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Childhood's Creatures
| Article
# : |
12161 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1994 |
1,768 Words |
| Author
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Dwight G. Smith
Dwight G. Smith is professor and chairman of the biology
department at Southern Connecticut State University in New
Haven. His latest book, Plants, was released this summer by
Pearson Publishing Company of Boston. |
Somehow children always seem to notice the little things in life, and the little creatures as well. Probably every child who has turned over an old piece of cardboard or shuffled through a pile of leaves has found pill bugs scurrying for cover as fast as their multiple legs allowed. And certainly every child who has surprised these bugs has been endlessly fascinated by the way in which they curl tightly into little balls when lightly poked or prodded.
These timeless little creatures from childhood are not bugs at all but are really land-dwelling crustaceans whose relatives include lobsters, shrimp, crayfish, and crabs. They and their nearest kin--the sow bugs, beach lice, and fish lice--belong to a distinctive animal group called the isopods, referring to the fact that all of their legs are the same size. Pill bugs can be distinguished from other isopods by their habit of rolling into protective pill-like balls when disturbed.
Although seemingly simple animals, pill bugs are actually the end product of millions of years of changes in which the basic crustacean bodies of their remote aquatic ancestors were remodeled and reshaped for a terrestrial existence. During this long sea-to-land transition, ancestral gills were shed for a more efficient system of breathing tubes for air, and the entire body was shielded in a protective, armadillo-like covering of armor. These new body features enabled pill bugs to successfully compete with those other highly developed land-dwelling arthropods, the insects.
Pill bugs are now a common and widespread group of animals, almost worldwide in distribution. They occur in a variety of habitats, from equatorial rain forests to deserts, steppes, tundra, and mountains. Although wide-ranging, almost all pill bugs are small and oval, averaging about half an inch or so in length. Most are colored in drab shades of metallic gray, a pattern well suited for camouflage in their darkened hiding places. Architecturally, the dominating feature of pill bug bodies is their outer covering, or exoskeleton, comprising several layers of protein. The shieldlike body envelops the softer abdomen and seven pairs ofjointed legs. The legs also double as breathing devices; they contain branching air tubes, called trachea, that promote gas exchange. For guidance pill bugs have a small pair of compound eyes, but most rely on chemical sensors for smell and taste that are located in their feet and at the base of their antennae.
The best-known and most frequently encountered pill bug is Armadillidittm vulgare, the common pill
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