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Slaters Pillbugs and Woodlice: Isopoda

Behavior And Reproduction

Terrestrial isopods usually come out at night to feed. During the heat of the day, they instinctively hide in dark places beneath rocks, logs, and leaf litter where there is life-sustaining moisture. Under hot, dry conditions they release odors that are attractive to other isopods of the same species. They pile on one another as a defense of against losing precious body moisture. Some desert species actually live in family groups, with adults caring for their young in burrows throughout most of the summer. Thirsty pillbugs can take up water through their uropods and channel it through grooves along the sides of their bodies to the mouth.

The brownish and gray colors help terrestrial isopods to hide from predators. They also have special glands in their thorax that produce an unpleasant odor. The European pillbug has a marking similar to that of the European black widow spider. Predators usually avoid this venomous spider. The similarly marked isopod fools potential predators into thinking that it is also a dangerous spider. Pillbugs can also protect themselves from predators and from drying out by rolling up into a ball to protect their delicate undersides.

Like all crustaceans, isopods must molt, or shed their exoskeletons, to grow. Isopods are unusual in that they molt one half at a time. The rear half of the body molts first and is followed two or three days later by the front half. It is only during this time that the eggs of the mature female can be fertilized. Both males and females are usually required for reproduction. But in some parasitic species, young adults start out as males and later become females.


The Socorro isopod, Thermosphaeroma

thermophilum, is an endangered freshwater species that resembles a pillbug. They are only found in Sedillo Spring at an abandoned spa near Socorro, New Mexico. They disappeared in the wild in 1988 when water stopped flowing to the spa. Thanks to a colony at the University of New Mexico, the species survived. Small populations exist now only under artificial conditions in four laboratories and the spa.

Males climb on the backs of females to mate. The males curl the abdomen around so that the underside comes into contact with hers. Males use their pleopods to transfer the sperm to the genital openings of the females. After mating the female releases a dozen to several hundred eggs into the marsupium. They will remain there from eight to twelve weeks. Depending on the species, one or two broods are produced each year. The pale juveniles leave the marsupium and resemble the adults. However, they lack the last pair of thoracic legs, or pereopods. The juveniles become larger and darker with each molt, eventually gaining the full number of limbs.

Most isopods live one or two years, but some may live as long as five. The longest-lived species, Armadillo officinalis, is known to live up to nine years.

Additional topics

Animal Life ResourceMollusks, Crustaceans, and Related SpeciesSlaters Pillbugs and Woodlice: Isopoda - Physical Characteristics, Habitat, Diet, Behavior And Reproduction, Conservation Status, Common Pill Woodlouse (armadillidium Vulgare): Species Accounts - GEOGRAPHIC RANGE, SLATERS PILLBUGS WOODLICE AND PEOPLE