The pill bug, commonly known as the roly-poly, is a familiar creature found beneath rocks and decaying leaves. While they are terrestrial animals, how they breathe is often confusing, leading people to assume they possess lungs or gills. Pill bugs use a highly specialized, air-breathing respiratory system that is neither a true lung nor a typical aquatic gill. These unique structures allow them to extract oxygen from the air while retaining a strong link to the water-dependent breathing of their ancestors.
The Terrestrial Adaptation
The pill bug’s respiratory structures are known scientifically as pleopodal lungs or pseudotracheae. These are not lungs in the mammalian or insect sense, but rather highly modified abdominal appendages called pleopods. In their aquatic relatives, these pleopods function as true gills, absorbing dissolved oxygen from the water. For the pill bug, the surfaces of these appendages have adapted to draw oxygen directly from the air.
The pleopodal lungs are visible as white, branched, or sponge-like structures located on the underside of the abdomen. Gas exchange occurs across the thin, delicate membranes of these surfaces. This adaptation allows for the efficient uptake of atmospheric oxygen.
Misclassified Creatures
The pill bug’s unique breathing system is linked to its biological classification. Despite their appearance, pill bugs are not insects. They belong to the Order Isopoda, placing them firmly within the Subphylum Crustacea, making them relatives of shrimp, crabs, and lobsters.
This crustacean lineage explains why their respiratory apparatus is unusual for a land-dwelling animal. Most crustaceans are entirely aquatic, relying on traditional gills. The pill bug is one of the few species to have successfully adapted its entire life cycle to land. Their armored, segmented body and ability to roll into a protective ball (conglobation) are adaptations that helped this group survive the move from sea to soil.
The Requirement for Humidity
The functional compromise of the pleopodal lungs is their absolute requirement for a moist environment. Unlike the true lungs of land vertebrates or the spiracle and tracheae system of insects, the pill bug’s respiratory surfaces are not covered by a thick, water-retaining cuticle. This lack of protection means the delicate membranes are prone to rapid desiccation.
If the air becomes too dry, the respiratory surface cannot maintain the moisture film needed for oxygen to dissolve and be absorbed, leading to suffocation. This limitation dictates the pill bug’s behavior and habitat selection. They are almost exclusively found in damp, dark microclimates, such as under decaying logs, stones, or leaf litter, where the humidity is consistently high. Their nocturnal activity is also a behavioral strategy to minimize water loss by avoiding the heat and low humidity of the daytime sun.