Ants and their guests

Of which some behave very badly

I met Bert Hölldobler in 1972 at the 14 th International Congress of Entomology in Canberra, the capital of Australia. He then told me he was appointed to be professor at Harvard University.  Harvard University! A PhD-student at that time, I was surprised that a man as young as Bert would be a Harvard University Prof, but reading his studies on ants, I was no longer surprised.

There is no other insect group than the 10,000 or so species of ants in terms of different social behaviours and lifestyles: Ants are social insects; solitary species do not exist. In the all-female populations (as in bees, males only need to fertilize a virgin queen; after that they can die) different castes varying in size and appearance have different functions. Although no ant species is marine, there is an intertidal one (Polyrhachis sokolova) and one that scavenges food from inside a pitcher plant’s insect trap (Camponotus schmitzi). Some species collect seeds or feed on other insects and small vertebrates; some clean carcasses of birds and mammals and army-ants are famous for their raids, separated by temporary bivouacs constructed by and with the ants’ own bodies. Honeypot ants have castes so round and heavy with honey that they can no longer move, while ‘aphid-tending’ ants protect (and ‘milk’) their aphids like cattle. Well-known are also weaver ants Oecophylla smaragdina (with nests of sewn-together leaves) and fungus-growing leafcutter ants. There, too, are at least 35 ant species that keep slaves of other species.

Anyhow, Hölldobler’s work that intrigued me most, dealt with unwanted beetle guests in ant nests; guests that behaved badly and had devious ways to sneak into an ant colony and not get removed. For the little antlike staphylinid beetle Atemeles to enter the nest of a Formica ant, it smells and behaves like an ant. Its larva possesses the same smell as an ant larva and for this reason remains unattacked. The beetle larva also knows how to beg for food and when it taps the mouth of a passing ant, the ant responds with regurgitating a drop of fluid, which the beetle larva ingests. The ants do not prevent the beetle larva killing and devouring the ants’ babies, but continue to feed the cheat. However, the beetle larvae’s disguise is so perfect that they themselves are in danger to be eaten by another beetle larva: these beetles are cannibals! At the end of summer the beetle larva pupates, yielding an adult beetle in autumn. The Formica host ant colony, however, stops reproducing in winter and the new beetle generation would starve  – unless, and that’s what they’re doing, they emigrate to find a nest of a Myrmica species that has eggs and larvae throughout the winter. To get into a Myrmica nest, the adult beetle secretes from its abdomen an appeasement fluid, which turns the Myrmica guards into drugged ‘friends’. Once inside the Myrmica nest the beetles have enough food, mature sexually and in spring bid good-bye to the Myrmica host and seek a Formica nest again to start a new generation.

There are even “highwaymen” beetle species that locate the ants’ foraging trails by scent and then ambush ant workers, tickle their mouthparts and steel what they spit out of what they had in their crops. Of course, harmless ‘tenants’ like some mites or springtails also exist in an ant nest; yet, I bet there are still many more species of the bad guys that await discovery. But it takes a lot of patience and good eyesight to spot them: believe me I looked for them and failed.

© Dr V.B. Meyer-Rochow and, 2021.
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