Formica is a genus of ants. Species of the Forelian formica group, whose type is the Red or Southern Wood Ant Formica rufa, are generally referred to as the wood ants. Many of the better known species have common names that usually include the words "wood ant", although F.rufa is also known as the "horse ant". The commonest species in any particular area is likely to be referred to locally simply as the "wood ant". As their name implies, they tend to live in wooded areas where there exists no shortage of material with which they can thatch their mounds. However, sunlight is most important to F.rufa group Formica species, and colonies rarely survive for any considerable period in deeply shaded, dense woodland. The most shade tolerant species is F. lugubris.

There are many species of Formica (ITIS records nearly 200). Some species, including Formica rufa, which is common in Southern England, make large visible nests of dry plant stems, leaves, or pine needles, usually based around a rooting stump. Wood ants typically secrete formic acid; F. rufa can squirt the acid from its acidopore several feet if alarmed. They can be relatively large: F. rufa workers can reach a maximum of around 10mm in length.