A few weeks ago, I wrote about what happens when you slip spiders and other animals recreational drugs. A tidbit that didn’t make it into the post was that scientists working with capuchin monkeys in South America had often seen the monkeys grabbing certain types of millipedes, crushing them and then massaging the dead insects into their fur. Sometimes it was a social event—four or five monkeys would share the same millipede, rubbing it all over themselves and then passing it to a friend. Afterwards, they started drooling and their eyes sometimes glazed over. Maybe, some of the scientists speculated, the millipedes were mildly psychoactive and the monkeys were getting high off of their secretions.

When a team of researchers actually analyzed the chemicals that the millipedes produced, though, they realized that the monkeys weren’t catching a buzz, but getting rid of one. The millipedes produced two chemicals, both compounds called benzoquinones, that happen to be excellent mosquito repellents. The monkeys were using the millipedes like bug spray. A later study supported that idea by placing the millipedes’ secretions between some hungry mosquitoes and a container of human blood. The mosquitoes landed and fed less, and flew around at a distance from the container more, when the benzoquinones were present than when they weren’t.

After discovering what the millipedes were for, one of the zoologists who worked on the second study began giving benzoquinone-soaked napkins to the capuchins at the Smithsonian National Zoo, where he worked. After a few rub downs with the napkins, the monkeys would start to abandon their regular keeper when they saw the zoologist coming and run towards him with outstretched arms. There’s good reason for that sort of reaction. Mosquitoes are always annoying, but during the South American rainy season, they can descend on a poor capuchin in thick clouds. Along with an itch, their bites might also leave behind the eggs of the parasitic bot fly, which will develop under the monkey’s skin and create a festering cyst that eventually bursts open with maggots.

Given how much fun that sounds like, the side effects of a millipede rubdown don’t seem as bad. Some benzoquinones are toxic and carcinogenic, and contact with them can cause irritation of the eyes, skin, and mouth that leads to a glassy-eyed look, drooling, and pain—plus, from a human perspective, an overall impression that a monkey might be stoned. For these reasons, the researchers who did these studies don’t recommend that you self-medicate with millipedes to keep the bugs away. One scientist who copied the capuchin technique of crushing a bug with his teeth to release the chemicals fell to his knees in pain when the benzoquinones got into his mouth.