JULY 2, 1997--As children chase twinkling insects and the setting sun throws long shadows across the backyard, consider this: Light cues keep predators from snacking on baby fireflies, University of Delaware scientists report in the new Journal of Insect Behavior.
"A flashing neon sign may lure hungry humans to an all-night diner," says Douglas W. Tallamy, professor of entomology and applied ecology, "but the bioluminescence of firefly larvae sends a very different message to would-be predators."
The UD study is believed to offer the first laboratory-based evidence of an insect using bioluminescence--rather than coloration--as an "aposematic display," which warns predators of an unappetizing or hazardous meal. Bright colors, such as the orange and black patterns on a monarch butterfly or the yellow stripes on a wasp, are far more typical examples of aposematic display, Tallamy notes. Baby fireflies (Coleoptera: Lampyridae) clearly use light signals to ward off predators, UD researchers found.
This new insight into firefly behavior may serve as an educational tool for both adults and children this summer, Tallamy says. "The more people understand about their natural world, the more they are likely to appreciate why it must be preserved for future generations," he explains. "And, children who understand why fireflies are flashing may get hooked on science."
Decoding light signals
Since at least 1952, researchers have known that adult fireflies use light patterns as part of a mating ritual, Tallamy says. Because baby fireflies are not mature enough to reproduce, researchers have speculated that younger specimens might use light cues for survival, rather than reproduction. Without laboratory evidence to support the theory, however, the messages sent by firefly larvae have remained a mystery--until now.
With graduate student Todd J. Underwood and John D. Pesek, an associate scientist in UD's Department of Food and Resource Economics, Tallamy tested the aposematic display theory on ordinary house mice raised in a laboratory. But first, the UD researchers needed to find out whether mice think firefly larvae taste bad.
In previous laboratory studies, vertebrate predators have consistently turned up their noses at lucibufagins, compounds present in adult fireflies. But, Tallamy notes, "only anecdotal evidence suggested that larvae are also distasteful." So, mice were offered a choice of either a firefly or a mealworm--a delicacy for rodents. As expected, all mice rejected the bitter fireflies, Tallamy says, even when they were still hungry enough to eat more mealworms.
Next, the UD researchers tested the ability of mice to associate light with a bitter taste. At one end of a Y-shaped maze, they placed a single piece of crispy rice cereal. A second piece of cereal was soaked in a stomach-turning concoction of quinine sulphate and mustard powder before being placed on the other side of the maze, which was rigged with a light-emitting diode. Though mice initially entered the maze "with a bias toward the glowing branch," they quickly learned to steer clear of the bitter-tasting tidbit, UD researchers say. Within eight to 47 runs, all mice had selected the darkened side of the maze at least seven times in a row.
"Our study answers a fundamental question that entomologists have been pondering for some time," Tallamy says. It also suggests an interesting topic for discussion between parents and children, he adds.