The pill bug, often called a roly-poly, is a familiar creature found in backyards and under damp logs worldwide. This segmented animal is recognizable by its tough, gray armor and its signature defensive habit of curling into a perfect sphere. Despite its common name, the pill bug is not an insect, which leads to curiosity about its basic anatomy, especially its visual organs. The number of eyes this humble detritivore possesses reveals much about its lifestyle and survival strategies.
The Visual System
Pill bugs possess two unstalked eyes, one located on each side of their head. These are simple compound eyes, sometimes referred to as ocelli, composed of a small cluster of light-sensing units called ommatidia. The visual capability provided by these structures is limited, offering poor visual acuity. They are not capable of forming detailed images or perceiving motion. Instead, the pill bug’s eyes are primarily used for phototaxis—the ability to sense the presence or absence of light, which guides the animal toward the dark, moist environments necessary for its survival.
Navigation Through Other Senses
Since sight is rudimentary, the pill bug relies heavily on its other senses to navigate and interact with its surroundings. Their primary sensory organs are their two pairs of antennae, one pair of which is longer and more visible. These appendages are covered in tiny sensory hairs, or setae, which function as the main interface with the world. The antennae are constantly flicked to detect airborne chemical signals through chemoreception (smell), allowing them to locate food sources and identify mates. They are also crucial for hygrotaxis, the detection of moisture, guiding the pill bug toward the high humidity required for breathing, and their highly developed tactile sense allows them to detect objects and feel vibrations.
Classification and Unique Biology
The pill bug is a terrestrial crustacean, not an insect, belonging to the order Isopoda. This classification places them more closely with aquatic animals like shrimp and crabs. Their crustacean ancestry means they possess specialized respiratory structures called pleopodal lungs, which function like gills. These gills require a constant source of moisture to function properly, driving their behavior and their nocturnal, hidden lifestyle. Their defining characteristic, the ability to roll into a tight ball (volvation), is a defensive mechanism that also serves to reduce water loss when exposed to dry air.