The rollie pollie, also known as a pill bug or woodlouse, is a terrestrial crustacean belonging to the suborder Oniscidea, making it more closely related to shrimp and crabs than to insects. These small decomposers play an important role in the environment, primarily by consuming decaying organic matter. While they are often observed simply rolling into a defensive ball, distinguishing the sex of an individual requires a close, careful inspection. Gender identification relies on observing specific, permanent anatomical features located on the underside of the creature, which often requires a magnifying glass and patience.
Essential Anatomical Indicators
Determining a pill bug’s gender involves viewing the ventral, or underside, surface of the animal. This requires gently turning the isopod over to examine the structures near the abdomen and thorax. The appendages on the abdominal segments, known as pleopods, are the primary markers for male identification. In males, the first two pairs of pleopods are distinctly modified into elongated, pointed reproductive structures called gonopods.
These male structures are typically angled and noticeably thinner than the corresponding plates on a female. They function to transfer sperm to the female’s genital openings during mating. If an individual clearly exhibits these two pairs of slender, specialized projections on the underside of the abdomen, it can be identified as a male.
Conversely, the female’s internal anatomy is marked by the presence of thin, membranous plates on the underside of the thorax. These plates are known as oostegites, and they are present even when the female is not actively breeding. The female’s pleopods, located further back on the abdomen, remain relatively flat and unmodified, often appearing rectangular.
While the appearance of the pouch itself changes with the breeding cycle, the underlying plates are always present. By focusing on the shape of the pleopods and the presence of the oostegite plates on the thorax, one can reliably determine gender outside of the breeding season.
Identifying the Female Brood Pouch
The most visually obvious, though temporary, indicator of a mature female is the presence of the marsupium. This structure is formed by the overlapping oostegite plates. The female releases fertilized eggs into this pouch, where they are protected and incubated for several weeks.
The visibility of the marsupium is directly tied to the female’s reproductive cycle. When the pouch is full of eggs or newly hatched young, called mancae, the female’s underside will appear visibly swollen or bulging. Broods can be substantial, with females carrying anywhere from seven to over 200 eggs at a time.
The presence of the marsupium reinforces the pill bug’s classification as a crustacean, as this pouch maintains a moist, aquatic-like environment for the developing embryos. The pouch serves a crucial function by protecting the vulnerable young from dehydration and predators. Identifying this bulging structure is the quickest way to confirm a female during the breeding season.
Size, Color, and Other Unreliable Markers
Many people assume that size or coloration can determine a pill bug’s gender, but these characteristics are generally unreliable for accurate identification. While it is true that in some species, adult males may be slightly longer or thinner, and females wider to accommodate the marsupium, this is not a consistent rule. Overall size is heavily influenced by the age of the individual, the specific species, and local environmental conditions, making it an inconsistent metric.
Similarly, differences in external color, patterns, or overall darkness are typically linked to genetics or the timing of a recent molt, not to gender. Though rare exceptions exist where a specific species exhibits a slight color difference between sexes, relying on these visual cues will often lead to misidentification. The distinctive ability to roll into a tight sphere, earning them the nickname “roly-poly,” is a trait of the Armadillidium genus (pill bugs) and is present in both males and females.
Behavioral observations, such as how quickly an individual moves or its tendency to seek shelter, are primarily driven by the need to maintain body moisture, as they breathe through gill-like structures. These behaviors fluctuate with humidity and temperature, not with the individual’s sex. For a definitive gender determination, visual inspection of the specialized ventral appendages remains the only reliable method.