A Blog by Ed Yong

Ants disinfect their young by drooling backside poison

Personal hygiene is paramount for insects, as it is for us. They too must contend with a cavalcade of bacteria and fungi that threaten to soil their food and infect their bodies. And cleanliness is one of their most important defences against such infections. This goal is familiar to us, but the methods are… unorthodox.

Take the emerald cockroach wasp. Its larva disinfects its food, which happens to be is a zombie cockroach that the young wasp is devouring from the inside out. Its cleaning technique involves slobbering antibacterial fluids inside its living cradle-cum-larder. I’ve written more about the wasp’s behaviour at Nature News (including the see-through windows that scientists installed in cockroaches so they could watch the larva). Carl Zimmer also has you covered here at Phenomena.

For now, I’m going to deal with another recently discovered case of insect hygiene, involving ants that suck poison from their backsides and drool over their brood.

Ants, bees and other social insects live in dense colonies that could easily be wiped out by a single outbreak of disease. To prevent this from happening, the entire colony often acts as a giant immune system that works together to protect their health. Bees, for example, will sometimes buzz in tandem to raise their body temperature, creating a “social fever” that heats bacteria to death. Ants will groom each other to remove the spores of parasitic fungi.

Sylvia Cremer from the Institute of Science and Technology in Austria has spent years studying these hygienic behaviours, and she has found a new one in the garden ant Lasius neglectus. These ants are known to lick fungal spores off their larvae, which they store in pockets in their mouths. Later, they spit these out in small pellets. Now, Cremer has found that the ants have another trick: They slather a poisonous fluid over their denuded youngsters, turning their mouths into temporary “chemical disinfection chambers.’’

Cremer’s team members Simon Tragust and Barbara Mitteregger noticed that the ants don’t manage to clean every spore from their larvae. But even those they miss are much less likely to germinate. The secret to this suppression lies in the ants’ back ends, and specifically in a hole called the acidopore. If the team plugged this opening, the ants were only half as good at suppressing the growth of fungal spores.

The acidopore connects to an ant’s anus, pheromone gland, and poison gland. It’s that last one that matters. When Tragust ad Mitteregger gently poked the ants from behind, they would release their poison as a droplet. Depleted of this chemical supply, they lost their antifungal powers.

The droplet that comes out of the acidopore contains 37 chemicals, but mostly formic acid—a pungent substance that many ants use to defend themselves from predators. Formic acid even gets its name from ‘formica’, the Latin word for ‘ant’. It makes up 60 percent of the acidopore droplets, and accounts for 70 percent of its fungus-busting powers. The other substances, such as vinegary acetic acid, did very little on their own but gave an extra boost to formic acid’s antifungal properties.

Some of the ants would spray the poisonous cocktail directly onto the larvae, but most of them took an indirect route. They ‘licked’ their acidopore and sucked up their own poison into their mouths. They then cleaned their brood, and slathered the acidic poison over them in the process.

This method has many benefits. Tragust and Mitteregger think that it allows the ants to apply their precious chemicals more accurately and evenly. It also means that they disinfect their own mouths, since the acid kills the spores that they lick off the larvae, and stops any pellets they spit out from germinating.

The garden ant and the cockroach wasp are just two new examples of insects acting as their own pharmacists—using self-made chemicals to defend themselves against bacteria. In the simplest strategies, they groom themselves with defensive chemicals. Ants and termites do this, as do rove beetles groom themselves, which use a substance called stenusine to walk on water as well as repel fungi and bacteria.

Some species use their chemicals to clean their homes, relatives, or food supply. Some bees and wasps incorporate their venom into the building materials for their nests. Fire ants apply antibacterial chemicals onto their eggs, and liberally spray the stuff into the brood chambers, where the eggs are kept. The European beewolf—a  type of parasitic wasp—embalms the honeybees that it provisions for its larvae, by covering them with an oily secretion that stops water from condensing and makes it harder for fungi to grow.

At a time when many human societies lack decent sanitation, and others have only enjoyed it for a few centuries, it’s sobering to remember that insects have been practicing careful hygiene for millions of years.

Reference: Tragust, Mitteregger, Barone, Konrad, Ugelvig & Cremer. 2012. Ants Disinfect Fungus-Exposed Brood by Oral Uptake and Spread of Their Poison. Current Biology http://dx.doi.org/10.1016/j.cub.2012.11.034

Herzner, Schelcht, Dollhofer, Parzefall, Harran, Kreuzen, Pilsl & Ruther. 2013. Larvae of the parasitoid wasp Ampulex compressa sanitize their host, the American cockroach, with a blend of antimicrobials. http://dx.doi.org/10.1073/pnas.1213384110

5 thoughts on “Ants disinfect their young by drooling backside poison

  1. Correction. Digestive waste does not exit through the acidopore — only the products of the venom and DuFour’s glands. B.t.w., colony founding fire ant queens were demonstrated to disinfect their eggs with venom, very gently applied with the sting, then spread with the mouthparts. There is no formic acid in fire ant venom, but rather, a slurry of toxic alkaloids and histolytic enzymes.

    [James, the paper says that the hindgut opens into the acidopore as well as the two glands you mention. Is that wrong, or have I misinterpreted it? – Ed]

  2. @James & Ed: Interesting. I’d like to know — I have always assumed that the rectum of the (formicine) ant opened to the same aperture as the venom, which would seem to be the acidopore. Male ants, regardless of not having venom, often have “rectal tubes” which may extend well out of the body and over the genitalia. Do the formicines lower their terminal sternite then to relieve the acidopore of defecation duty, and thence using a different opening?

  3. *thence _use_ a different opening. (It’s hard to review what you wrote in this tiny comment box, besides the fact that there is no edit option.)

  4. The sting and venom apparatus are anatomically derived from the genitalia, which are anterior to the anus, but everything is so “scurnched” together in ants that this is hard to see, especially in formicines, which have lost the sting. So yes, the anus is located, in effect, dorsal to the acidopore, and the acidopore must be tucked forward (sort of like a pooping dog, to put it in readily visualizable terms), in order for the ant to defecate.

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