The roly-poly, also known as a pill bug or woodlouse, is a common terrestrial crustacean found under rocks and logs. Unlike insects, this animal belongs to the order Isopoda, sharing a lineage with marine creatures like shrimp and crabs. Identifying a female roly-poly that is “pregnant,” or gravid, requires understanding its unique reproductive process and recognizing specific visual changes.
Understanding the Marsupial Pouch
The female roly-poly utilizes a specialized structure for reproduction called the marsupium. This fluid-filled sac is located on the underside of her body, situated between the first five pairs of legs. The pouch is created by overlapping plates known as oostegites, which extend from the base of her legs.
Once fertilized, the female deposits her eggs directly into the marsupium, where they remain until they hatch. This structure provides a protected, moist environment for the developing brood, which can number from a few dozen up to 200 eggs. The eggs and newly hatched young are sustained by a marsupial liquid while they incubate for several weeks.
Visual Identification of Gravidity
The most reliable sign of a gravid female is a noticeable change in the shape of her underside and overall body profile. When the marsupium is full of developing eggs, the area between her legs becomes significantly swollen and distended. The typically flat belly plates will appear puffy, giving the female a visibly plump look compared to a male or non-gravid female.
Observing the female from the underside may reveal the eggs or newly hatched young through the thin membrane of the pouch. This visible mass often appears as a pale, whitish, or yellowish cluster of small specks near the center of her body. This cluster makes the area appear much lighter or more opaque than the surrounding segments. A female carrying a brood may also move slower due to the weight and bulk of the fully formed marsupium.
Appearance After Hatching
The eggs hatch inside the marsupium into miniature versions of the adult, known as mancae. These young typically remain within the protective pouch for a few days or weeks before departing. This period allows them to continue their early development in a secure, high-humidity environment.
Once the mancae leave the marsupium, the female’s underside changes appearance rapidly. The oostegites retract and flatten, causing the distended area to shrink back to its normal size. The female may appear temporarily thinner after releasing her brood. She will then be ready for a subsequent mating and brooding cycle, potentially producing up to three broods annually.