THE last stragglers of a mass moth migration are now flitting south through CostaRica as part of a periodic exodus from Mexico to South America. Millions of day-flying moths migrated through Costa Rica in August on a periodic, one way exodus from Mexico, making their way toward Panama and the northern reaches of South America at speeds of up to 20 kilometers (12.4 miles) per hour.The moth, Urania fulgens, known in Costa Rica as the colipato verde, is medium- sized with striking black, metallic, green-banded wings that sweep into tails, lending the moth an aspect that competes with butterflies for beauty pageant points. The moths are indigenous to Costa Rica, Panama, parts of southern Mexico and northern Central American countries. Once every four years during August a swarm of them descend from the northern countries through the warm tropical regions, laying eggs on a particular woody vine, the Omphalea diandra.SCIENTISTS did not identify the most likely reason for the migration until the early 1980s.Apparently, the Omphalea diandra vine will not be bullied by Urania larvae: according to a 1983 article by Neal Smith in The Florida Entomologist, the moths migrate in search of more plants on which to lay their eggs because Omphalea reacts to grazing by becoming toxic. The female moths lay orange clutches of 80 eggs on the plant. After they hatch, the larvae shimmy to the newest leaves on the stem to dine. In an experiment Smith conducted with razor blades and a hole punch to simulate the effects of 80 grazing larvae, he found that after three periods of leaf destruction, the plant raised its toxicity and killed, or slowed the growth of, many of the new larvae. If left alone, however, it reverted to its benign state, which is delicious to baby Urania.So, hordes of moths migrate to find fresh, nonpoisonous outcroppings of Omphalea, laying eggs and scouting their own food. They travel for several months; with a three-month life expectancy, many die of old age on the way.THE adult moth feeds on nectar from guaba flowers, vinegary tree secretions and wet sand, Costa Rican entomologist José Montero told The Tico Times. Montero works in the National Biodiversity Institute Park (INBioparque), a large research center in Heredia, north of San José.(Information about visitation and other aspects of the park is available on the Web in English and Spanish at www.inbio.ac.cr) Besides the mass migration every four years or so, the moth makes internal migrations from the Atlantic to the Pacific Costa Rican coasts. These depend only on the availability of food for the moths and their larvae, Montero said.While adult moths do not eat from the plants that host their eggs and larvae, they can detect the toxicity of those plants through sensors in their feet.BESIDES the natural defense the Omphalea puts in the moths’ path, humans play a role in the moths’ migration as well, Montero said. The thinning- out and destruction of the plants moths need to survive spurs them on in search of new eateries.The Urania is one of the brilliant- colored and day-flying moths that blur the line between moths and butterflies. They are both from the same order, Lepidoptera; both go through the same life cycle of egg, larva, cocoon and winged adult; and the differences between them are not always obvious. In fact, the Butterfly Farm, a Costa Rican operation in La Guácima, northwest of San José (www.butterflyfarm.co.cr), explains there is little that differentiates them. Moths are usually night fliers, seen swarming streetlights and bug zappers, and butterflies are usually more colorful and wander by day, but, like the Urania, there are exceptions.The one telltale standby is the antennae:Butterflies’ are always straight and club-ended, while moths’ vary in shape and texture and are usually much wider.