- The Washington Times - Wednesday, February 9, 2005

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.

“One of the features that characterizes insects and arthropods is that its exoskeleton limits growth. Periodically, it needs to be shed,” he says of the process called molting.

The exoskeleton initially is soft and pliable, leaving the insect vulnerable to attack.

“Typically, they don’t molt out on the middle of the floor; they seek out a protective place. At night, they’ll do it [because] fewer predators are around.”

The fully grown exoskeleton helps protect an arthropod from attack, but it doesn’t completely heal if damaged.

“They don’t get the same kind of tissue regeneration,” Mr. Fell says.

All arthropods go through the molting process as they grow to adulthood, though some perform an abbreviated version of the maturing transformation.

Incomplete metamorphosis includes the egg, nymph and adult stages of development, a pattern followed by the stink bug and other insects. A complete metamorphosis, which creatures including the butterfly, ant and beetle undergo, involves four main stages — egg, larva, pupa and adult.

The overwhelming majority of arthropods stop molting when they reach their adult stage. One exception, Ms. Kester says, is the mayfly, which continues molting for its entire life span, which can be as short as one day.

Gary Hevel, public information officer with the Smithsonian’s department of entomology, says most modern textbooks contend that exoskeletons contain three or four layers, depending on how researchers break them down. All reference three main layers, the epicuticle, endocuticle and exocuticle.

The epicuticle features a wax surface that waterproofs the creatures, Mr. Hevel says.

The cuticle layers, whether considered three or four distinct parts, are made of hard sugars, some of which can be particularly strong, depending on the amount of proteins packed into them.

Insects may be small, but their muscular structures and exoskeletons can make them formidable prey to others within their scale.

Not all exoskeletons offer an armorlike defense mechanism, however.

Mr. Havel says caterpillars, which have very soft exoskeletons, contain an average of 2,000 muscles that attach to the skeleton’s interior to help the segmented creatures move.

University of Maryland entomology professor Raymond St. Leger says exoskeletons are a major reason so many insect species exist — more than 500,000 types of beetles alone.

“[The exoskeleton] provides a very light skeleton of great strength compared to the internal skeleton we have,” Mr. St. Leger says. “It’s a remarkable achievement.”

The material’s strength, in part, is thanks to its composite nature, which Mr. St. Leger compares to plywood or concrete.

Exoskeletons also may have once given way to an insect’s appendages, such as their wings, Mr. Hevel says. Scientists speculate that some insects’ wings began as flaps in their exoskeletons, which soon were joined by similar flaps in that area, which developed later into full-fledged wings. The wings provided the insects the ability to fly to find food and mates and to escape dangers.

A select group of arthropods use their exoskeletons to detect heat sources. Jewel beetles, for example, are believed to detect heat through their outer shells. The creatures often can be found near the site of forest fires, Mr. Hevel says, as female jewel beetles like to lay their eggs in burned tree bark.

The study of exoskeletons doesn’t just let scientists understand how arthropods tick; it also could benefit mankind some day.

The field of biomimetics, otherwise called bio-inspired technology, looks at how replicating what occurs naturally can be exploited for human benefit.

Mr. St. Leger says work already has begun that was inspired by insects’ exoskeletons.

Scientists have created sturdy miniaturized robots, shaped not unlike cockroaches, that can crawl into nooks and crannies that otherwise would be hard to reach. The devices often come equipped with small cameras to film from vantage points otherwise impossible.

“We look at insects, and we use them as an inspiration,” Mr. St. Leger says.

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