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

up at B, the new location. When crowds of bees had done the same, we removed one station and put the remaining one into the middle, between the two original sites. Next we moved our feeder to site B. Most of the bees, such as 30 green with blue and 39 green with red, who had been at the previous site, showed up. That is, we had trained bees who had been at one site to come to the second site, so we knew they now knew two sites and could potentially use either as a reference site to return home.

      For the planned experiment it was important that the bees forget the intermediate sites that had been instrumental in getting them to go to the two widely separated sites. So, for the rest of the day, we alternately fed the bees, first at site A, then at site B, and monitored which individuals were showing up at both sites (most of the individuals continued to forage at either one or the other site).

      We were now, near the end of the day, finally ready to move from training to trials. The experimental plan was to select one of the bees who knew both sites. This bee would, after feeding at one site and getting ready to leave, be captured in a dark box and thus “blindfolded” and then brought to a third feeding site where she had never been before. Here she would be released after being equipped with a radar-tracking transponder. We presumed that she would do at least one of three things: she might recognize where she was and fly straight home; she might instead fly off in her original (now wrong) direction; or she might immediately know that she was at an unknown location and search until she found one of her two feeding sites and from there take a direct beeline home. Knowing her exact flight path would allow us to distinguish among the alternatives, which would be essential to ultimately decoding her homing mechanism. Setting up this experiment had taken a long time, but I would now, possibly, be treated to an exciting demonstration of bee homing, one I could never have imagined possible.

      Menzel picked up his walkie-talkie to call the radar station: “Mike, we’re now going to put a transponder on a bee — are you ready?” Mike had spent some years in the army where he was trained on radar, and he was now working part-time while getting a university degree in computer technology. He replied yes, he was ready. Menzel then took me to feeding station A, where a whole lineup of bees was coming and going.

      “Which one do you want?” Menzel asked me. I wanted a bee that I had gotten to know over the course of the day, so I chose 39 green with red-tipped abdomen. We waited for her to arrive and let her feed for a while. As planned, Menzel then held a glass vial over her while she was distracted sucking up syrup. When she was full, she walked up into the vial, and Menzel corked it shut and darkened it by wrapping his hand around it. We then took her to a site distant from both feeders, a place where she had not previously fed and from where we would now release her.

      The vial holding “39 green” had a plunger at the bottom with a wide-mesh screen at the top. Menzel gently pushed the plunger in and forced the bee up against the screen, held her there, and picked up a tiny transponder (a wire holding a diode with a sticky pad at one end). With fine tweezers, he deftly removed the protective paper from the sticky pad and glued the transponder onto the top of the bee’s thorax. “Ready?” he radioed Mike.

      “OK.”

      Menzel removed the plunger and held the vial with the open end up, for the bee to crawl up. She hesitated at the lip of the glass, groomed her antennae, and then lifted off. She showed no strain in flight. (The transponder’s weight is twenty milligrams, and a bee can fly with double her body weight, carrying a hundred-milligram load of nectar in her honey stomach plus two pollen packets on her hind legs.) However, she flew only two or three meters before dropping down into the grass, stopping to preen herself some more. But a couple of minutes later, she finally took off again. Mike, who was now monitoring her flight, radioed us. At intervals we heard: “She is heading south-north-east-north-west-south.” Then, finally, Mike continued: “Now her path is straightening out — now she is heading directly for her hive!”

      She had suddenly oriented correctly. This was the crucial point: she had apparently recognized something that had “placed her on the map,” so that she then “knew” in what direction to fly to reach home. Assuming she had taken a path she had never taken before, did her successful homing suggest a “map sense”?

      I ran over to the radar tent where Mike showed me the radar screen and the dots where the three-second successive readings traced the bee’s path. A computer screen, where software had converted the time and directions of the bee’s flight path into different-colored images for easy reading, showed that the bee’s original flight direction was toward where the hive would have been had we not moved her from her feeding spot. In other words, she acted as though she didn’t know where she was when we released her. As expected, however, after she reached the area where her hive would have been, she flew loops in several directions. Then, after she had flown ever-farther away from both her real and “would-have-been” hive locations, she suddenly seemed to orient and flew directly toward the hive. Amazingly, it was along a route that had not been her normal foraging route from her two feeding sites. Had she perhaps seen a blue or a yellow tent and, having learned their relationship to each other during previous orientation flights, transposed that information to fix her new location? Only more bees could tell.

      Other bee homing experiments with hundreds of bees were ongoing. And in the group’s final publication two years later, the thirteen-author research team headed by Menzel concluded that honeybees incorporate information for flight direction from both their previously learned flights as well as landmarks and from the flight directions learned from hive mates within the hive. But they can redirect their flight vectors to and from the hive and perform novel shortcut flights between the learned and the communicated vectors.

      “The” homing instinct, recognized and traded on by every American beeliner to get honey, and used by von Frisch to decipher the bee language, is a source of fascination and mystery still. Von Frisch had likened it to a “magic well” from which the more you take, the more runs back in. The “well” is still doing that, three-quarters of a century after his prophetic pronouncement.

       GETTING TO A GOOD PLACE

      THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter — about a half centimeter — she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.

      The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.

      On the second day after I found the tent, May 1, there