butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.
In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).
Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.
One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.
Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.
These behaviors get the animals to a good place (for overwintering or for reproduction). Like the long-range movements with specific endpoints on the map, homing to a good place is not always easily distinguished from moving out of a bad place. The behavior is a mechanism with deep evolutionary roots. Indeed, insect wings (and metamorphosis) may themselves have been an original adaptation for dispersal, to colonize temporary pools, animal carcasses, or other temporary resources. The first individuals to reach the resource won the competition to use it and multiply there, and these were more likely to be the ones that flew, and flew far and wide, rather than those that walked at random.
Wings and metamorphosis have lesser value in constant conditions. Some insects are able to respond in real time to the changes in conditions they experience (especially crowding), in that when they don’t “need” to disperse they either don’t grow wings (some aphids) or the muscles to power the wings are broken down and the amino acids from the protein are used instead to make more eggs (in some Hemiptera bugs). Often there are discrete “dispersers” versus “non-dispersers” in any given insect population, and the percentage of each depends on the quality of the home habitat and hence the relative cost/benefit ratio of moving versus staying.
Dispersing to “anywhere but here” generally applies to nonmigratory species that have no encoded or learned directions to go to but may have innate instructions to move in more-or-less straight lines rather than potentially going in circles in order to achieve distance. In Africa, dung-ball-rolling scarab beetles race away from their often thousands of competitors at a dung pile at night by using the swath of stars of the Milky Way galaxy as a reference. Swarms of insects feeding at dung and carcasses also attract predators, and as soon as they finish feeding, many distance themselves from those predators. I’ve observed blowfly larvae at animal carcasses keeping to almost perfectly straight lines in their getaway at dawn, by steering directly toward the direction of the rising sun. Mass movements sometimes observed in some rodents, such as lemmings and gray squirrels (as in 1935 in New England) following a population explosion after a superabundance of food, may be another example of dispersal to get to a better place, though not necessarily a predetermined one.
On the other hand, “true” migrants are able to utilize ideal conditions in two places, provided they vary predictably. Arctic terns, Sterna paradisaea, breed throughout the Arctic, then fly to Antarctica to escape winter when food availability declines and to arrive in spring and food again, a round-trip distance of nearly seventy-one thousand kilometers. Gray whales, Eschrichtius robustus, also feed in the Arctic in the summer but then travel eight thousand kilometers along the coastline to Mexico to bear their calves in warm waters.
Dispersers are not neatly differentiated from migrants, although the first commonly rely on passive mechanisms as opposed to the migrants, which move to specific goals by their own powers of locomotion. There are all gradations in between. Each case is unique, and there are thousands. Let’s look at more ways of getting to a good place, as represented by eels, a grasshopper, aphids, and ladybird beetles.
Eels. There are many species of eels, but the American, Anguilla rostrata, and the European, A. anguilla, the species with which we are most familiar, live most of their lives in freshwater ponds and lakes. For reasons that are not clear, though, they do not reproduce in their home areas. To the contrary, both disperse (or “migrate”) thousands of kilometers on a one-way trip to spawn and then die in the mid-Atlantic.
Just as birds and some insects use air currents, eels use water currents to help them leave their lifetime homes. After eels leave their freshwater homes and head for the ocean to spawn and die, their larvae then drift in the ocean currents for years. But eels’ dispersal behavior is anything but passive. Eels fatten up to prepare to leave their home haunts in the bottoms of freshwater lakes and streams. Before they head for the sea, they absorb their digestive tracts and transform themselves by greatly enlarging their eyes and turning silvery on their ventral side. The latter transformation produces counter-shading that reduces their visibility to predators below them in the open ocean waters.
The eels’ dramatic changes in behavior, morphology, and physiology that enable them to switch from living in freshwater habitat to open ocean highlight the operation of strong selective pressures. But why do they leave their homes in freshwater ponds where they grew up and lived most of their lives? Their one-way, once-in-a-lifetime migrations to their Sargasso Sea spawning grounds can’t be to find a better feeding ground, or to escape competition. However, I suspect what the behavior accomplishes superbly is that the adults, which are predators, do not come in contact with and feed on their own young. Is it a mechanism that has evolved because it reduced predation on themselves?
There are over six thousand publications on eels, but the life cycle of these economically important food fish is still murky and has a long history of speculation. For centuries, nobody