By Eve Broughton
What animal has a belly pouch for incubating its young, has its lungs on its rear legs, came here from Europe as a stow-away and can roll up into a ball for self-defense?
Yes, it’s that fun little bug (actually crustacean) you played with as a kid, the rollie-pollie, officially known as Armadillidium. There are many other types of woodlice, sow bugs or pill bugs, but only Armadillidium can roll up.
Woodlice are the only successful terrestrial crustaceans. The other members of their order, Isopoda, are marine, two of which live in our coastal waters. Isopods are distantly related to krill and mantis shrimp.
Isopods are rather flat from top to bottom and shaped like a long oval. Their heads sport two pairs of antennae; the larger two are distinctively bent. Two compound eyes and four pairs of mouthparts are also on the head segment. Behind the head are seven large segments each with a pair of walking legs (14 total), and beyond that five small segments with paddle-shaped pleopods, followed by the last segment with uropods, which are sensory and defensive appendages.
Several adaptations have allowed land dwelling. The gill-like appendages of marine isopods have changed into lung-like structures. These “lungs” (pleopods) must be kept moist for gas exchange, which is why woodlice live in moist sub-surface environments. The pleopods also absorb water vapor from the air and soil. In males, the first two sets of pleopods are long and used to pass sperm to the female’s genital opening at the base of the pleopod.
After mating the female develops a fluid-filled pouch on her underside, the marsupium, in which she lays the fertilized eggs. There they incubate for several days and then hatch into small white juveniles. Each brood may have several hundred eggs and the female may reproduce twice a year from May to September.
The young absorb fluid in the marsupium for a few hours and then break out. At this time they have only six pair of legs; the seventh pair will appear with the first molt. Molting begins with the rear segments. Several days later the front segments molt. They will molt every few weeks for their remaining lives, usually two-to-three years.
What else is exciting about the woodlouse? Its metabolic rate is dependent on the surrounding temperature, so they cannot tolerate freezing temperatures. They excrete ammonia through their exoskeleton and can drink water with both their mouthparts and their anus. The oxygen-carrying molecule in their blood is hemocyan, which uses copper rather than iron. This makes their blood blue. The woodlouse excretes the copper, so eats its feces to replenish the supply. And when ill with an iridovirus, the woodlouse turns blue.
Woodlice feed on decaying vegetation, wood, leaf litter, moss, algae and bark. Their excretions are then further decomposed by fungi, bacteria and protozoa into essential soil nutrients such as phosphates and nitrates. They should be considered a friend to the farmer.
Written by Eve Broughton. Eve lives in Whitethorn and was educated at UC Berkeley.
This column is sponsored by the Lost Coast Interpretive Association as part of their environmental education mission. For information about the organization, visit facebook or the website, lostcoast.org. Interested in submitting an article? Email firstname.lastname@example.org.
The Armadillidium, better known as the rollie-pollie.