Spiders turn on their stripes at night to lure prey

November 26th, 2007 - 6:54 pm ICT by admin  

Washington, Nov 26 (ANI): Researchers have discovered that during night, many spiders flip themselves over and display hidden abdominal stripes to lure unsuspecting prey.

The discovery by Dr I-Min Tso and colleagues at Taiwan’s Tunghai University helps explains why arachnids, like the giant wood spider and the orchid spider, eat large amounts of carefully wrapped prey on their webs just before sunrise.

It also suggests that entomologists may have underestimated the activities of some spiders when darkness falls.

A sign to detect night-prowling spiders is colouration. Dark grey or brown spiders are usually nocturnal, since their drab colours serve to reduce their visibility during the day. However, many spiders have different patterns and colours on their abdomen.

“During the day, these spiders usually perch on twigs or barks near their webs with their brown-coloured [back side] facing upward and the yellow abdominal markings obscured. However, at night while the spiders are sitting on their orb webs hunting, they fully expose the yellow markings,” researchers said.

They added that other studies suggest moths may perceive such colourful markings as potential flowers or new leaves, both food sources.

In the study, researchers focused on brightly coloured orchid spiders, which have green bodies, silver stripes on their backs and yellow or bright green stripes on their abdomens. They first monitored this spider’s hunting behaviour with video cameras 24 hours a day.

They found that the number of active night hunters was “two to three times higher than [those orchid spiders that hunted] during the day”.

It was found that their snagged prey, mostly moths, also tended to be bigger at night.

The researchers experimented with various colour manipulation tests that involved painting over the silver, yellow or green parts of the orchid spider.

Obscuring the yellow stripes considerably reduced moth catches, providing evidence that these colours probably function as night lures.

According to the researchers, the spider’s silver back stripes might even function “as a thermal regulator by reflecting sunlight during the hot hours of the day”.

The determinations could apply to many other spider species, especially those that share similar flip-side colouration.

Dr Steve Heydon, senior scientist and collection manager at the Bohart Museum of Entomology in the US, said that while moths “can’t see red and don’t see colours the way that we humans do”, the findings are possible.

He said that moth vision shifts more to the ultraviolet end of the spectrum at night.

He is more sceptical about the proposed thermoregulation theory concerning spider silver stripes.

“These would probably sparkle and shine from the moth’s perspective, proving to be dazzling lures when the stripes are exposed,” he said.
The findings were published in the journal Animal Behaviour. (ANI)

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