SOME CATERPILLARS ARE SCAVENGERS

by Whit Gibbons

July 15, 2007

Imagine an animal that depends on finding the carcass of a species on the endangered species list before laying its eggs. The gopher tortoise moth does just that, and its caterpillars dine strictly on the material that holds the shell of the gopher tortoise intact.

The gopher tortoise is federally protected in its western range of Alabama, Mississippi, and Louisiana. In Georgia, South Carolina, and Florida, the tortoise is only afforded state protection but has been proposed for federal listing. The gopher tortoise is the largest burrowing reptile in the Southeast, making its winter and nighttime home by constructing underground tunnels up to 10-feet deep and 30 feet long. The gopher tortoise moth, though relatively poorly known, is an equally fascinating species as noted by Mark and Nancy Deyrup of the Archbold Biological Station in Florida.

Lepidopterists (people interested in butterflies and moths) work with some of the most intriguing animals in the world. Some focus on familiar butterflies such as the elegant swallowtails, the nomadic monarchs, and the little blue, white, or yellow varieties that flit across the landscape. Although typically more attractive to us, butterflies don't hold a candle to the moths in terms of abundance. For example, butterflies have fewer than 20,000 species worldwide compared to more than 140,000 species of moths. The more than 11,000 kinds of U.S. moths outnumber the U.S. butterflies by more than 10 to 1.

The most obvious lepidopterans are butterflies hovering around pretty flowers on sunny days and moths practicing suicide missions around outdoor lights at night where hungry spiders, bats, and treefrogs forage. But their caterpillars hold the most unusual and bizarre secrets. Moth caterpillar lifestyles include those that protect themselves with venomous bristles (urticating hairs). Some caterpillars have dramatic, scary looking eyespots on their front or back end that might keep a bird from trying to make a meal of it. Monarch butterfly caterpillars feed on milkweeds and retain the toxic alkaloids of the plant in their own bodies. Thus, both monarch caterpillars and adults are protected from potential bird predators by being poisonous.

Although over 90% of caterpillars are herbivores, the diversity of plants eaten and the ways in which they feed seem near endless. A small proportion of the world's caterpillars are carnivorous, subduing and eating other animals, including flies, spiders, and ants. Some even eat other caterpillars, and a Hawaiian species eats snails after capturing them in a silk trap. But aside from herbivory, the most common caterpillar feeding category is that of the scavengers that make a living by eating dead animals. The gopher tortoise moth qualifies as a scavenger specialist, along with more infamous members of the same family, the clothes moths whose caterpillars eat wool and fur.

Opportunities for feeding on dead tortoises may be getting rarer as the big reptiles disappear from more and more areas with development, but the gopher tortoise moth has a plan when it finds one. The female moths are among the last scavengers to arrive at a tortoise shell lying on the ground, the muscles, bone, and major organs already scavenged by flies and beetles. The moths lay their eggs at night on or near the carcass, to which the hatching caterpillars are attracted.

The material of interest to the caterpillars is the protein known as keratin (our fingernails are keratin), which binds the tortoise's large shell plates together. Keratin provides the food for the caterpillars. But what keeps these soft-bodied little potential prey from become meals themselves to predatory invertebrates, such as ants, roaming the woods? Each gopher tortoise caterpillar secretes a protective tube made mostly of silk and covered with grains of sand that is long enough to extend from below the ground to the various seams on the tortoise. The tube protects the creature from predators and buffers it from hot or cold weather as it crawls to one end to eat and to the other to retreat underground.

I said above that lepidopterists have many opportunities for startling discoveries. Wonder what you might find in your own backyard if you look closely enough at the caterpillars or other insects you find?



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