Only Caterpillars Know Some Moth Secrets

by John Rawlins

Moths are everywhere! These nocturnal relatives of day-flying butterflies are found at temperatures above freezing in nearly every terrestrial habitat. In one superfamily aloneùthe Noctuoidea, which includes such popularly known species as tiger moths, gypsy moths, cutworms, moths and owletsùover 7,200 genera and nearly 70,000 species have been described. Numerous species are still undiscovered and the actual number may exceed 100,000.

For many decades scientists have based their study of moths on specimens in the adult stage, for at this stage they are visibly easy to identify, collect and preserve. Unfortunately, the general characteristics of adults, which allow such common functions as flight and feeding, often fail to provide the researcher with sufficient information to determine evolutionary relationships. Moths can actually be understood better in evolutionary terms by examining earlier developmental stagesùsuch as the eggs, pupae and especially the caterpillars. Noctuoid caterpillars, for instance, display consistently the presence of two microscopic hairs on the thorax. Such seemingly trivial traits can be decisive for identification, as well as for recognizing patterns of descent among the bewildering array of features exhibited by caterpillars and adult specimens alike.

This is why since 1980 I have emphasized the need for better caterpillar specimens for research and have developed an unprecedented program of rearing larval stages of all Lepidoptera (moths and butterflies), assisted by fellow curator Chen Young. This effort has produced thousands of caterpillars that are preserved from species in Asia, Africa, tropical America and right here at home.

Caterpillar specimens in museum collections have not usually been accurately identified and adequately preserved. The few specimens available for research are usually in poor condition or tentatively identified. For most genera, especially tropical ones, there are no specimens at all. Hence the birth of the project at the Carnegie Museum of Natural History. Our program uses eggs laid by a single female. Once they're hatched, the moth's immature stages are reared in isolation through pupation, and the specimens are preserved and photographed for later study.

The practical problems of doing this are many. For example, the larval foodplant is usually unknown and must be inferred by ecological associations or educated guessing. Likewise the special techniques for maintaining living cultures are as diverse as the lineages being reared. Despite these practical difficulties, the Carnegie collection of eggs, caterpillars and pupae has become known internationally as an exceptional resource for entomologists.

Unexpected relationships between species have been uncovered when caterpillars were closely examined. In several cases, the features of caterpillars have shown that the moths had been incorrectly classified in distantly related families. The presumed African tiger moths in the genus Carpostalagma, whose larvae and pupae disclose that they are actually owlet moths, are now seen through Carnegie specimens to belong to an entirely different family. In other cases, caterpillars show that genera considered for more than a century to be a close kin are really only distantly related. Caterpillar structures of Old World wasp-moths (tribe Syntominae) reveal that they are not closely related to seemingly similar New World wasp moths (tribe Ctenuchinae). This suggests that the adult similarity is due to convergent evolution of color and behavior to mimic similar kinds of venomous wasps.

Carnegie caterpillars have also provided evidence about well-known North American species. A good example involves pitcher-plant moths in the genus Exyra. Known for a century for its risky habit of feeding only on the leaves of carnivorous pitcher-plants (Sarracenia), these moths were considered closely related to small owlet moths called "acontines." But after rearing a species of Exyra, I was amazed to observe features unique to caterpillars of another well-studied group of noctuids (Plusiinae) that includes common plant pests such as the celery looper. Because of this evidence, entomologists now accept a connection that would have been considered preposterous by experts a century ago.

Studies now show the importance of researching all of the developmental stages for phylogenetic investigations. Recently I summarized the current knowledge of noctuoid caterpillars based mostly on Carnegie specimens. In collaboration with Ian Kitching of the British Museum, a specialist in adult moths, we have hypothesized a novel classification for Noctuoidea that incorporates available information from all life stages. A definitive treatment may be impossible for the thousands of genera, but our efforts will provoke testing by others, and will encourage them to study caterpillars as well as moths.

The humble caterpillar crossing your path deserves a close look, for it may reveal secrets about the evolution of moths.

John Rawlins is associate curator of Invertebrate Zoology, Carnegie Museum of Natural History.