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Sunday, September 23, 2001
Nature

They change colour to merge with flowers
Nutan Shukla

LIVING among flowers, the crab spider is expert mimic. It adorns its body with such colours and patterns that they match completely with its surroundings. The result is it merges so well with the flowers that it is almost impossible for the casual observer to spot it. Wearing this cloak of anonymity, the arachnid is able to ambush its prey without much difficulty. Instead of spinning a web or chasing the prey, as most of its cousins do, crab spiders sit motionless amongst the petals and anthers of insect-pollinated flowers and wait for insects, such as butterflies and hoverflies to alight.

When any pollinator touches down upon the flower, the camouflaged predator pounces upon the unsuspecting visitor and grabs its head and bites into its mouth parts, reversing the normal flow of nutrients, and sucks out the body fluids. Desiccated butterflies are often left resting on the flower.

The cryptic coloring is important. In an experiment with dandelion flowers, a scientist once placed yellow pebbles on half the flowers and black pebbles on the rest. Insects were very reluctant to visit the flowers with the conspicuous black pebbles, indicating that without the flower-matching colours a crab spider would have a very lean time indeed.

EARLIER COLUMNS
They do not pursue their prey
September 2, 2001
They attack with stinging cells
August 19, 2001
Predators that appear harmless
August 5, 2001
The chameleonís deadly weapon
July 22, 2001
A colour for every occasion
July 8, 2001

Manakins sing duet to entice females
June 24, 2001

Sperm whales have a whale of a time
June 17, 2001
They use projectiles to catch prey
May 27, 2001
These insects lure the prey with light
May 20, 2001
Predators in the deep seas
April 29, 2001
They know how to entrap
April 15, 2001
Small creatures with a big sting
April 1, 2001


Flowers, of course, do not last very long and so, when its flower is spent, the spider moves to another bloom and can even adjust its colour to match its new home. There is, however a curious postscript to the crab spider story. Insects see different wavelengths of light than we do, and are able to see ultraviolet light reflected from flowers. Significantly, flowers look very different when viewed at these wavelengths. Striped nectar guides, like airport runway landing lights, show pollinators the fast way to the reservoirs of nectar, and crab spider shows up quite clearly. In theory, then, insects should be able to see the spiders. It is a mystery why they donít avoid them.

In the tropics, there are colourful species of praying mantis that mimic the flowers on which they sit. These are the flowers mantids, and although they are much larger than the crab spiders, they are only visible when they move. One of the most beautiful is the so-called devilís flower, an African mantid with red-and-white legs. White orchids in South-East Asia have their own deadly orchid mantis that resemble white petals.

Praying mantids also mimic leaves and sticks, and they get their name from the prayer-like attitude of their first pair of hinged, grasping limbs that can extend forward instantly in one-twentieth of a second to snatch a meal. They have insatiable appetites and are skilled predators. One specimen was seen to snatch and eat 10 cockroaches in a span of only 3 hours. Mantids come in all shapes and sizes, some as large as 6.5 inches long, and catch small frogs and lizards.

Perhaps the most unlikely predators to lie in wait on a branch are the larval stages of moths. The Hawaiian looper caterpillar, along with 20 related species, is killer caterpillar. It rests on a leaf or stem, grasping it with its rear suckers, and extends itself out at an angle, looking to all the world like a straight, green twig. If however, fly would come close and touch sensitive hairs on its back, the caterpillar bends rapidly and seizes the prey in its grasping forelegs. With the victim firmly caught, it straightens out once more and devours the prize.

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This feature was published on September 16, 2001
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