The cells in Drosophila melanogaster that produce pheromones are located in the abdomen. Here, the cells are marked by a green fluorescent protein.

View a video of fruit flies displaying unusual courtship behavior.

Fruit flies linger over a bowl of rotting fruit. To untrained eyes, the flies may look like a swarming nuisance, but scientists have found that flies’ swoops and buzzes are ways to send signals through the crowd. Another, less obvious way these insects communicate is through chemical signals called pheromones. (It’s easy to think of these chemical signals as being similar to smells.)

Scientists have long known that pheromones may play an important role in reproduction — certain pheromones may attract a potential mate, for example. But in a surprising new study, scientists found that male fruit flies are particularly attracted to other flies — male and female — that don’t put out any pheromones at all.

The researchers also found that fruit flies without pheromones are attractive to males of other species. This research suggests that pheromones may be even more complicated — and important — than scientists thought. Besides telling other insects to come a little closer, pheromones may also be used to say, “Back off!” That message is important for keeping up barriers between species.

There are many different types of fruit flies, no matter how similar they all look as they swarm over a rotting tomato. Scientists have wondered how fruit flies can tell each other apart. Appearance may play a role. So may sound — the mating song of each different kind of fruit fly is different, for example.

Scientists suspect pheromones may also help fruit flies find potential mates of the same species — but there are 30 or more pheromones to choose from. In the new study, which was led by Joel Levine, the scientists wanted to figure out what messages the different flavors of pheromone were each sending. Levine is a neurogeneticist at the University of Toronto at Mississauga. (Neurogenetics is the study of how genes affect the development and function of the brain and the nervous system.)

His team genetically altered fruit flies so that the flies no longer made pheromones. Then the researchers watched the mating behavior of the insects, and observed that males went after the flies that didn’t have pheromones.

“Males are only after one thing. They want to mate,” Levine told Science News. Females, on the other hand, preferred males with pheromones to the males without. “She will not go for the guy who has no odors,” Levine said. He and his team also used the scentless flies as a starting point for other experiments. They were able to identify one particular pheromone, for example, that kept flies of different species from breeding.

These chemical signals help flies tell males from females, and help tell members of different species from each other. The new research suggests that pheromones may be more important than sight or sound in that crowd of flies hovering over the fruit bowl. “We expected the chemicals would play a role,” Levin said, but “we had no reason to think that the effects we saw would be so strong.”

Strange Attraction from Science News on Vimeo.

In the absence of pheromones, flies engage in unnatural courtship behavior. In this movie, two males attempt copulation with each other’s heads.

Credit: Jean-Christophe Billeter et al, Nature 2009

Going Deeper:

Silk threads can be dyed bright colors and then woven into beautiful fabric.

iStockphoto

Since then, researchers have unraveled many of silk’s mysteries, but they still don’t fully understand how silkworms, spiders, and other small creatures create what turns out to be one of the toughest materials known.

Silkworms weave silk into white cocoons.

iStockphoto

But Ann Terry, a physicist and a visiting professor at Oxford University in England, thinks that she and other researchers are closing in on that remaining mystery. Terry and other experts hope that current research into silk will lead to a new generation of fabrics that are lightweight and superstrong. Such materials would be useful for medical and military purposes and also could help astronauts and clothing-makers.

Strong stuff

The silk industry still depends on silkworm silk, but scientists have lately focused their attention on spider silk because it’s much tougher. (Toughness describes how much energy it takes to break a material.)

Spiders use a particularly tough type of silk to form the “arms” of their webs. These arms, like spokes of a wheel, run outward from the center of the web, where the spider in this photo is.

iStockphoto

Spiders can spin different types of silk, some of which are tougher than others. In a classic orb web (like the kind you’d expect to see in a haunted house), the toughest type of silk forms the arms of the frame. These arms, like spokes of a wheel, stretch outward from the center of the web, says Gareth McKinley, a scientist at the Massachusetts Institute of Technology in Cambridge. Another type of silk, which is sticker, forms the spirals that connect the arms of the frame. This sticky silk helps the spider capture its prey.

Spider silk can be “strong stuff,” McKinley says. To test silk’s strength, scientists hang weights from the frame threads of an orb web, then measure how much weight those threads can hold. The researchers have found that spider silk can be as much as 100 times tougher than the same amount of steel. It is about twice as tough as Kevlar, a synthetic fiber used to make sturdy objects such as bulletproof vests and boats.

Slippery and sticky silk

Spider silk starts out as a goopy, yellowish liquid inside the animal’s body. So, how do silk-spinning creatures turn this liquid into one of nature’s toughest solids?

To better understand how that happens, McKinley and colleagues tested two properties of spider silk: slipperiness and stickiness. To test slipperiness, they used a microscopic device that mimicked the motion of a thumb and forefinger sliding back and forth against each other, with a glob of liquid in between. The stickiness test mimicked a thumb and forefinger pulling a glob apart over and over again.

Shawna Liff, a graduate student at the Massachusetts Institute of Technology, works with a synthetic material that is similar to spider silk in its strength and stretchiness.

Donna Coveney/MIT

Sliding the glob quickly, the researchers found, made spider silk 30 times as slippery as it was to start with. And pulling made it more than 100 times as sticky.

Those results help explain what happens when a spider squeezes out liquid silk through the narrow channel in its abdomen. First, the silk becomes slippery. This allows the silk to flow more easily as the spider excretes it. It’s so sticky that the spider can hang from it—like a person dangling from a bungee cord. When clinging to a strand of silk, a spider can change how fast it drops by varying how quickly it draws out its silk, McKinley says.

This microscopic picture shows what happens when a synthetic material that’s like silk gets stretched. The stretched part of the material is near the bottom of this picture. It appears to be brightly colored because the molecules are lined up.

Courtesy of Gareth McKinley Lab/MIT

Scientists have already figured out how to extract liquid silk from a spider’s body and to use it to spin fibers. But these human-made threads are never as tough as the ones spiders spin on their own, Terry says. Scientists are still trying to figure out exactly why. They’ve found out, for example, that the protein molecules that make up the silk line up and form parallel chemical bonds inside a spider’s body. This adds an extra measure of toughness, Terry says. Spiders regulate the amount of water and other molecules that go into their silk supplies, which can also affect the silk’s quality.

Supersilk soon?

Unfortunately, farming spiders for their silk is impractical because the creatures produce only very small amounts of liquid. They are also too territorial to tolerate living closely with other spiders. Silkworms are easier to breed and keep in captivity.

The spinning process that silkworms use may explain why their silk isn’t as tough as spider silk. Unlike spiders, which draw silk out of their abdomens, silkworms draw silk out of their mouths. They move their heads in a figure-eight pattern as they do this. In a recent study in which researchers kept the worms’ heads from moving, the worms produced fibers that were just as tough as spider threads.

This finding suggests that silk manufacturers might someday be able to use the silk from silkworms to make spider-strength thread.

Researchers are also looking for more efficient ways to make silk. Some experiments have involved inserting the spider’s silk-making gene into alfalfa, goats, and other organisms to have them produce silk proteins. These proteins could then be harvested and spun into silk.

Ultimately, understanding the biology of the silk-making process and the physical qualities of silk should help researchers make even better materials, Terry says. Knowing how each factor affects the final product will give scientists more control over the process. And that control could open a wealth of potential uses for silk—from the manufacture of lighter, stronger protective gear to the ability to help repair torn ligaments in people.

Spiders make silk spinning look easy, but they’ve had millions of years to figure it out.

“Nature still beats us,” Terry says. “We have a lot to learn from nature.”

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