Rolly pollies, also known as pill bugs or woodlice, are commonly found in damp garden environments. Despite their presence in moist areas, these small creatures are surprisingly vulnerable to water. They cannot swim and will drown if left in standing water, a unique aspect of their biology.
What Rolly Pollies Are
Rolly pollies are not insects, but terrestrial crustaceans, making them distant relatives of marine animals like shrimp, crabs, and lobsters. They belong to a group called isopods, unique as the only crustaceans fully adapted to land. These creatures typically measure between one-quarter and one-half inch in length, possessing a distinctive oval, segmented body with seven pairs of legs.
A notable characteristic is their ability to curl into a tight, protective ball when disturbed, a defense mechanism known as conglobation. This behavior shields their softer undersides from predators and helps them conserve moisture. Their armored appearance, with seven overlapping plates on their thorax, provides the flexibility for this defensive posture.
Why They Don’t Swim
The primary reason rolly pollies cannot survive in water relates directly to their respiratory system. Unlike aquatic crustaceans that use gills to extract oxygen from water, rolly pollies breathe through specialized gill-like structures. These structures, sometimes referred to as pseudotrachea or pleopodal lungs, are located on their paddle-shaped hind legs.
These terrestrial gills are adapted to absorb oxygen from moist air, not from water. When a rolly polly is submerged, these delicate respiratory organs become saturated and cannot function effectively to transfer oxygen. This lack of oxygen exchange leads to suffocation, causing the creature to drown.
Their Natural Habitat and Survival
Rolly pollies thrive in environments that offer consistent moisture without being waterlogged. They are frequently found in damp, shady places such as under rocks, logs, leaf litter, and within compost piles. Their preference for high humidity is a direct consequence of their gill-based respiration and their tendency to lose water through their bodies.
To prevent desiccation, rolly pollies are often nocturnal, becoming active during the cooler, damper night hours. They absorb moisture from their surroundings, including from damp surfaces and through their mouthparts or abdominal appendages. This reliance on environmental moisture means they actively seek out and remain in humid microhabitats, avoiding open, dry areas or standing water.