See also: http://www.abc.net.au/science/news/stories/s1611172.htm (more accessibly written).
‘Scent of a woman’ tells male redback spiders to find a mate
Female Australian redback spider eating male spider
Image: Ken Jones, UTSC
A redback male gets only one mating opportunity — immediately after he has sex for the first time his partner – who is 100 to 200 times his body weight – will eat him alive.
When males can smell females, they develop rapidly, trading a smaller body size and less fat reserves against getting to virgin females sooner. The competition is tough – there are often up to eight males on a female’s web. If the male can mate first, he will be able fertilize 90-100% of the female’s eggs — so being able to sense population density and respond to it by accelerating development is a definite advantage.
This is the first time such a phenomenon has been shown in any animal.