The Rolly Polly, also known as the pill bug or woodlouse, is a familiar creature often found tucked beneath rocks and decaying leaves in gardens. Despite their common name, these animals are not insects but terrestrial crustaceans belonging to the order Isopoda, making them distant relatives of crabs and shrimp. Their tough, segmented armor and tendency to roll into a protective ball—a behavior called conglobation—have long fascinated observers. Given their nocturnal, low-light habitat, curiosity arises regarding their sensory capabilities: do these small isopods possess eyes, and how well can they see?
Answering the Central Question: Do They Have Eyes?
The simple answer is yes, woodlice do have eyes. They possess a pair of small, dark eyes located on the sides of their head, or cephalothorax. Unlike the complex compound eyes of flying insects, which can contain tens of thousands of individual light-sensing units, the woodlouse eye is significantly reduced. This structure provides the animal with just enough visual information to navigate its dark, damp environment.
Anatomy of the Woodlouse Eye
The visual organs of the common pill bug (Armadillidium vulgare) are unstalked compound eyes, but they are extremely simplified. Instead of thousands of facets, their eyes are composed of a small cluster of individual light-gathering units, called ommatidia. The eye of a common pill bug may contain a mere 15 to 50 ommatidia on each side of the head. This limited number of units means the eye lacks the necessary resolution to form a sharp, detailed image of its surroundings.
The structure is more akin to a collection of simple eyes fused together than a fully developed compound eye. Because they are unable to focus light precisely, these eyes are incapable of producing detailed vision. This anatomical limitation means the woodlouse’s visual world is based almost entirely on variations in light intensity rather than on recognizable shapes or objects.
Vision Function: Detecting Light and Darkness
The primary purpose of the woodlouse’s eyes is to facilitate phototaxis, their automatic movement in response to light. These simple eyes are tuned to detect changes in illumination, such as a sudden shadow or the gradual onset of dawn or dusk. Woodlice exhibit negative phototaxis, meaning they are driven to move away from light sources and toward areas of darkness.
As crustaceans, woodlice breathe through gill-like structures, making them susceptible to rapid water loss and desiccation in dry or sunny conditions. Increased light intensity serves as a warning signal, prompting the animal to quickly seek shelter under logs or deep within leaf litter. Moving away from light guides the woodlouse toward dark, humid microclimates where it can safely maintain body moisture and forage.
Navigating the World Without Complex Vision
Because their eyesight is limited, woodlice rely on other sophisticated senses for finding food and navigating their environment. The most prominent sensory structures are their two pairs of antennae, which act as non-visual probes. The larger, more visible pair of antennae is equipped with specialized hairs that function in mechanoreception, allowing them to detect vibrations and physical obstacles.
The antennae also contain chemoreceptors, which are sensitive to chemical cues in the air and substrate. These chemical sensors are the main tools for locating food, such as decaying plant matter, and for recognizing pheromones left by other woodlice.
Woodlice also possess an ability called hygrotaxis, the capacity to sense and respond to gradients in air moisture. This non-visual sense, processed through receptors on their body and antennae, guides them to their preferred damp habitats. Hygrotaxis compensates for their poor eyesight and ensures their survival on land.