Banner by Shirin Kaye

It's a Bug's World After All

By Shirin Kaye

Thirty thousand mahogany drawers. Fourteen million “bugs.” All labeled and pinned in neat rows alongside fellow members of their species. This collection occupies the third floor of the Carnegie Museum of Natural History (CMNH), right next door to the University of Pittsburgh’s main campus. The displays include specimens ranging in size from smaller-than-my-pinky-nail to looks-like-it-could-eat-my-toe-for-snack—and ranging in color from soil-brown to iridescent. The amount and variety of “bugs,” both in the public exhibit and private labs, is awe-inspiring. But what exactly is a “bug”?

While “bug” refers colloquially to all arthropods (as in the exhibit and this article), an organism must have several characteristics to technically be one. In the CMNH’s Invertebrate Zoology department, the staff entomologists specialize in arthropods—such as beetles, spiders, and shrimp. Their library of specimens is organized by Linnaean ranks—nested categories of organisms in the animal kingdom. Invertebrates are organisms with exoskeletons. Arthropoda—one of the 16 phyla and over 85% of the animal kingdom—consists of invertebrates and includes classes such as Chilopoda, Crustacea, and Hexapoda. Arthropods are characterized by their segmented bodies, jointed legs, and hard exoskeletons made of chitin (the main structural molecule). Insects fall within Hexapoda and notably have six legs and three distinct body segments. True bugs—like bed bugs and aphids—are insects whose mouths are designed for the piercing and sucking necessary for them to feed.

An arthropod by which I’ve always been fascinated is the centipede, an elongated organism with two legs per body segment that I’ve often seen scurrying around parks—or in my house. The class Chilopoda contains thousands of centipede species, whose members can range from one-half to over 12 inches long, with 15 to 191 pairs of legs—though always an odd number. These little guys tend to live in humid environments, because their bodies cannot retain moisture, and they can live long—up to 10-plus years! Centipedes predate on a variety of smaller invertebrates and, in turn, are preyed upon by larger organisms. Their first pair of legs are forcipules, venomous claws used on prey before eating. In contrast, millipedes have toxins inside their bodies and bright colors to deter predators. These many-legged creatures have smaller mouths, rounder bodies, and can continuously grow more body segments and legs throughout their lives. Millipedes are part of the class Diplopoda and, despite their name, only one species has thus far been found to have over 1,000 legs: Eumillipes persephone, deep in the aquifers of Western Australia.

The CMNH’s collection includes arthropods from six continents and over 100 years, with its oldest specimens being the butterflies. From speaking with (or bugging!) staff members in the Bug Hall, I learned what entomologists do daily. The main role of a natural history museum is to store specimens for supporting research and identification. The collection contains holotypes (the main, defining example of a species) and paratypes (contemporaries of the holotype, kept as backups) that may be referenced by other entomologists and used in teaching. Much effort also goes into maintaining the collection; the museum staff must prevent pests, like carpet beetles, from eating the specimens’ exoskeletons. Additionally, new species are constantly discovered and must be cataloged. Such attention also helps track invasive species and notice structural, behavioral, and genetic changes that occur within species over time. At large, entomologists can work in agricultural pest management (controlling pests without pesticides), biosecurity (detecting invasive species at ports of entry and preventing their spread), and academic research. The CMNH performs contract work in taxonomy and biosecurity for the US Department of Agriculture and Forest Service.

I had the privilege of speaking with Dr. Ainsley Seago, associate curator for Invertebrate Zoology at the CMNH, about her work. She specializes in beetles and conducts research concerning biomimetic engineering. The fields of biomechanics and physics coincide with entomology when scientists investigate “ways to take insect technology and adapt it for human use.” Such projects are funded by the U.S. military, the Department of Defense, and private companies. For example, makech beetles are so tough that they could survive being run over by a truck; it is no coincidence that their exoskeletons have the “same structure as the composite materials used to make the outside of airplanes: super strong, super light.”

Beetles constitute Coleoptera, an order within Insecta that includes ladybugs, fireflies, and scarab beetles. There are about 300,000 known species of beetles, all characterized by hard sheaths that protect the wings beneath. A characteristic of beetles that is of particular interest to Seago is their structural color. The colors we perceive bugs to be can result from chemical pigments or the way light reflects off the microscopic crystals of their exoskeletons. Multiple structures can give rise to the latter type of colors: thin stacked layers of chitin, 3D photonic crystals, and diffraction gratings. For example, Seago pointed out a set of light green scarab beetles. When I viewed them through 3D movie glasses and closed one eye at a time, the color alternated between green and brown! The green is a structural color, resulting from constructive interference: “If layers of chitin are about a quarter of any wavelength of visible light apart, they will reflect that wavelength of light really brightly. [In this case,] the layers are laid down like a spiral, so the green light is circularly polarized. The light is twisting as it moves through space. We can switch that off with a circularly-polarized lens in the opposite direction, [leaving the] brown, unpolarized melanin color.” Yet unanswered questions include, how are these structural colors made? What evolutionary advantage do such complex colorings give bugs? Insights about structural color gleaned from insects can be applied in research to engineer photonic crystals or develop alternatives to plastic from chitin.

“There’s a beetle technology for everything: there’s chemistry, there’s physics, there are beetles that can make heat, beetles that can pull water vapor out of the air and suck the liquid water into their mouths, beetles that can change colors reversibly,” Seago enumerated while putting away display specimens, careful to handle them by the tops of the pins that pierce each specimen. After presenting lots of fascinating information, Seago remarked, “that’s the thing that’s so joyful to me about being an entomologist: there’s just so much to learn.”