One of the continuing fascinations in biology is the discovery of new tales of the graceful and slow combat between predators and prey.
One such sequence of move and countermove is an evolutionary duel between plants and their insect enemies.
In their leaf veins, milkweed and several other plants circulate latex similar to that in rubber trees. To insects that try to eat the leaves, it is repugnant, toxic and gums up their mouth parts. Most insects leave milkweed alone.
But not all. Beetles especially, including chrysomelids and some caterpillars, have discovered they can circumvent the toxin by doing some artful surgery, according to a report by David Dussourd of the University of Maryland and Thomas Eisner of Cornell University in the current issue of Science magazine.
They cut the veins so the supply of the white, sticky latex is halted to some areas of the leaf; they can then chew the greenery at leisure. Except that other insects, which would not normally go near the plant, have learned to hop on leaves whose veins have been cut.
Some plants, in a counter-counter move, have developed veins that do not just branch downward. They are a complete network; if one vein is cut, the latex can still be carried down the leaf by alternate routes. Another plant has developed spines, which protect the veins.
Over many millions of years, the slow rhythm of play and counterplay has carried this episode one level further, to triple-countermove. The sphinx moth caterpillar, faced with the networks of veins in the papaya leaf, has learned to cut a groove straight across the leaf from edge to edge to shut off the latex. In the plant with spines, one caterpillar has learned how to snip off the spines, then cut the veins.
How do insects know which plants need to have their veins cut? Not only do insects show the ability to snip veins on plants that need it, but they also can tell if the veins have already been cut.
Their eyesight is too poor to help, but possibly they sense a chemical given off by the latex-producing plants, Eisner said. What the chemical signal may be is the next question the researchers will take on.