The Homing Instinct

by Bernd Heinrich

  Let’s continue the story of honeybees by looking at their home-making. As soon as the colony moves into a tree cavity that will become its new home, the bees begin to work their mandibles along the walls, loosening any debris and rot, dragging it out, and flying off and discarding it. When solid wood remains, they smooth it by coating it with resins (collected from tree buds) mixed with wax. Small holes that might leak heat are plugged with the resin as well. At the same time, some bees begin to forage and return with their honey stomachs filled with nectar. A critical need now in the home-making is for wax to build the honeycombs.

  For over two thousand years, ever since Aristotle wrote on the subject, it was believed that honeybees gathered the wax to build their home from flowers, where they get their nectar and pollen. But after a long history we learned instead that young worker bees synthesize the wax, peaking in their production of it at around two weeks of age. The wax is made from sugar by glands between the abdominal segments. Foragers coming back to the nest without a place to deposit nectar indirectly sense the need for storage containers for honey and are stimulated to convert the sugar they carry into wax. It is extruded in little flakes that the bee picks up with a swipe of her hind legs and then transfers to the mouth to masticate. Then it is communally made into honeycomb.

  The precise regularity of honeycomb has been a source of wonder, if not astonishment. There has been debate about how the bees create the shapes of the cells. However, it is known that the workers require head movements to build the comb because if a bee’s head is experimentally immobilized (by gluing it to the thorax), she is unable to make comb. Other than that, and after 350 scientific papers about research on honeybee comb and after the publication of a book on honeybee wax, we still don’t have “the” full answer to how bees make honeycomb. We do know, though, the chemical composition of the wax, and that it has special properties with regard to temperature effects. Nest temperature is regulated at thirty-five degrees Celsius, and at that temperature the wax is solid but soft enough to be malleable, yet not so soft as to compromise mechanical structure.

  A typical honeybee home contains about a 2.5-meter-square comb surface with a total of about one hundred thousand “cells,” the hexagonal multipurpose units of the honeycomb. But no one bee makes a whole cell. Each adds onto what another has done, wherever it happens to be, and the result is one of the most beautifully and perfectly crafted structures in the animal kingdom.

  Forager bees regurgitate nectar-on-the-way-to-honey into an empty cell or one that already has honey in it, or they scrape off the pollen clumps from their legs and shove that into an empty cell or one that already has pollen in it. Honey production from the dilute nectar is accomplished mostly by climate control of the home. The high temperature that is maintained and the fanning by the bees to circulate air cause water to evaporate from the nectar until it becomes the thick, golden-colored honey, a product that is so concentrated that molds and bacteria cannot grow in it. It is the energy base that drives the hive activity in the summer and is “burned” (metabolized) by the bees in winter to produce heat by shivering.

  When by springtime the wax bins are emptied of honey and/or pollen, the queen inspects the combs and lays an egg into each empty one she finds (up to fifteen hundred per day), and it then becomes a “crib” for a single larval bee. When baby making is no longer on the agenda, the baby cribs revert back to food storage bins. Those cribs made for holding males (also used for honey and pollen storage) are larger than those for the sexually undeveloped females, the workers.

  Honeybee combs grow from the top down, and in the hive they are usually placed parallel to each other. The spaces between the combs, or “bee spaces,” serve as the crawl space where the bees travel around in the hive and also where they gather. In a hollow tree that is used as a bee home, the lower combs are preferably used as the nursery, and it is here that the dances occur, while the ones higher up in the “attic” of the home are used mainly to store honey.

  Our human home-making now is intimately related to the bees’, because honeybees are major food providers for us by crop pollination while also yielding us food directly. Many families do (or could) harvest all of their annual sugar needs in the late summer from one beehive, leaving enough to tide the colony over to the next spring. We have established a symbiosis with them. They give us honey and pollination, and we reciprocate by giving them home sites. Like most symbioses, ours started by exploitation.

  Access to the honey of bee trees in Europe had involved climbing the tree and hacking away the back of it to expose the hollow with the bee nest. After the honeycombs were taken out, the hole was covered with a door for future access. As the bee tree was periodically visited, the bees were sedated with smoke, new honeycombs were removed, and then the tree was sealed up again. Presumably the forest “beekeepers” left as much of the brood nest intact as possible. In the Bialowieza Reserve in Poland and Russia (possibly the only virgin forest left in all of Europe, where one can still find oaks, basswoods, and pines aged over five hundred years), there are still such centuries-old bee homes in some of the ancient trees once maintained by the now-long-gone forest beekeepers. In east Africa (Tanzania), wild bee management until very recently involved hanging hollowed-out logs horizontally from trees to serve as hives. In Egypt, from the days of the pharaohs, clay cylinders served and still serve the same purpose.

