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Studying the Praying Mantis with Nets and Isotopes

The praying mantis is an arthropod and a predator, a skinny tough guy (or gal) with jointed feet and an exoskeleton. It’s willing to attack larger prey, from mice to snakes to hummingbirds. There’s even been a report of a praying mantis eating a turtle.

Thrilling YouTube videos of the creatures aside, we know little about their lifetime dietary habits. Larry Hurd, Washington and Lee’s Herwick Professor of Biology, is adding to that bank of knowledge this summer by analyzing the stable isotopes of wild mantids and their prey, a procedure never before used on mantids.

“Are they eating other predators mainly? Are they eating phloem-feeding herbivores? Or are they eating things that eat vegetation?” asked Hurd, who has studied the praying mantis for more than 35 years. He thinks that mantids are frequency-dependent predators, eating whatever is most abundant in their habitat.

The project is a collaboration with Maj. Pieter deHart, an assistant professor of biology at VMI, and research assistance from sophomores Megan Shearer of Columbia, Md., and Joseph Taylor of Petersburg, Va.,, both Robert E. Lee Summer Scholars at W&L.

DeHart is the team expert on stable isotope analysis, a research method that involves parsing and weighing the basic chemical elements of an organism to determine its chemical signature. For the mantid study, the team is comparing the isotopes of wild mantids against the isotopes of other organisms within their habitat and within their probable food chain. They are examining lab-fed mantids as a control group.

Stable isotope analysis is groundbreaking when it comes to mantids because scientists have never observed their long-term eating habits in the field. Recording what mantids eat, over an extended period of time, may lead to a better understanding of how predators control biological diversity and species interactions within eco-systems.

The procedure is also more informative than an insect autopsy. “All that tells you is what’s sitting in its gut. It’s not telling you what that incorporating into its tissues,” said deHart. “Stable isotopes are the way to tell that because we’re essentially taking the whole organism, grinding it up, combusting it, running it through a machine and saying ‘What do these elements look like?’ ”

Once a week, Shearer and Taylor take handheld nets to several fields on campus. There is a fourth field in Norfolk, Va. “We sweep them back and forth, and it catches the insects,” said Shearer. “We catch maybe two or three mantids at each site.”

At W&L, the students sort and identify the captured mantids and insects, then carry them to VMI for further preparation. “We have to freeze-dry our sample first so that there’s no moisture,” said Taylor, “because if you grind it up and there’s moisture, it’s just a mess.”

The students grind the dried insects into a powder, which they then scoop into tiny cups with a micro-spatula for weighing. Once they weigh it, they send the powder to an out-of-state facility for processing by an isotope mass spectrometer.

Inside the spectrometer, air heats to a temperature of 2,400 degrees Celsius, which combusts the insect powder into a gas. A magnet then pulls out and sorts the insect’s basic elements, primarily carbon and nitrogen.

After the insects are processed in the spectrometer, the team will place data about the weights of these elements onto a graph, also known as a food web. By comparing the elemental weights, or chemical signatures, of wild mantids with those of other insects from the same habitat, the team can draw conclusions about the mantid’s food chain and eating habits. Animals higher on the food chain have a heavier elemental weight.

“As one of my colleagues puts it, it’s kind of hocus-pocus, like a magic box sort of thing. You put in there, it spits out a number, and you make sense of the number,” said deHart. “There’s both science and art with it, and you have to interpret the data based upon the best available literature.”

The students are looking forward to getting back the first results. “We’re doing something really new. No one’s mapped out the ecology of mantid ecosystems, so it’s pretty exciting to be doing something new to science that might contribute to knowledge,” said Taylor.

And for those still wondering how a praying mantis stuns a larger animal, the answer is straightforward: “They just grab it with their forelegs, and they hold it in front of them,” explained Hurd. “You’ll see them go after really huge wasps, and the wasp is trying to sting them, and the stinger keeps glancing off the armored prothorax. They just hold it like a crane and just eat the head off. Once they have something, it’s toast.”

— by Amy Balfour ’89, ’93L