Of Booger Beetles and Fecal Shields
Sometimes topics for VPM’s “What’s Bugging You?” radio show were inspired by comments and suggestions from listeners. Case in point, a neighbor of entomologist Dr. Art Evans shared a photo of a strange looking beetle resembling a crusty bit of mucus. Learn more about the unusual defense strategies employed by the clavate tortoise beetle, Plagiometriona clavata (Fabricius) with Dr. Evans and VPM radio producer Steve Clark.
The clavate tortoise beetle, Plagiometriona clavata, was first described by Danish zoologist Johan Christian Fabricius in 1798. It is recorded from much of North America, save for the Pacific Northwest. Their oval, tortoise-shaped body has thin, broadly expanded margins that are partly translucent. The dorsal surface is contrastingly marked and distinctly bumpy, or tuberculate. In fact, it is the only tuberculate tortoise beetle in the United States. It has been suggested that these beetles resemble a bit of crusty mucous upon first glance. When viewed from the side, clavate tortoise beetles have a cone-like peak on their backs. All of these features combined make them harder to see among their food plants. Both adults and larvae feed on the leaves of solanaceous plants, including eggplants, peppers, and tomatoes.
The spikey green larvae are somewhat flattened. The last abdominal segment bears a “fecal fork” to which the larva attaches its own feces. When carried over its back like an umbrella, this dried mass of waste known as a fecal shield is thought to camouflage the larva as it grazes the leaves of its food plant. Fecal shields afford another kind of security, too. The larvae co-opt the chemical defenses of their food plants for their own protection. By concentrating these potent compounds in feces used to construct their shields, the larvae arm themselves with a powerful feeding deterrent that repels potential predators. Fecal shields also contain bits of caste exoskeletons that are shed by the larvae as they grow. Larvae deftly construct their shields with the aid of a turret-like anus that is both muscular and extendable.
Some species of tortoise beetles are iridescent thanks to layered nanostructures in their exoskeletons that reflect intense colors that shift with relation to the quality of light or changing angles of view. Others have the ability to change their colors. For example, the shiny metallic surface of the aptly-named golden tortoise beetle, Charidotella sexpunctata, is caused by sunlight reflecting off pockets of liquid pigment within the exoskeletal layers. In response to external stresses, they can temporarily change their colors from brilliant gold to a shiny red or golden orange, sometimes with black spots, by moving pigment through microscopic ducts inside the exoskeleton.
In the world of insects, looks can be deceiving. Whether you are working in your garden, walking in a city park, or hiking along a woodland trail, take a moment to appreciate the smaller things in life, especially if it looks like a booger with six legs!
While we are on the topic of things that look like they are something they’re not, check out this story on fecal mimicry.