  Over most of the world, bees are now handled for agricultural pollination and honey production by providing them with prefabricated homes that accommodate the bees’ own extraordinary architecture. Such hives, the 1851 invention of the Rev. Lorenzo Langstroth of Philadelphia, consist of sections called “supers” that are set (like removable floors of a multistory house) one directly on top of the other. This arrangement allows one to enlarge (or contract) a bee home in successive stages as each super gets filled, and then also to disassemble it to examine the contents and remove the honey. But the main innovation of the Langstroth hive is that it channels the bees to build their combs into frames that can be moved, rather than as formerly when the honeycombs were solidly cemented into and throughout their home.

  Each super usually has ten removable frames that hold the bee comb that, as in a wild hive, may alternately serve as a brood nursery in the spring after it is emptied of honey, and then again as a storage facility for pollen and honey in late summer and fall. The frames can be pulled up by the beekeeper and the honey extracted by centrifuge, so the combs are used over and over again. Otherwise the combs would be destroyed each time we took the bees’ honey, and a typical bee home requires the bees to use up seven kilograms of honey to build about one kilogram of wax—and that does not include the labor of making the honeycomb from the wax. For most of our history before the Langstroth hive, and well into the present in some locations of the world, examining the inside of a bees’ nest and getting the honey out required the equivalent of destroying the house to get into the pantry. Giving bees a good home makes it easier for them to make the surplus honey that we take from them for our use, because they need to expend less work in house construction, and because we move their homes to areas where there are many flowers with nectar and pollen.

  The Langstroth hive has multiplied the honeybee population to extend to an ever-larger scale, and to distribute it to an ever-larger geographical range. It was made possible by our sensitivity to specific details relevant to the bees. Our relationship to bees extends our already unique stature as the only animal to be symbiotic by giving homes to diverse animals, including cattle, dogs, cats, horses, chickens, turkeys, and geese.

  The Langstroth hive gives bees an almost ready-made home and has made honeybee management possible on an industrial scale, as well as accessible to the amateur. Bees are now part of our home environment. They benefit all and strengthen the whole fabric of our existence. Perhaps one can exist without them, but what is the point of existence without seeing them working on flowers, hearing the music of their buzzing, tasting their honey
, and drinking their mead? (As if to answer my question, I received an e-mail almost immediately after I had written these words, informing me that the Harvard Microrobotics Lab has a team of computer scientists and engineers making a robobee to pollinate fields of GMO crops, because these robots would be immune to the heavy doses of pesticides and herbicides that Monsanto sells. The robot’s reputed “biggest drawback” is that “at the moment” it needs external power; it “flies” on a tether to a power source. Drawback indeed. In anything modeled on nature, power is the central ingredient. All else is window dressing.)

  Solitary animals also use mobile homes. Over eight hundred species of hermit crabs (both on land and in the water) avail themselves of the benefits of protection from predators and the elements by carrying discarded snail shells around on their backs. But good snail shells are often hard to come by, and some animals have evolved to construct their own mobile homes, marvels in miniature. Bag moth caterpillars make themselves little portable shelters that they stay in for nearly a year and do not leave until they are adult moths, and in some species they do not even leave them then. Their shelters are made of silk, smooth and white inside but bristling on the outside with all sorts of material, depending on the species. Bag moth homes may resemble a pine cone, or a miniature elongated log cabin with two entrances. The top entrance is where the larval caterpillar can reach out with its head and legs, and at the other end is an exit hole for disposing of wastes. The outside of the bag may be made of any kind of debris the caterpillar encounters: bits of grass, twigs cut to length, or other materials. The construction materials may be affixed either in longitudinal (“log cabin”) or perpendicular lines around the length of the bag. At the end of the summer, when the caterpillar has attained its mature size, it closes the front “door” by fastening the front end to a support. It then turns around inside its bag to face the other entrance, and then it pupates. The moth emerges from the pupa the next spring. If it is a female, having no wings and looking like a little sausage, she comes to the back-door entrance on the proper schedule to broadcast sex pheromone and wait for a male to mate with. If successful, she turns around and, with a long extension on the tip of her abdomen, reaches back into the back door and deposits her (up to several hundred) eggs into her just-vacated pupal skin. She may then return inside to die there and leave her body as a loose plug at one end of the sac. In some species, eggs never leave the female’s ovary, bypassing the stage of shedding the pupal skin.

  A home made by a bag moth caterpillar that is (as here) also used by its pupa. At left is the house made of about one hundred pieces of chewed-to-size twigs. The caterpillar inside leaves an entrance at the top from where it reaches out with head and legs and takes its house with it as it feeds. But after achieving full growth, it seals that entrance and attaches its house with silk to a solid substrate, such as a twig. It then turns around inside, molts, and becomes the pupa. The caterpillar’s former waste-disposal door will now provide the eventual exit door when the moth emerges, months later. This species (collected in Suriname) had an added feature: it enclosed the whole log structure in silk before pupating (center).

