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If you have ever gone into thick deciduous woodland during June, July and August where Formica rufa abounds, you will have noticed a peculiar type of rain falling on top of you. This rain is peculiar because it falls when there are no clouds above and is composed entirely of live wood ants rather than water. The rain results from wood ants taking the quickest way to the ground following a day in the aphid fields, or if they are disturbed and lose their footing.

According to research (Haemig, P.D. (1997) effects of birds on the intensity of ant rain: a terrestrial form of invertebrate drift. (Animal Behaviour. 54, 89-97) about a tenth of descending ants can use this method and if they are disturbed by birds it can rise to over 30%. From my own experience this form of rain can be extreme, directly above a F. rufa nest, following ants on the surface spraying formic acid at you which rises quickly into the air, alerting the aboreal foragers to your presence. Coupled with attack from below climbing up your trouser legs, and noxious formic acid fumes choking the life out of your lungs, a wood ant nest can be a very unhealthy place to hang around. If you want to observe wood ants in the wild I thoroughly recommend approaching a nest very slowly on a non-populated side as vibration from your footsteps can alert a whole nest to your presence. Then, depending on your height and the size of the nest dome you can either get a grandstand view of events on the populated side or a pain in your back and neck!

Anyway, back to the ant rain. Alot of the raining ants have extended abdoman indicating they were milking aphids, far fewer have caterpillars or flies in their grip showing they were foraging in the canopy for protein. If there are no birds in the neighbourhood and you want to assess the population of wood ants in trees, try hitting the base of one with a thick dead branch - only remember to bring an umbrella because a hailstorm will result!

Tags: Formica | rufa | Keeping Ants | Evolution | Behaviour

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