JUNE
2008

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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06-26-08

Download the audio of On Six Legs: MP3, WMV.

Caterpillars on Parade

Late spring and early summer could be called caterpillar time. At least in most parts of the world, that is when human and caterpillar encounters are most likely to occur.

Caterpillars are immature forms of the insects that in their adult stage are called moths and butterflies. Humans frequently encounter caterpillars because many of these insect larvae feed on leaves out in the open where we can see them.

There are many sizes and shapes of caterpillars, but the most common form has a conspicuous head and six legs on the thorax, just as do adult insects. But many caterpillars also have five pairs of leg-like structures on the abdomen. Scientists call these fleshy protrusions prolegs. The prolegs have hooks at the end that caterpillars use to hang on to soft material, such as leaves, stems and, if given the opportunity, your finger or shirt.

Scientists have identified more than 11,000 different species of Lepidoptera, the insect order that includes the moths and butterflies. That means there are lots of different kinds of caterpillars and many are difficult, if not impossible, to tell apart.

A number of caterpillars feed on plants that humans call crops. We dub such caterpillars pests and try to get rid of them. Since most caterpillars are not pests, accurate identification of specific caterpillars has been an important aspect of insect control decisions.

Historically, most insects were classified according to adult forms. In 1948 entomologist Alvah Peterson published the first scientific key for use in identifying caterpillars. This was a technical key based to a great extent on chaetotaxy, the arrangement of spines on the body of the caterpillar. Such an approach is useful to insect scientists who worry about things like spines on caterpillars, but it is not of much value to other people.

Almost everyone who sees a caterpillar seems to want to know what it is, even if the insect is not causing damage to a crop or a cherished plant in the flower garden. For this reason, there have been popular guides produced for use in identifying caterpillars. There are also Web sites designed to help put a name on caterpillars.

Most popular guides to caterpillar identification begin with questions like, "Was the caterpillar feeding inside the plant or on the leaves?" The inside feeders are generally known as borers and include such things as European corn borers and squash vine borers. Most borers are out of sight and, therefore, out of mind to most people, unless the borer is a pest species.

On the other hand, most leaf-feeding caterpillars are out there in plain sight as they do what they do best -- eat! A few of the leaf-feeding caterpillars build webs or tents or tie leaves together. These caterpillars produce silk, which is used to build such structures and, especially in moths, is used to produce a structure called a cocoon, which protects the pupal stage of the insect. Such is the case with the silkworm moth, the insect that is the source for the silk that humankind has learned to fashion into thread for making cloth.

Other groupings for caterpillars are based on how the caterpillar looks. For instance, some caterpillars have a single horn protruding from the rear end. These caterpillars are appropriately called hornworms. These include the tomato and tobacco hornworms, which are common pests of tomato and tobacco. In the adult stage, hornworms are moths called hawk moths or hummingbird moths. These moths feed on nectar from flowers and delight gardeners everywhere with their aerial antics as they feed.

Some caterpillars are hairy or fuzzy. The wooly bear caterpillar, widely purported to be able to predict the severity of an upcoming winter, is one of these. Other caterpillars have a body covering that could be called bristly, sort of like a two-to-three-day growth of hair on a human. Other caterpillars are covered with even coarser growths called spines. Watch out for such caterpillars. The bristles and spines might produce a sting if touched.

Many caterpillars have smooth skin, especially those that become butterflies. Some have smooth skin that is adorned with knobs; others have fleshy protrusions or tufts of hair. Grouping caterpillars in broad categories such as smooth skin or hairy is a first step in putting a name on the next caterpillar you see on parade!

 

Writer: Tom Turpin
Editor: Olivia Maddox