The American bolas spider attracts different moths at different times of night.

A spider lures two moth species to their doom by mimicking both their sex pheromones, researchers have discovered. The spider changes the blend through the night, to attract species that are active at different times.

A bolas spider doesn't build a web. It slings a single line of silk across a gap and waits on it, dangling another piece of silk with a sticky glob at the end.

Next the spider releases chemicals that imitate the sex pheromones of female moths. Male moths come in search of female company; the spider swings its silky lasso and hauls in a meal.

Cunning, but limited. Different moth species produce very different pheromone blends - not surprising, given the importance of mating with one's own species. So spiders have to choose which species to attract.

And a combination of pheromones often turns a male moth off, even if his own species is in the mix. So the spiders trade-off the effectiveness of their mimicry of individual species against the number of species they can attract.

Let us prey

Different bolas spiders catch different combinations of moths. The American bolas spider Mastophora hutchinsoni catches two unrelated moths - the smoky tetanolita (Tetanolita mynesalis) and the bristly cutworm (Lacinipolia renigera).

The two species fly at different times of the night. Cutworms are active - and hunted - before about 22:30. Tetanolitas take wing from about 23:00 onwards.

Kenneth Haynes of the University of Kentucky and his colleagues shifted the moths' body clocks so that spiders hunting one species met the other, and vice versa.

Moths are liable to this kind of invasion of privacy Thomas Eisner , Cornell University

The spider caught both throughout the night, showing that it produces a mixture of the two species' pheromones, perhaps compromising its ability to catch either one.

The spider also reduces the amount of cutworm pheromone as the night goes on, the researchers found. Male tetanolitas don't like cutworm pheromone, Haynes' team discovered. But male cutworms aren't discouraged by a whiff of tetanolita.

"To specialize on different moths at different times of the night is amazing," says chemical ecologist Thomas Eisner of Cornell University, Ithaca, New York. Hunting two species gives the spider some variety in its diet. But it has still painted itself into an evolutionary corner, says Haynes.

"The evolutionary trajectory towards specialization seems a risky one," he says. "In years when the prey is scarce, the spiders will have little food." Bolas spiders are scarcer than web-spinners that catch a wider variety of prey, Haynes says.

Moths' scaly wings allow them to slip out of spiders' webs. But their pheromones are a weak point, says Eisner, as they are simple, easy-to-manufacture chemicals. "Moths are liable to this kind of invasion of privacy," he adds.