From Turtle Shell to Poop Umbrella, This Is One Unusual Group of Beetles
By Leslie Mertz
Tortoise beetles are a bizarre group. They look somewhat like tiny turtles with a colorful, or in some species translucent, carapace (the upper shell in turtles), and they engage in some unusual behaviors, including carrying an “umbrella” made of their feces.
“I adore these beetles — everything they do is kind of wonderful to me,” said Caroline Chaboo, an entomologist at the University of Nebraska State Museum.
In fact, the direction of her academic career came from a chance encounter with a tortoise beetle. Before beginning her graduate program in 1993 and while working at the American Museum of Natural History, she recalls struggling to settle on one insect group to study because “they are all so cool.” She took a break to visit her parents in her native Trinidad, and just outside their house, she looked under a leaf and “fell in love.”
What stole her heart was a black-speckled, rust-colored tortoise beetle with a highly sculptural carapace. It was a female Acromis spinifex, and she was with her young.
“The mother actually stays with her brood and guards them from the eggs through hatching, all the larval stages, pupation, and until the teneral (just-molted) young adults disperse,” she said.
Another interesting feature of this species is that the males possess huge prongs that they use to fight for females.
“Having such an interesting species right on my doorstep made a big difference in my decision to study tortoise beetles and other members of the leaf beetle family,” she said. “This was something cool and interesting, and it pushed me over the edge in my decision to study this group. I was a lost cause after that.”
One of the most distinctive features of the tortoise beetles is their domed carapace that looks much like a turtle’s upper shell. The beetle’s carapace is a combination of the elytra (hard hindwings) and the pronotum (plate over the thorax). Depending on the species, the carapace may be smooth and round with the seams between elytra and pronotum only barely visible. In other species, projections on the pronotum or elytra may clearly separate them, or the domed carapace may be covered in spines.
Some, like the clavate tortoise beetle (Plagiometriona clavata) of the eastern and southern United States, have a translucent carapace with an opaque pattern within. The clavate tortoise beetle’s pattern looks a bit like a short-necked gingerbread man. Some other tortoise beetles are solidly and vividly colored with combinations of spots, stripes, and metallic hues.
Many also have the ability to change from shiny to matte, or from one color to another in a matter of seconds.
“The color-changing properties appear to be due to the movement of fluid within the cuticle,” Chaboo said. “For instance, there’s a species in Florida called the Geiger tortoise beetle, Eurypepla calochroma, that can do that. If my shadow falls across the beetle, I can see it change colors from a silvery greenish to a full brownish color and back within seconds.”
Chaboo added that it is unclear if the color switch is a response to danger, has significance in attracting mates, or has some other purpose.
Another curious characteristic of tortoise beetles is the “umbrella” that many of the larvae make from feces (or frass).
“It’s a remarkable thing,” said Chaboo. “They stack feces as well as the exuviae, or their cast skins, into what can become an incredibly ornate structure that is built like a ladder: the urogomphi (unusual paired structures that extend from the larva’s rear end) function as an internal ladder, the horizontal ‘steps’ of the ladder are the exuviae, and the feces fills in, often coming off as long branches or as clumps.”
The larva carries around the structure like an umbrella over the body through all of its instars, and “if the exuviae are there, you can count how many shed head capsules are present and determine the age of the larva.”
Interestingly, the larva can also adjust the position of the fecal umbrella so that it will face a predator. Chaboo speculates that it could provide protection from predators by not only camouflaging the larva below, but also by providing a smelly barrier.
“We think the atmosphere is full of volatile chemicals coming off of the feces, which may create a little bubble of offensive compounds around the larva,” she said. “These are just some of the hypotheses that my colleagues and I will be exploring with a National Science Foundation grant over the next two years.”
Co-principal investigators on the project are Dr. Ken Keefover-Ring of the University of Wisconsin-Madison, and Dr. Paula A. Trillo of Gettysburg College in Pennsylvania and the Smithsonian Tropical Research Institution in Panama.
Chaboo’s decision to study leaf beetles has taken her around the world.
“There are many species of tortoise beetles that are only found in the New World, and many that are only found in the Old World, so to understand their biology and diversity, I have to go to different places to sample the species that are there,” she said.
That includes such widespread reaches of the planet as Costa Rica, Panama, Nicaragua, Australia, South Africa, and particularly Peru.
“I find tortoise beetles fascinating and a microcosm of the evolution and beauty of life on Earth, so I am curious about all dimensions of their biology,” she said.
When she gets back to Trinidad, she continues to peek under leaves to find the species that first enthralled her.
“It is still cool to see them,” she said. “They are like friends. It is nice to say ‘hi’ and wish the family broods good luck.”
Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.