Caterpillars, the larval stage of butterflies and moths, often face numerous threats from predators. Many caterpillar species employ a defense mechanism known as “playing dead,” or thanatosis. This involves entering a temporary state of immobility to deceive potential threats. Thanatosis is a widespread defensive tactic observed across various animal groups, serving as a last resort when other escape methods are unavailable.
The “Playing Dead” Behavior
Thanatosis in caterpillars involves a sudden change in their posture and activity. When disturbed or sensing a direct threat, a caterpillar might instantly cease all movement. This immobility is often accompanied by dropping from its perch to the ground below. Some species may also curl their bodies tightly, resembling a dried-up leaf, debris, or even bird droppings, further enhancing the illusion of being inedible or lifeless.
This state of feigned death is not permanent. The caterpillar remains motionless for a period, which can range from a few seconds to several minutes, until it perceives the threat has passed. Once the danger is gone, the caterpillar will gradually uncurl and resume its normal activities, like feeding or moving. For example, tiger moth caterpillars are known to curl up and play dead when disturbed. The common oak moth caterpillar also exhibits this behavior, sometimes drawing its head under its body when mildly alarmed.
Why Caterpillars Play Dead
Thanatosis in caterpillars avoids predation. Many predators, including birds, spiders, and other insects, are visually oriented and hunt live, moving prey. By suddenly becoming still and falling, the caterpillar can disappear from the predator’s sight or be mistaken for something unappealing. This sudden cessation of movement can confuse a predator, causing it to lose interest.
The effectiveness of this defense lies in the predator’s instinctual avoidance of dead or diseased prey. A motionless caterpillar might be perceived as unappetizing carrion, reducing the likelihood of an attack. This strategy is useful when the caterpillar cannot flee or employ other defenses, acting as a final attempt at survival.
Beyond Playing Dead: Other Survival Tactics
Caterpillars utilize a diverse array of other survival tactics to evade predators. Camouflage is a widespread strategy, where caterpillars blend seamlessly with their environment by mimicking leaves, twigs, bark, or even bird droppings. Some species, like the Giant Swallowtail caterpillar, resemble bird droppings, making them unappealing to predators.
Chemical defenses are common. Many caterpillars possess specialized glands that release foul-smelling secretions when threatened. The Black Swallowtail caterpillar, for instance, extends an orange, forked organ called an osmeterium that emits a strong odor to deter predators. Some caterpillars acquire toxins from the plants they consume, like Monarch caterpillars ingesting milkweed toxins, which make them unpalatable or poisonous to predators, often advertised by bright warning colors. Other visual deterrents include large eyespots that mimic the eyes of larger animals, such as snakes, to startle attackers.