Chemicals in spider silk repel ants

Golden orb spiders are vulnerable to coordinated ant attacks. Image: Professor Daiqin Li

Golden orb web spiders add a chemical to their web silk to prevent ant invasions.

Golden orb spiders are vulnerable to ant attacks- while they can defend themselves against a single ant by wrapping it in silk, a coordinated attack by three or more would be a problem. However, despite their abundance, ants rarely appear on the spiders’ webs.

Researchers from the National University of Singapore and the University of Melbourne set out to discover why the ants aren’t venturing onto the webs to steal the spider’s prey or attack it.

“A spider is kind of a sitting duck… in the middle of its web,” said University of Melbourne zoologist Mark Elgar. “(They’re) quite vulnerable to attack by voracious hordes of ants.”

To find out why the ants were avoiding the webs, the researchers encouraged Golden orb web spiders to create webs in the laboratory. They then removed the surface chemicals from the spider silk to determine what the active chemicals were.

The study, published today in the journal Proceedings of the Royal Society B, revealed that the compound produced by the spiders is a well-known pyrrolidine alkaloid. Other pyrrolidine alkalides are used by various insects as a predator deterrent.

“Once we knew what the compound was, it made the (behavioural) experiments that much easier,” said Elgar.

The researchers then constructed bridges from spider silk that had been stripped of its chemicals and coated with distilled water. They also built bridges using the untreated silk and observed that the ants would not cross these bridges, but would walk on the treated silk.

However, further observations revealed that only the larger orb spiders produce the chemical. Professor Elgar suggests that this is because the ants can walk on the thicker silk produced by these large spiders, but the thin silk produced by smaller spiders wouldn’t support them.

The study has given rise to a number of questions, such as the mechanism that enables the spiders to produce the chemical and use it to coat the almost liquid silk as they produce it. “(It’s) not a bi-product of making silk,” said Elgar.

Another mystery to be solved is why the ants avoid this chemical i.e. which receptors are being activated. “Ants live absolutely in a chemical world, so for them taste is everything.”

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