The ways that animals disguise themselves as ants are as numerous as the ways that kids dress up as ghosts for Halloween. Two main differences are 1) everybody knows that the “ghost” is a child in a costume, but those disguised as ants fool a lot of animals, including real ants, and 2) animals that are disguised as ants are that way for life—not just once a year like trick-or-treaters.
Mimicking ants is so common that there is a term for it—myrmecomorphy—which means “ant form”. There are advantages to looking like an ant—primarily that many would-be predators will leave you alone. Ants bite and sting, they are full of bad-tasting toxic defensive chemicals, and they can work as a group to defend against a predator trying to eat any one of them. As a result, a lot of birds and mammals avoid ants and won’t eat them.
Spiders are among the most common ant mimics, with over 300 species that do it. Spiders that look like ants typically have a false waist (skinny middle section), and their bodies look more like the three-segmented bodies of ants than the two-segmented ones of spiders. It’s common for them to have colored patches around their eyes that make their simple eyes resemble the compound eyes of ants. Some ants are transformational mimics, looking like different ants throughout their lives, but always resembling an ant of the proper size as they grow. Spider species whose males have large chelicerae (think of them as spider jaws) hold them in a specific position so that they resemble ants that are carrying something.
Not only do myrmecomorphic spiders look like ants, many also act like them. It’s common for them to wave their front pair of legs near their head to look like ant antennae. (Spiders don’t have antennae, but they have one more pair of legs than ants do.) They also move in an erratic manner far more typical of ants than of spiders.
Perhaps the most amazing aspect of ant mimicry is the olfactory deception used to pass as an ant. Many spiders live inside ant nests, which requires that the spiders smell like their ant hosts. Ants recognize intruders by their foreign smell, and are famously rough on them, killing and ejecting them from the nest. Ants can tell who their nestmates are because all members of a colony have what’s called a “colony odor”. Spiders acquire this chemical signature from the nest material or from the ants themselves, and are therefore accepted as members of the society.
There are many benefits to living in an ant colony. Ant colonies provide a stable microenvironment with many resources. Ant-mimicking spiders eat other arthropods that live with ants, the ant larvae themselves, or the food that the ants have collected and carried to the nest.
Spiders are closely linked with Halloween because of their role in folklore as evil companions of witches, but their disguises should make them winners of even the most competitive costume contests.