In the world of invertebrates, survival often depends on unique adaptations. One example of this is spiders that look and act like ants. This strategy of mimicry is an evolutionary tool, allowing these arachnids to navigate a world full of predators by posing as a different, and often less appealing, insect.
What Are Ant-Mimicking Spiders?
Ant mimicry is a strategy employed by over 300 species of spiders, though it is most common in the jumping spider family (Salticidae). These spiders achieve their ant-like appearance through specialized physical modifications. Their bodies are often elongated and slender, which helps to obscure the typical two-part spider body plan and create a more convincing three-segmented look of an ant.
To further enhance the illusion, many of these spiders have constrictions on their bodies to create a false “waist,” known as a petiole. Their exoskeletons are frequently shiny and colored to match the specific ant species they imitate, ranging from black to yellow or gold. Some even have dark patches near their heads that resemble the large, compound eyes of an ant.
The mimicry extends beyond physical appearance to behavior. A hallmark of these spiders is the way they move, often adopting the quick, winding walk of an ant. Most distinctly, they hold and wave their front pair of legs in the air, simulating the constantly moving antennae of an ant. These front legs are still used for locomotion but are lifted into an “antennae” posture when the spider is still.
The Purpose of Ant Mimicry
The primary driver behind this disguise is defense, a concept known as Batesian mimicry. Ants are not a preferred meal for many predators, such as birds, wasps, and even other spiders. This is because ants are well-defended; they can bite or sting, and many species possess unpleasant chemical defenses. By impersonating an ant, the spider gains protection from predators that have learned to avoid them.
This mimicry can be effective enough to fool predators that hunt by sight. However, some predators, like certain wasps, use chemical cues to identify prey at close range. Some ant-mimicking spiders have evolved to produce chemical signals that smell like ants, providing another layer of defense against predators that hunt by scent. This allows them to live in close proximity to the ants they copy.
In some cases, the mimicry serves an offensive purpose. This strategy, called aggressive mimicry, allows the spider to infiltrate ant colonies. By looking and smelling like an ant, the spider can move among them without triggering an alarm. Once inside the nest, it can prey on the unsuspecting ants or their eggs and larvae.
Distinguishing Spiders from Ants
Despite the convincing costume, there are clear biological differences that can distinguish an ant-mimicking spider from an actual ant. The most definitive feature is the leg count. All spiders, including mimics, have eight legs, whereas ants have only six. The spider uses its front two legs to imitate antennae, so a careful count of the walking legs will reveal the difference.
Another identifier is the number of body segments. Spiders have two main body parts: a fused head and thorax called the cephalothorax, and an abdomen. Ants have three distinct segments: a head, a thorax, and an abdomen. While some spiders have constrictions to simulate three parts, the underlying two-part structure is a fundamental difference.
Finally, examining the eyes provides a good clue. Spiders in the jumping spider family have large, forward-facing eyes that give them excellent vision. In contrast, ants have smaller, compound eyes located on the sides of their heads. The prominent, forward-looking eyes of the spider are a giveaway that you are not looking at an ant.
Common Types and Habitat
The most well-known group of ant-mimicking spiders belongs to the genus Myrmarachne. This name combines the Greek words for “ant” and “spider.” Species within this genus are found worldwide but are most diverse in the tropics. A few species, like Myrmarachne formicaria, are found in more temperate regions, including North America.
These spiders live in the same habitats as the ants they mimic, which is necessary for the disguise to work. They are found on the leaves of plants, on tree trunks, and in leaf litter on the ground, always in close proximity to their ant models. The specific ant species a spider mimics can even change during its lifetime; some have been observed mimicking one type of ant as a juvenile and a different species as an adult.
These spiders are harmless to humans. Their venom is not considered medically significant, and their primary defense is their disguise, not aggression. They are reclusive hunters focused on small insects and other arthropods, posing no threat to people.