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.
