The specialized relationship between certain butterfly larvae and ants is one of the most intricate partnerships in the insect world. This unique biological bond is known as myrmecophily, meaning “ant-loving,” and is particularly common among the caterpillars of the Lycaenidae family, which includes the blues, coppers, and hairstreaks butterflies. Approximately 75% of the over 6,000 species in this family have some form of association with ants, ranging from loose coexistence to complete dependency. The partnership is an evolved interaction driven by a complex exchange of chemical, tactile, and acoustic communication. The caterpillar uses a sophisticated arsenal of biological tools to manipulate the ants, which, in turn, provide protective services for the vulnerable larva.
Caterpillar’s Chemical and Acoustic Arsenal
The butterfly larvae possess specialized organs that facilitate communication with ants, primarily centered on chemical and acoustic signals. The most conspicuous is the Dorsal Nectary Organ (DNO), a gland situated on the seventh abdominal segment that secretes a carbohydrate and amino acid-rich fluid. This sugary reward is the main commodity offered to the ants and is often referred to as “honeydew” or “nectar.” The caterpillar carefully controls the release of this secretion, often only producing droplets when an ant antennates the area, reinforcing the ants’ attendance.
Chemical mimicry is another powerful tool the caterpillar uses to gain acceptance from ants, which are normally hostile toward non-colony members. The caterpillar produces cuticular hydrocarbons (CHCs), the waxy compounds ants use to identify their nestmates and colony. Some caterpillars acquire the specific, colony-matching CHC profile of their attendant ants, cloaking themselves in the ant colony’s scent to avoid aggression. Other specialized epidermal glands, called pore cupola organs, also secrete appeasement pheromones that help subdue the ants’ aggressive behavior.
Caterpillars also employ acoustic communication through stridulation, rubbing specialized structures together to produce vibrations. These substrate-borne sounds are audible to the ants and can mimic the distinct acoustic signatures of ant workers or queens. The sounds are believed to influence ant behavior, such as encouraging tending or increasing the ants’ aggression toward potential threats. This multimodal signaling, combining chemical appeasement, nutritional reward, and acoustic manipulation, allows the caterpillar to integrate itself into the ant society.
The Protective Exchange Between Species
The mutualistic exchange is founded on the bargain of food for defense, where the caterpillar’s secreted fluid serves as a reliable, high-energy resource for the ants. Since the carbohydrate-rich fluid from the DNO is metabolically costly to produce, its value must be balanced by the benefits received. The reward is so attractive that it can manipulate the ant’s neurochemistry; for instance, the secretions of some species reduce dopamine levels in the attendant ants’ brains. This lowers their locomotory activity and increases their fidelity to the caterpillar, ensuring the ants remain close to their food source.
In return for the secretions, the ants act as highly effective bodyguards, surrounding the caterpillar and aggressively defending it from natural enemies. This protection is especially valuable against specialized parasitoids, such as parasitic wasps and flies, which lay their eggs on or inside the vulnerable caterpillar. The presence of a swarm of ants often deters these small, flying attackers. The protection also allows the caterpillar to feed on the tender, nutrient-rich terminal foliage of plants, which it might not otherwise be able to access safely.
Parasites and Specialized Relationships
Not all myrmecophilous relationships are mutualistic; some represent a parasitic end of the evolutionary spectrum. This is particularly evident in certain species of the genus Maculinea, often called Large Blue butterflies, which have evolved a highly specialized “cuckoo” strategy. After the initial larval stage, these caterpillars drop to the ground and secrete chemicals that closely mimic the cuticular hydrocarbons of the host ant’s larvae. A foraging ant, deceived by the chemical signature, picks up the caterpillar and carries it into the ant nest, mistaking it for a member of its own brood.
Once inside the ant nest, the Maculinea caterpillar assumes a high social status, exploiting the ant colony’s resources. In predatory species, the caterpillar consumes the ant’s own larvae and pupae. Other “cuckoo” species, such as Maculinea alcon, are fed directly by nurse ants through trophallaxis, the mouth-to-mouth transfer of food. These caterpillars are so well-integrated that they are often prioritized over the ant’s own young, even during times of food scarcity. Acoustic mimicry of the host queen’s sounds further elevates the caterpillar’s status, ensuring it receives the highest level of care and protection.