Woodlice are frequently encountered in gardens and homes, often leading to the common question of whether they are insects. Despite their appearance, woodlice are not insects. They belong to a different group of animals entirely, the crustaceans, making them more closely related to crabs and lobsters than to typical garden bugs.
Identifying Woodlice
Woodlice are classified as terrestrial isopods, belonging to the suborder Oniscidea within the class Malacostraca and the phylum Arthropoda. Their bodies are characterized by a segmented, dorso-ventrally flattened shape, protected by a hard, armor-like exoskeleton composed of seven plates. This exoskeleton is shed in two stages as the woodlouse grows.
A distinguishing feature of woodlice is their seven pairs of legs, attached to each of their seven body segments. Unlike insects, woodlice breathe through specialized gills, which must remain moist to function. These gill-like structures absorb oxygen from the thin layer of water covering them. This reliance on moisture explains why woodlice are typically found in damp, dark environments such as under rocks, decaying wood, or in compost heaps. Females carry their fertilized eggs in a pouch called a marsupium on their underside, where the young, known as mancae, develop before hatching.
Defining Insects
Insects are members of the class Insecta, also within the phylum Arthropoda. Insects are characterized by a body plan divided into three distinct segments: the head, thorax, and abdomen. The head typically bears a single pair of antennae and mouthparts, while the thorax serves as the attachment point for their appendages.
All adult insects possess three pairs of jointed legs connected to the thorax. Many insect species also have one or two pairs of wings, likewise attached to the thorax. Like woodlice, insects have an exoskeleton, but their segmented body and specific number of legs and antennae set them apart from crustaceans.
Common Misconceptions
The frequent confusion between woodlice and insects arises from several superficial similarities. Both groups are small, segmented arthropods covered by a hard exoskeleton, and they often inhabit similar terrestrial environments. The common names for woodlice, such as “pill bug” or “roly-poly,” further contribute to the misconception that they are a type of insect.
Despite these shared traits, the biological differences are substantial. Woodlice possess seven pairs of legs, while insects consistently have three pairs. Woodlice breathe through gills, necessitating a moist environment, whereas insects utilize a system of internal tubes for respiration. Furthermore, woodlice are not parasitic, unlike some actual lice, which are indeed insects. These distinctions highlight that despite their similar appearance and habitat, woodlice belong to a separate branch of the animal kingdom.