When a “rolly polly” is gently touched, it often curls into a tight, armored sphere. This distinctive action, which gives these animals their popular name, is a fascinating adaptation that serves multiple purposes. Understanding this behavior reveals insights into their unique biology and their place in the natural world.
What Are Rolly Pollies?
Despite their common name suggesting they are insects, rolly pollies, also known as pill bugs, are actually terrestrial crustaceans. They belong to the order Isopoda, making them more closely related to marine creatures like crabs, lobsters, and shrimp than to typical garden insects. These animals measure between one-quarter and three-quarters of an inch in length and have an oval, segmented body covered by a hard exoskeleton. They possess seven pairs of legs and two pairs of antennae.
Pill bugs thrive in moist, shaded environments, commonly found hidden under decaying leaves, logs, rocks, and other organic debris. Their preference for damp conditions is directly linked to their physiology. These creatures are primarily nocturnal, becoming most active at night when humidity levels are higher, further protecting them from drying out.
Why They Roll Up
The primary reason pill bugs roll into a ball, a behavior termed conglobation, is for defense against predators. When disturbed or threatened, they rapidly contract specialized muscles, pulling their segmented plates tightly together to form a compact sphere. This action effectively protects their soft, vulnerable underside and delicate legs, leaving only their tough, armored exterior exposed to potential threats. The behavior is a reflexive response triggered by sensory input like touch or vibrations.
Beyond predator avoidance, conglobation also serves an important function in water conservation. As crustaceans, pill bugs breathe through specialized gill-like structures located on the underside of their abdomen. These respiratory organs require a moist surface to function properly and absorb oxygen from the air. Since pill bugs lack the waxy cuticles of insects, they are highly susceptible to desiccation, or drying out.
By rolling into a ball, they significantly reduce the surface area exposed to dry air, minimizing evaporative water loss from their bodies and respiratory surfaces. This behavior helps them retain moisture and keeps their gills hydrated, allowing them to survive in terrestrial environments despite their reliance on humidity. Pill bugs cannot last longer than a few days in dry conditions.
Their Ecological Role
Pill bugs play an important role in ecosystems, primarily as decomposers. They feed primarily on dead or decaying plant matter, such as fallen leaves, rotting wood, and other organic debris found in the soil. Through this feeding activity, they break down organic materials, which helps to return valuable nutrients back into the soil. This process contributes to soil fertility and overall soil health, making them beneficial inhabitants of gardens and natural landscapes.
Beyond nutrient cycling, pill bugs can help manage heavy metal deposits in the soil by consuming them. They also serve as a food source for various animals, including birds, spiders, centipedes, and some amphibians, integrating them into the broader food web.