pseudoscorpions secure themselves with “safety harnesses” they fashion from silk from their claws, and the males that managed to stay on the beetle compete for the attention of the female passengers and mate with them. The females usually disembark at the next branch, but the males may stay on the beetle if new females climb on and use the bug as a mobile love shack for a few days.