The Homing Instinct: Meaning and Mystery in Animal Migration. Bernd Heinrich

       The young black cherry tree showing relative locations of a tent caterpillar moth egg cluster (C) from which the clutch of just-hatched caterpillars emerged and traveled to start making their home (H) in a crotch of the tree, and their first travels as a group (T) to feeding places

      At noon they came out and crawled onto the outside of their tent, waving their heads back and forth, apparently weaving silk from their salivary glands to enlarge it. Another hour later they were again all massed inside the tent and perched, immobile, tightly against the bark, where they were barely visible through the thin gossamer veil of silk.

      The caterpillars stayed in their tent through the night, and I expected them to go at sunup to the same branch where they had been the day before. But instead, this time they all followed an entirely different path, going directly up the tree instead of down as on the previous day, and without taking another side branch. I could not detect any silk on their so-far two different foraging trails, and this time they went even farther — a distance of 130 centimeters. After their one meal the day before, they were already noticeably larger. A few were the same size as the day before, but most had probably doubled in weight. There were many tiny fecal droplets in their web. So they had fed, even though it seemed hardly possible that they had anything to feed on at the barely opening bud.

      On the third day the buds had opened and the tree was replete with new small leaves pushing out of the buds. But it had been a cool night — there was again frost on the ground at dawn — and the caterpillars made a slow start.

      The pattern soon became clear: the caterpillars spent most of the night and most of the day when they were not feeding in their home. The time spent on tree branches was brief, and it could not have been just to keep warm that they stayed in their home because they went back inside just as quickly after feeding regardless of temperature or time of day.

      Having found and watched the caterpillars of one tent, I then observed others for more clues to their homing behavior. One of the surprises to me was that as they grew larger, they foraged independently of one another, no longer going to and from feeding areas in groups. Furthermore, after they were about half grown they left their tents, not to return at all but still to continue feeding before eventually searching for a spot in which to spin their flimsy cocoon. Tent caterpillars usually choose a bark crevice to pupate, although commonly they also choose the cracks in the sides of buildings. But why were the young caterpillars strongly homebound and the older ones not?

      I suspect the young ones’ web-making behavior may have evolved in part as an anti-predator response. The tents were visited by red wood ants, Formica rufa, and right after the caterpillars hatched, these ants often loitered alongside them on their trails. I tore a nest open on one side to find out if it served as protection. It must have, because ants entered, though frequently wiping their antennae as though irritated. Nevertheless they tarried inside the damaged nest, and I saw them grab and walk off with caterpillars. No ants entered an intact nest of the several I watched, each of which consisted of several successive layers of silk. Thus, the webbing of the tent acts as a deterrent to predators such as ants. Staying inside the home most of the day and night, as these caterpillars appear to do when they are small, probably reduces mortality from parasitic flies and ichneumon wasps as well. When they are larger, the caterpillars are probably protected from the ants, as well as from most birds, by a layer of fine spines. They pupate without having to bury themselves to escape frost, because the adult emerges long before there is any frost.

      Because these caterpillars are protected from predators in the summer homes they build and by the spines they wear, because they mature early enough in the summer for the pupae to avoid the cold of winter (by early emergence of the moth), and because the eggs and young larvae are immune to freezing because of the antifreeze they contain, “everything” in the life of the tent caterpillar moth may be found within a few meters. The adults that emerge in late June are not far from the apple or cherry trees where the parent left her eggs, and their life cycle can be completed without their having to go far from home, unlike some other insects which traverse a continent to be able to satisfy all their needs.

      Monarchs. Of all the insects, the travels of the monarch butterfly, Danaus plexippus, are perhaps most famously spectacular in both scale and scope. Dr. Lincoln P. Brower of the University of Florida in Gainesville (now at Greenbriar College), who has studied this butterfly and its migration for over forty years, records the rich history of the emergence of our knowledge of monarch migrations. Early naturalists saw “immense swarms” in the prairie states where the caterpillars fed on the leaves of the many native species of milkweed (Asclepias) and the adults fed on the nectar of their flowers. Monarchs declined when later industrial agriculture destroyed many of their food plants, but in the nineteenth century they resurged in the East due to land clearing and the spread mainly of one milkweed, A. syriaca. Millions of them were seen passing for hours, even in Boston. This was a phenomenon that is hard to imagine now and it ignited much interest then. Charles Valentine Riley, the entomologist who first hypothesized that these butterflies engaged in a birdlike migration, cites people seeing them in the fall in swarms that extended for kilometers and obscured the sun, “blurring day into night.” Huge lines of them passing Boston in 1880 were described as “almost beyond belief.” Now, with reforestation, plowing, and then the use of Roundup and other weed killers that eliminated their food plants in agricultural fields, the monarch is but a shadow of what it was. In the past several years in the East, it seems to have almost disappeared. For the first time, I saw not a single one in late summer of 2013. But our knowledge of the scope of the monarch migration has blossomed.

      Monarchs migrate on their own power for thousands of kilometers, and, unlike many other insect migrants, the population (though not the individuals) has a regular two-way migration, although as with the other insect migrants, the individuals that come back are not the same ones that left.

      Unlike most of the other North American butterflies and moths, which overwinter in New England as eggs, larvae, pupae, or adults, monarchs cannot survive there through the winter in any stage. The population that normally now graces fields all along eastern North America overwinters at around three thousand meters’ elevation in dense fir groves on the southwest slopes of volcanic mountains thousands of kilometers to the southwest, near Mexico City. The monarchs find shelter in those fir stands from rain, hail, and occasional snow. It is not cold enough for the butterflies to freeze there, but it is cool enough for them to conserve the energy resources that they have accumulated on their way south.