A Bug's Diet: How It Can Help You

Most of us have heard it a thousand times: you have to eat a wide variety of foods in order to get all of the nutrients your body needs. In this respect, we're not alone in the animal kingdom; many other omnivores (animals that eat both plants and meat) as well as herbivores (plant-eaters) also require a diverse diet in order to ensure proper nutrition.

But while we're busy trying to make the right choices at the supermarket, some animals seem wholly uninterested in-or incapable of-being picky eaters. For example, carnivores (animals that feed primarily on other animals) are pretty much limited to eating whatever they can kill. And some predatory invertebrates, such as web-building spiders, seem to have almost no control over what they eat, as they can't even chase after their prey but rather have to wait for it to get caught. (An invertebrate is an animal, such as a spider or insect, that does not have a backbone.)

For these reasons, scientists had assumed that predators simply get the right balance of nutrients from their prey, and don't have any reason to be selective eaters. But a recent study has turned this idea on its head-at least in the case of certain insectivorous (insect-eating) invertebrates. According to the study, which was published in the January 7, 2005 edition of the journal Science, these animals do actively select food to create the most nutritious meal.

The study involved three species of insectivorous invertebrates: Agonum dorsale (a ground beetle), Paradosa prativaga (a wolf spider), and Stegodyphus lineatus (a web-weaving desert spider). Ground beetles are highly mobile and can scurry after whatever prey they choose. Wolf spiders are not as mobile as ground beetles; they are "sit-and-wait" predators, meaning they wait for potential prey to come near them before attacking. Finally, the web-weaving spiders simply have to settle for the bugs that get stuck in their webs.

To find out if any of these species have control over their nutrient intake, the scientists first subjected them to a nutritional imbalance. For one or two days, they fed the beetles and spiders food that was either high in protein and low in lipids, or low in protein and high in lipids. (The beetles were fed powder made from crushed locusts and enriched with either lipids or proteins. The spiders were fed fruit flies that had been raised on foods that led to either a protein-rich or lipid-rich body composition.) Both proteins and lipids are crucial parts of these species' diets.

Then, the researchers carried out tests on the invertebrates' subsequent eating behaviors, and found that all three species could compensate for their nutritional imbalance. For example, after being fed just one kind of food for a couple of days, the beetles were offered a choice between protein-rich and lipid-rich food. The scientists observed that, in this situation, they selected the kind of food that they had not previously been fed; in other words, they went for the food that contained the nutrient they were lacking.

A similar thing happened with the wolf spiders. After the nutritional imbalance had been established, the spiders were offered either the same kind of food they had been given previously, or the alternative kind. (The researchers offered the spiders only one kind of food because they wanted to find out how the spiders respond when they end up in an area where only one kind of prey is available.) The researchers found that the spiders who were given the alternative kind of food ate more than spiders who were given the same kind of food. For example, spiders that were first fed lipid-rich flies and then offered protein-rich flies ate more than spiders that received the same kind of food in both steps of the experiment. Thus the wolf spiders compensated for a lack of choice of prey by varying the amount they ate.

"Wolf spiders don't build webs but sit and wait for prey to appear and then ambush them, so we didn't think they would be able to go out and select their diet," David Mayntz, a co-author of the study and zoologist at Oxford University in England, told National Geographic News. "They have to deal with whatever they catch. But we found what they eat from the prey they do catch will depend on how much protein is in the prey and what [the predator's] last meal was."

Even web-weaving desert spiders--which simply loiter near their web until they catch something-can take control over what they put in their bodies, the study revealed. As with the other species, the scientists fed the desert spiders either protein- or lipid-rich food. After the spiders had been on this diet for a day, the researchers gave them either the same kind of food or the alternative kind, and then analyzed the remnants of the prey after the spiders had fed on it. They found that, regardless of what kind of food they had been given, the spiders extracted higher levels of either protein or lipids from it, depending on what they needed. The researchers think that this selective digestion is accomplished with help from enzymes that the desert spiders secrete to break down their food. (An enzyme is a protein that facilitates chemical reactions.) "The spiders let the enzymes work for a while, then suck up the half-digested juice," Mayntz said. "Apparently they are able to take more protein out via the enzymes if the last prey they ate was poor in protein."

"All of them are trying to get the right nutrients, but in different ways," Mayntz told the New York Times.

How, exactly, do these predatory bugs figure out what's the right nutritious food? Scientists don't know, although they speculate that some may rely on a taste mechanism, while others may use smell. Researchers now plan to work their way up the food chain, testing larger predators-such as minks and cats-to see how they manage to achieve a well-balanced diet.

Sources

Fountain, Henry. "Eat or Be Eaten (But Eat Right)." New York Times, January 11, 2005, page F3.

Mayntz, David, Stephen J. Simpson et al. "Nutrient-Specific Foraging in Invertebrate Predators." Science, January 7, 2005, page 111.

Owen, James. "Spiders Watch Their Diets Too, Study Says." National Geographic News (January 6, 2005) news.nationalgeographic.com / news / 2005 / 01 / 0106 _ 050106 _ spider _ diet.html.

"Predators Look for Quality Over Quantity." Press release from Oxford University (January 7, 2005) www.admin.ox .ac .uk / po / 050107.shtml.

Schmid, Randolph E. "Predatory Insects Change Diet for Balance." CBSNews.com (January 7, 2005) www.cbsnews.com / stories / 2005 / 01 / 07 / tech / main665469.shtml.