Skeletons on the outside

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It isn't easy preparing arthropods for display at the Smithsonian's National Museum of Natural History.

Entomologists can stick a pin through the exoskeletons, the rigid outer surface of all arthropods, of some the creatures to be displayed with little trouble. It isn't so easy with those whose outer casings are either so soft they must be soaked in alcohol to stiffen them or so hard they require a pair of forceps to force the pins through.

Arthropods, invertebrates with segmented bodies and hollow, jointed legs, all have exoskeletons of various densities that protect and support their soft tissue.

These rigid shells are made chiefly of chitin, a polysaccharide similar to cellulose. Fibrous chitin strands are laid out in different directions, which helps the casings distribute stress more evenly.

Karen Kester, associate professor of biology at Virginia Commonwealth University in Richmond, describes an exoskeleton as nature's way of keeping away predators and environmental foes such as pathogens.

The outer shell also keeps vital moisture from escaping, a characteristic crucial to desert-dwelling creatures such as the darkling beetle.

"It prevents the insect from drying out," Ms. Kester says. "Some desert creatures can go very long times under very dry conditions."

An insect's stiff shell is segmented to allow the creature to move, says Richard Fell, professor of entomology at Virginia Tech in Blacksburg.

"In simple terms, it's a series of hardened plates stuck together. For flexibility, you need some areas that are membranous," Mr. Fell says.

The outer shell itself also starts out far less rigid than it ends up as part of the arthropod's growth process, he says.