  One bagworm caterpillar I found on an acacia bush in Kenya had made its home by chewing off the bush’s long (about five centimeters), tough sharp spines and using its silk to glue them longitudinally into a tube around itself. It lived in this thorn tube and, like the bagworm mentioned previously, could extrude its legs and head from the opening left at the top in order to hold onto the bush, travel around on it, and feed on the leaves. When threatened, it pulled itself up tight against a branch. Still another bag moth home that I found in Suriname had an added feature to the otherwise similar longitudinal short sticks “log cabin” model. This caterpillar had covered or enclosed the whole “log” structure in a sheet of tough canvaslike silk. I cannot begin to fathom what its function might be, but it must be important because some very fancy acrobatics must be involved in applying it. It is easy to see how a caterpillar can make a silk case surrounding itself. But how does the caterpillar manage to surround something it’s already in?

  In their defensive mobile homes, these caterpillars can feed in the presence of predators, unlike most others that protect themselves either with poisons or hairlike sharp spines, or with exquisite mechanisms of hiding that greatly constrain when and how they are able to feed.

  Despite being strictly aquatic, the larvae of caddisflies (Trichoptera) make almost the same kind of mobile homes that the bag moth caterpillars do. Each caddisfly species has its own specific kinds of building materials and its own often very unique way of assembling them to build its home. In some, the materials used, such as cut pieces of live plants, dead twigs, pebbles, flattened stones, crustacean shell pieces, or pieces of leaf, are assembled in random (radial) tubes. But one species, Lepidostoma hirtum, makes a square case by silking rectangular leaf panels into four rows to make a four-sided tubelike box. These artistic creations have also inspired art; the French artist Hubert Duprat “produces” (sort of) variations of the naturally made caddisfly cases when he relocates larvae to aquaria in his studio and provides them with flakes of gold and precious stones, and they then build their cases entirely of these materials.

  In Alder Brook next to my home in Maine live at least five common caddisfly species, each making a very specific “case.” There the larvae have a large choice of available materials, and they exert their specific preferences. One of these species constructs a flattened case from sand grains that rests on the bottom of sandy stream areas. Another uses small pebbles; a third, long thin pieces of bark glued together lengthwise into a round tube home; and a fourth makes its house by cutting grass into short lengths and silking them crosswise rather than longitudinally like those of the bagworm moths described earlier. In each instance, the house mimics some feature of its surroundings and allows the occupant to move around freely. Because of its home, the caddisfly lives where it would otherwise, within minutes, probably be eaten by this brook’s trout and minnows.

  Samples of the “cases” made and used as portable homes by the larvae of different species of caddisflies. The four cases on the left are mobile and found in still water. The three on the right, found in flowing water, are fixed in place and also serve as food traps.

  One might suppose that the ultimate home is one that you do not need to either leave or travel to. And indeed, the larvae of one caddisfly species, Neureclipsis bimaculata, achieves both. But its strategy of making the home a food trap rather than going to the food requires a different diet and habitat, namely, flowing water containing plankton. This caddisfly larva extends and flares the entrance of its house with silk, so it acts like a seine. This home entrance faces the water current, and the larva stays in its home at the bottom of the net to catch prey that drifts in.

  A promethea moth caterpillar (Collosama promethea) making a home, the cocoon, for its eventual pupa. The caterpillar begins by attaching bands of tough silk that connect the petiole of a leaf to the twig. The leaf is rolled up and the inside is lined with a thick, tough layer of silk. During the winter the cocoon remains attached to the twig and mimics a dried leaf.

  A cecropia moth caterpillar (Hyalophara cecropia) may pull several leaves together (top), make a loose papery cocoon, and fill it with a thick layer of fluffy, cottony silk and a tough second shell inside. An exit hole is left for the eventual moth to escape in the spring. The caterpillar may also attach the cocoon without wrapping it to adjacent leaves (bottom), by simply attaching it to the sides of a twig. The caterpillar (center) finishes making the inside layer of its cocoon before it molts to pupa

  Some of the more obvious homes made by larvae, and most easily taken for granted, are the cocoons that house the pupae of moths. They serve as a safe place for the pupa, the immobile stage of the animal between the caterpillar and the adult stages. The majority of moths in the Northern Hemisphere spend about nine months in this stage, during which they are helpless and depend for survival almost enti
rely on their home. In some moth species, for example, sphinx and owlet moths, the larvae burrow underground and create a small cavity there that serves the same function as a cocoon. Many other moths create homes out of silk and stay aboveground. These may be fortresslike solid structures and may employ “tricks” that that hide the otherwise helpless pupa within. The silk used to make the cocoon of one species, Bombyx mori, is well known as the raw material with which we produce fine fabric.