“Candy” Scales Discovered in Dozens of Moth Species
By Leslie Mertz
While studying moths in the rainforests of Costa Rica and Peru, entomologists noticed odd-looking groups of scales at some of the males’ scent organs. They took a closer look with a scanning electron microscope and were surprised to see scales that were unlike anything they had seen before. “These scales remind me of a piece of candy wrapped in cellophane,” described Michael Boppré, who is the lead author on a new Journal of Insect Science paper that details the discovery. The finding provides insight into—and leads to new questions about—moth and butterfly (lepidopteran) communication and scale development.
Typical scales in butterflies and moths are complicated layered structures with posts called trabeculae, crossribs, and blank spaces called windows. The new scales, however, had none of those features, he said. Instead, they looked like thin, flexible envelopes (the “cellophane”) that were filled with viscous material that the researchers now believe is some type of sugar (the “candy”). Once the moth dies, the material crystallizes, and since the researchers first noticed them on dead specimens, they dubbed them crystal scales or crystal macrosetae.
The unusual configuration of the crystal scales appears to be due to arrested development occurring in the pupal stage, Boppré said. Typical scales develop completely and form structures that reflect or absorb light, and cause the striking colors and patterns of their wings and bodies, but the crystal scales are different. Although they have the universal scale features of sockets and stalk-like petioles to help hold scales in place, crystal scales “are not differentiated, so it looks as if the moth produces the scales but stops their further development into typical scales, and instead uses them as a bag for storing chemicals,” he said. “Finding these scales creates curiosity about the ontogeny of lepidopteran scales development during the pupal stage.” He speculated that these chemicals are a type of sugar because the scales not only “cause a sweet sensation on the human tongue,” but also are highly hygroscopic (attract considerable amounts of water), which is also a characteristic of sugars.
The researchers spotted the crystal scales during a study of communication mechanisms in male tiger moths in the subfamily Arctiinae. Specifically, they were interested in the role of the androconial organs, which are common to many male moths and butterflies. These organs are everted during courtship—then resembling hairy tubes or spheres—and emit pheromones that stimulate females. In all, the researchers identified the crystal scales at or near the androconial organs in moths from 80 species representing more than 30 genera from five different subtribes, so the crystal scales weren’t oddities restricted to a few closely related species, Boppré said. “That was another striking find.” Other researchers might have seen crystal scales in other moths, he added, but they haven’t pursued studies of them.
The best way to learn more about the crystal scales is to study them in live moths, and to do that, Boppré hopes to one day rear the moths in the lab. That, however, is not an easy task. “The general rule in the tropics is that you have a very high diversity but a very low abundance of species,” he said. In other words, researchers can find a few individuals from many species, but not many individuals from any one species. In addition, while Boppré and his team have been able to attract males to their lights and traps, they rarely find females of the respective species. On top of that, little if any natural-history information is available on any of the species, and that includes the identity of the larval host plants. So even if the researchers were able to collect females and receive eggs, they couldn’t raise the caterpillars.
Nonetheless, Boppré remains optimistic. He remarked, “Eventually one day, by chance, we hope to find the foundation for breeding one or two or three species of arctiine moths with these sugary scales, and this will permit detailed experimental studies on a variety of questions that are perfectly obvious but only investigatable with bred specimens.”
He added, “You need to have patience and be resistant to frustration to work in the tropics. It is complicated and difficult, but that’s the reality. And, of course, it is absolutely rewarding.”
Journal of Insect Science
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.