Isopods

Woodlice - pill bugs and sow bugs - are small earth-colored creatures generally encountered on the sidewalk or in the garden. Pill bugs are often called roly-polies because of their habit of curling up into a ball when disturbed. Sow bugs look similar but do not roll up. Most people would consider these creatures, if not insects, certainly by the more generic term, ``bugs''. While the woodlice belong in the same phylum as insects, Arthropoda, biologists have placed them in a different class, Crustacea - the same class that contains the crayfish, lobsters and crabs. Sow bugs and pill bugs have seven pairs of legs and are terrestrial members of the order Isopoda. (The Pill Millipedes resemble the pill bugs even curling into a ball when disturbed but have more than seven pairs of legs.)

In general, pill bugs and sow bugs are omnivorous, feeding on young plants of all kinds, fungi, decaying plant material, and detritus. Most are not well adapted to the land and prefer to remain in damp areas so that they do not become dehydrated. Their distribution and activity is affected by the moisture content and temperature of the soil and the air. (Lawn watering saturates the soil forcing them to the surface.) Water is acquired mainly from the food consumed and absorption through the surface layers of the body.

Examination shows that they have a segmented armour shell similar in appearance to that of an armadillo. As in all arthropods, growth proceeds in a series of molts during which the exterior skin is shed. For these animals however, the molt occurs in two stages - the back half goes first and sometime later, perhaps several days, the front half is shed. (Hence, molting woodlice should, for a short period, be bigger at the back than at the front but I don't know how noticeable this difference is.) Just after molting, the new lighter-colored cuticle is soft, shiny and sticky. During the next few hours growth takes place until the new cuticle hardens enough that it can no longer expand. Females have special molts in which a marsupium or brood pouch forms. The skin which is shed is normally eaten so that valuable moisture and minerals are not lost.

In the ``Insects of the Los Angeles Basin (1993)'', Charles Hogue indicates that the most common local species is the Common Pill Bug, Armadillidium vulgare. This introduced species is probably the commonest one in San Diego county as well. The female produces 1-3 broods per year after she is 2 years old. Slightly more than 100 eggs are carried in the closed marsupium from which 30-60 larvae or mancas emerge. As usual the exact numbers depend on geographical location, season, female size and other factors. The maximum lifespan of this species is between 3 and 4 years. (Of the isopod species studied, the maximum is around 9 years.) In general, mortality is determined mainly by environmental factors such as droughts and floods.

A related group of isopods are the rock slaters. These marine isopods feed mainly on seaweed and can be found along the rocky shoreline of Gunpowder Point.

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201