“Wood bug” is a common, non-scientific term for small, segmented creatures found in damp places, particularly near decaying wood. The animals most often referred to by this name are terrestrial isopods, which are land-dwelling crustaceans, making them more closely related to shrimp and crabs than to actual insects. Specifically, this colloquial term usually refers to the two most common types: the pill bug and the sow bug.
General Physical Characteristics
Terrestrial isopods share several physical traits that distinguish them from true insects. Their bodies are typically oblong or oval-shaped and possess a noticeably arched or convex profile. The size of an adult usually ranges from about one-quarter to three-quarters of an inch in length, with a coloration that is usually dark gray, brown, or slate blue.
The body is covered by a rigid, segmented exoskeleton that is divided into three main regions: the head, the thorax, and the abdomen. The thorax is the largest section and is composed of seven distinct, overlapping plates. Unlike insects, these creatures possess seven pairs of jointed legs, with one pair attached to each of the seven thoracic segments.
These animals breathe through gills located on the underside of their bodies, a trait that necessitates a constant need for a high-moisture environment. They lack the waxy outer layer that helps insects retain water, which means they are highly susceptible to desiccation. This physiological requirement influences their behavior and where they are most commonly found.
Distinguishing Between Pill Bugs and Sow Bugs
The most reliable way to visually distinguish between the two primary types of “wood bugs” is by observing their body shape and their reaction when disturbed. Pill bugs are famous for their defensive strategy called conglobation, or the ability to roll completely into a tight, nearly perfect sphere. This rolling ability is facilitated by their highly arched, dome-like body structure and the seamless fit of their body segments. When unrolled, the pill bug has a smooth, rounded rear end.
Sow bugs are physically unable to roll into a ball as a defense mechanism. Instead of a smooth, rounded posterior, sow bugs possess two prominent tail-like appendages, known as uropods, that project noticeably from the rear segment of their body. Their body shape is generally flatter and more oval compared to the pill bug’s dome. When a sow bug is disturbed, it will typically scurry away rather than attempt to curl up.
The sow bug’s flatter profile and permanently exposed uropods are the clearest visual identifiers separating it from the pill bug. The pill bug’s ability to completely enclose its vulnerable underside into a “roly-poly” ball is a behavior exclusive to its species.
Common Habitats and Behavior
Both pill bugs and sow bugs rely on moisture and decaying plant matter. They are primarily scavengers, or detritivores, meaning their diet consists mainly of decomposing organic material such as rotting wood, leaf litter, and mulch. This feeding habit makes them beneficial decomposers that play an important role in recycling nutrients back into the soil.
These isopods seek out cool, dark, and damp environments to prevent their gills from drying out. They are often found hidden beneath stones, logs, flowerpots, and any other debris lying on the ground. Their preference for these locations explains why they are frequently encountered when gardening or overturning objects in a yard.
Both sow bugs and pill bugs are nocturnal, meaning they are most active at night when the humidity is higher and the risk of dehydration is lower. If they are found indoors, it is typically an indication of a significant moisture problem, as they will not survive long in a dry environment. They usually enter homes through cracks in the foundation or around doors while seeking shelter.