Stuck in the red

Natural History, June, 2008 by Stephan Reebs

The leaves of pitcher plants form deadly, watery traps for insects, which beef up the verdant carnivores' nutrient supply. The traps, or pitchers, often sport red streaks or entirely red exteriors that contrast sharply with background greenery and probably serve to lure in insects. Indeed, the redder the pitcher, the more insects it catches, according to several studies.

But redder pitchers also hap pen to bear more nectaries, an additional temptation. Can an eye-catching red color alone bait bugs, without the lure of a sticky-sweet last meal?

To find out, H. Martin Schaefer of the University of Freiburg in Germany and Graeme D. Ruxton of the University of Glasgow in Scotland devised a simple experiment. They painted the traps of twenty Nepenthes pitcher plants either entirely red or entirely green, then set the plants out near ponds.

After a few days, the red pitchers had caught twice as many insects as the green ones. Apparently, as the two scientists write, the pitchers really do "roll out the red carpet" for insects.

That might explain why red is common in most carnivorous plant species--though why they're not all red remains an open question. (Biology Letters)

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