Saturday 19 June 2010

Insects That Can't Beat Them Scare Them (Via HerpDigest)

Insects That Can't Beat Them Scare Them
6/14/10 by Sean B. Carroll , New York Times

Imagine that you are a one-half-ounce, two-inch-tall insect-eating bird foraging for dinner on the dimly lighted floor of a Costa Rican rain forest. You come face to face with a pair of beady eyes. Study them for a moment.

If those eyes belonged to a snake, that moment of study would mean that you would be dinner by now.

The face, however, is not a snake's, but the chrysalis of a skipper butterfly. An uncanny resemblance - but, as it turns out, not a unique disguise.

In one area of Costa Rica alone, a team of researchers led by Daniel H. Janzen and Winnie Hallwachs of the University of Pennsylvania and John M. Burns of the Smithsonian National Museum of Natural History have discovered hundreds of species of moths and butterflies whose caterpillars or chrysalises display false eye and face patterns that mimic those of snakes, lizards or other animals. In a study published this week in The Proceedings of the National Academy of Sciences, they propose that this plethora of counterfeit patterns has evolved to exploit birds' innate instinct to avoid potential predators.

The idea is a fresh twist on the well-established phenomenon of mimicry among animals. First described by the British explorer Henry Walter Bates in the 1860s (the subject of my column on Feb. 16), the original insight was that harmless, edible species could gain protection from predators by resembling distasteful, noxious species.

Bates assumed that for this mechanism to work, the potential predators had to learn which prey in their range were to be avoided. And the potential prey - say, a large, colorful adult butterfly - must closely resemble the inedible species it mimics.

But when it comes to a deadly encounter with another species, there may be no second chances, no opportunity for learning. Hence, natural selection would favor instant recognition, and hard-wired rapid responses, in a close encounter with potential danger. Harmless creatures that evolved some general resemblance to the variety of creature features to be avoided (eyes, scale patterns) would then gain some protection.

Dr. Janzen and colleagues have cataloged a delightful assortment of striking false eye patterns on the front and rear ends of caterpillars and the front ends of chrysalises.

Their bounty and insights are the product of a dedicated, and somewhat accidental, long-term study of the denizens of the Área de Conservación Guanacaste, or A.C.G., in northwestern Costa Rica.

It began in 1978, when Dr. Janzen broke some ribs falling into a ravine while conducting field studies in the region. The road to the hospital was too rough to navigate, so he wrapped his sore rib cage and confined himself to a chair for a month.

Unable to explore the rain forest, he soon went a bit stir-crazy. The field station had only two hours of electricity each night, and just enough power to run a 25-watt light bulb. Fortunately for Dr. Janzen, that was a bumper year for moths, which were attracted to the light. So he passed the time building a moth collection.

When he recovered enough to wander back into the rain forest, he discovered that it was also a bumper year for caterpillars. The challenge was to identify which of the many different kinds of caterpillars belonged to which species of moths or butterflies. Now 71, he told me from his field station 32 years later that "my private insanity was to find all of the species before I die."

To accomplish his goal, he had to set up a system of collecting the caterpillars, photographing each of them, raising them into adults, then identifying each of the species, at least half of which had not been described previously. He started by himself, then was joined by his wife, Dr. Hallwachs, an expert on rodents and now caterpillars. The operation continues to this day, 365 days a year, with the help of 33 trained Costa Rican assistants.

In an area of about 77 square miles, more than 450,000 caterpillars have been studied. As of a few years ago, the team had identified more than 12,000 species. That number ballooned to 15,000 species when the team discovered, through the use of DNA typing, or "bar coding," that many of the species were actually made up of multiple distinct species, as many as 11 in one case. The total number of species in just this one region equals that of all of the moths and butterflies species of North America.

With caterpillars and chrysalises coming into the station at the rate of more than one hundred a day, Dr. Janzen began to discern a trend. In species belonging to many different groups, he saw caterpillars or chrysalises that bore all sorts of paired eyelike markings of various color schemes, with round or slit pupils. The variety of patterns suggested that the bugs do not have to match exactly the appearance of any particular predator for the ruse to work.

Moreover, the distinct behavior of many caterpillars when handled underscored that the whole game was to startle the many species of insect-eating birds that foraged in the dry, cloud and rain forests of the conservation area. Some eye patterns became visible only when the caterpillars were molested and expanded part of their body, and some large specimens wriggled and rattled like snakes.

Dr. Janzen and his colleagues estimate that a typical foraging bird might encounter tens to hundreds of false-eyed bugs each day. It is unlikely that a bird encountering such a spectrum of patterns could learn to discern which specific ones were safe and which were not, especially when one mistake would mean its demise. It would be better to leave suspicious items alone and to quickly move on.

For two centuries, naturalists have sought to catalog and make sense of the dazzling diversity of life, particularly as found in the tropics. Often, new insights have come from asking very simple questions, like "Why does this small caterpillar look like a snake on one end?"

But the answer to such questions requires finding many more creatures and understanding where and how they live. And that requires a special breed of human willing to live far from the comforts of home and eager to look at 450,000 bugs for 32 years.

for slide show of insects trying to look like snakes.

No comments:

Post a Comment

You only need to enter your comment once! Comments will appear once they have been moderated. This is so as to stop the would-be comedian who has been spamming the comments here with inane and often offensive remarks. You know who you are!

Related Posts with Thumbnails