When you're feeling sick, your mom or dad gets you medicine. But what does a butterfly do？
The monarch butterfly, famous for its long migration and bright orange and black wings, seeks out a cure for its caterpillars（幼虫） by laying eggs on medicinal plants.
Wait, you might be saying, "Don't butterflies only eat milkweed（乳草属植物）？" You'reright, but the species of milkweed matters. These plants contain poisonous chemicals called cardenolides, and tropical milkweed has more poisons than swamp milkweed. Monarchs are not harmed when they eat the poisons; instead, they build up the poisons inside their bodies, making
them poisonous, too！
But even a poisonous insect has problems. The parasite（寄生虫） Ophryoeystis elektroscirrha infects monarch caterpillars. The infection lasts through the transformation into butterflies, and can be passed onto future young. The more poisonous a caterpillar for butterfly is, the better it will be at fighting off the parasite. So it's a good idea for caterpillars to eat tropical milkweed instead of swamp milkweed.
But caterpillars don't know what's good for them. Jaap de Roode, who heads a monarch butterfly lab at Emory University in Georgia, tried giving the caterpillars a choice between tropical and swamp milkweed, and the baby butterflies showed no preference.
Of course, Mom knows best. Female monarchs infected with the parasite preferred to lay their eggs on tropical milkweed, while uninfected females didn't care. "These very small animals have the ability to make a distinction between plants that will be good for their children and plants that will be bad for their children," says Jaap de Roode, "Somehow, they know that they are infected, and they know what to do about it. " Those cured caterpillars will be all ready to fly to Mexico.
67. How do the monarch butterflies deal with the parasite Ophryocystis elektroscirrha？
A. The monarch butterflies get rid of the parasites through their long migration.
B. The monarch butterflies lay their eggs on tropical milkweed.
C. The monarch butterflies lay their eggs to kill the parasites themselves.
D. The monarch butterflies pass on the infection to their caterpillars.