ABSTRACT.Major workers ofOecophylla longinodaemit venom from the tip of the abdomen, as it is brought immediately above the head. The sources of the venom are (a) the poison gland, which contains formic acid, and (b) Dufour's gland, which contains hydrocarbons, including n‐undecane and other n‐alkanes, 4‐tridecene, 8‐heptadecene and 4, 7‐heptadecadiene. The venom elicits a ‘mass attack’ response in other major workers. Formic acid and n‐undecane presented together experimentally also evoke this response, the mixture being considerably more effective than either compound tested separately. These compounds act in combination with the mandibular gland secretions to form a complex alarm