Roly-polies, also called pill bugs or woodlice, are common backyard residents known for their ability to curl into a tight ball. These creatures are not insects; they are terrestrial crustaceans, making them closer relatives to shrimp and crabs. Like their marine cousins, roly-polies have a unique reproductive process. This process involves incubating their developing young internally in a protective structure, rather than laying traditional, exposed eggs.
The Specialized Brood Pouch (Marsupium)
The female roly-poly uses a specialized structure called a marsupium for reproduction, which is often mistaken for laid eggs. This brood pouch is located on the underside of her body, covering the thorax. It is formed by overlapping plates, known as oostegites, attached near the bases of the first five pairs of legs.
The marsupium functions as a humid, protected incubation chamber for the fertilized embryos. It is filled with a specialized fluid that provides the developing young with water, oxygen, and nutrients. Since roly-polies require constant moisture to breathe, the marsupium is an adaptation that allows them to reproduce entirely on land.
When the female is carrying a brood, the marsupium becomes noticeably swollen and may appear slightly distended. A gravid female may show a translucent, yellowish tint to the pouch, signaling the developing embryos inside. The female carries this protective structure for several weeks, providing continuous maternal care until the young are ready to emerge. A single female can produce multiple broods in a season, with clutch sizes ranging up to 200 embryos.
Appearance of the Developing Embryos
The actual “eggs” are the developing embryos contained entirely within the fluid-filled marsupium. These embryos are small, spherical structures, pale in color, often described as white or translucent yellow. They are incredibly small, reaching a maximum diameter of about 0.7 millimeters at full development.
The embryos are densely packed and remain immobile throughout the approximately three to four weeks of embryonic development. They are not visible externally unless the female is closely examined or the pouch is ruptured. The embryos rely completely on the nourishing, milk-like fluid secreted by the mother for sustenance and hydration.
Emergence of the Young
After the initial incubation period, the embryos hatch within the marsupium, transitioning into the first juvenile stage known as the manca. These manca are miniature versions of the adult, measuring only about two millimeters in length and appearing pale and whitish.
A distinguishing feature of the newly hatched manca is that they possess only six pairs of legs, lacking the seventh pair found on adults. They remain in the mother’s pouch for an additional one to two weeks, continuing to receive nourishment before venturing out. The mother gives birth to fully formed, walking miniatures.
The young manca undergo their first molt shortly after exiting the pouch, which is when they gain their seventh pair of legs. They continue to molt several times as they grow, gradually darkening in color and increasing in size. This process lasts until they reach full maturity.