Rannoch Wood Ants
There are four species of large ant that are resident in Rannoch. They are Formica aquilonia, Formica lugubris, Formica exsecta and Formica sanguinia.
When you are walking through the Blackwood of Rannoch or other areas of pine forest you are likely to see large conical nests made of pine needles, often on south facing slopes along side the tracks. The most likely species to have made these nests is Formica aquilonia which is the Scottish Wood Ant, although some could be Formica lugubris. The ants need to have sunlight on their nests so that they can warm up. They are cold blooded (more strictly poikilothermic) and require external heat to assist them in warming up. If the ants are cold they are forced to be inactive. The main conical nest may have so called small satellite nests connected to them by ant trails. There can be several queens in a nest, which is in contrast to the situation in bee or wasp nests where there is only one queen.
The wood ants have three castes, queens (breeding female), males, and workers (non breeding females). The workers leave the nest to gather food. This may be prey in the form of small invertebrates or it may be honeydew provided by aphids. The prey provides the ants with protein and the honeydew provides the ants with carbohydrate. Aphids are more commonly know as greenfly but few are green and there is a huge variety of species. In the Caledonian pine forest (such as the Blackwood of Rannoch) aphids feed on the trees. They have sharp mouthparts called stylets which are rather like hypodermic needles which are inserted into the phloem vessels. These vessels carry the sap which is rich in sugar. The aphids take in so much sugar that they need to excrete it and this is the honeydew that the ants feed upon. In return the ants defend the aphids from predators such as ladybird larvae. The ants are well equipped for this task as they have large biting mandibles and the ability to squirt formic acid from the tip of their abdomens. To do this the ant curls its abdomen underneath it and the jet of acid, which can travel up to six inches, is directed forwards. The ants are feisty enough to direct this jet at human beings although to little effect.
The worker ants guide themselves and others with the aid of chemicals called pheromones that are laid down as a trail. Ants are social insects and the males have only half the usual number of chromosomes (they are haploid). A complex genetic situation arises that makes it worthwhile for the workers to look after the offspring of the queen rather than reproduce themselves. The same situation exists in bees, wasps and hornets.
The ants in the Blackwood of Rannoch become active in April and increase their numbers of workers through the summer. In July winged queens and males are reared and these go on a marriage flight to mate. The males soon die and the queens drop their wings and either form a new nest or return to an existing one in the hope of being accepted. The queens store all the sperm that they need for the rest of their lives. If they use the sperm to fertilise and egg it will become a worker or another queen. If they lay an egg without fertilising it a male will be the result
A variety of other insects live in the wood ants nests. These are called inquilines (meaning lodger). The inquilines does not necessarily harm the ants although they may. There is a colourful green chafer beetle which I have seen making its way into the nest. The inquilines often produce chemicals which have the effect of pacifying the ants.
The wood ants themselves are preyed upon by greenwood peckers which make conical holes into the nest as they extract ants. Perhaps fortunately for the ants green woodpeckers are not common in Rannoch.
Formica exsecta makes smaller nests and is quite rare but has been found at Camgourhan. Formica sanguinea is interesting in that it makes slaves of other ants. The queen will attack the nests of other ants and then the larvae are raised as slaves.
If you find a wood ants nest do not disturb the surface as it may let the rain in and damage the colony. Just consider their ways.