Darwin's Backyard

by James T. Costa

  3. Graph a survivorship curve for all the seedlings in your sample, plotting percent surviving on the y-axis against census date on the x-axis.

  See also:

  K. E. James, “DNA Barcoding Darwin’s Meadow,” in Darwin-Inspired Learning, ed. M. J. Reiss, C. J. Boulter, and D. L. Sanders (Rotterdam: Sense Publishers, 2015), 257–270.

  J. McLauchlin, “Charles Darwin’s Lawn Plot Experiment,” The London Naturalist 88 (2009), 107–113.

  4

  Buzzing Places

  Insects always had a special fascination and charm for Darwin, especially beetles, his first love in natural history collecting. “Whenever I hear of the capture of rare beetles,” he wrote to his friend and protégé John Lubbock, “I feel like an old war-horse at the sound of a trumpet.”1 But in the development of Darwin’s evolutionary thinking, beetles played second fiddle to the Hymenoptera: ants, bees, wasps, and their ilk. Not for their diversity—this group is handily outdone in that department by beetles, the single largest taxon in the animal kingdom—but for their astonishing behaviors and even more astonishing life histories. Bumblebees first caught his attention, a group he knew as “humblebees.” Who is not familiar with the buzz and hum of bumblebees as they make their floral rounds, so evocative of lazy summer days in garden and meadow? In September 1854—the very month he wrote about feeling like an old warhorse—these bees caught his son Georgy’s attention while they were out one day. As Georgy, later Sir George, by then a distinguished astronomer, recalled many years later:

  I was once the unconscious means of making a discovery in natural history . . . When I was about 8 or 9 I was one day in August or September in the “Sand-walk” (so-called because at one time the path had been covered with red sand), & as my father paced round the walk I waited by the old oak tree at the end. On his coming round I told him that there was a humble bee’s nest in the tree. This he declared to be impossible, but I stuck to it that there was, & that the bees were going & coming from it. Accordingly we waited there & presently a bee came & buzzed about & went away, and then another and another. That there was no nest was obvious but the fact excited his curiosity & he determined to investigate it.2

  Georgy had discovered a bumblebee “buzzing place.” His father was delighted and puzzled in equal measure—he loved a natural history mystery. The “buzzing place” was a way station of sorts that the bees would briefly visit, dozens of them one after another. Darwin determined that the bees were male Bombus hortorum, the common garden bumblebee of Europe and Eurasia. He and Georgy started watching them, and soon discovered that this wasn’t the only buzzing place—the bees flew well-established routes along the hedgerows and paths of the garden, stopping to buzz at certain spots along the way. Just where they came from, where they were going, and what their way-station buzzing was all about were puzzles. They also found that keeping up with bees on the wing is not easy. Field assistants were needed, and the rest of the kids were soon recruited—besides Georgy there were Willy (15), Etty (11), Bessy (7), Franky (6), and even little Lenny, just 4.

  Over time they mapped 11 buzzing places over a distance of about 300 yards. In one investigation their father arranged them at intervals along the bees’ flight path, each near a buzzing place, so they could follow them as far as possible and also observe the buzzing places closely. “The routes,” he wrote later, “remain the same for a considerable time, and the buzzing places are fixed within an inch. I was able to prove this by stationing five or six of my children each close to a buzzing place, and telling the one farthest away to shout out ‘here is a bee’ as soon as one was buzzing around. The others followed this up, so that the same cry of ‘here is a bee’ was passed on from child to child without interruption until the bees reached the buzzing place where I myself was standing.”3 The insects were too fast to follow for long. To improve visibility Darwin tied a flour-dredger to the end of a stick, stationing himself at a buzzing place at one end of the bees’ route, and giving each bee a liberal sprinkling as it appeared. As far as they could tell the bees were unfazed by being flour-coated. George remembered that they “could see the bee much further when he was whitened all over. We traced the bees over a line of 300 yds or so, & then they disappeared like lightening [sic] over the corner of the kitchen garden wall.” Another time Darwin followed the bees into a dry ditch thick with brush, where he could just see the insects fly slowly along the ground between the thorn bushes. Too overgrown for an adult to get through, he marshaled the troops: “I could only follow them along this ditch by making several of my children crawl in, and lie on their tummies, but in this way I was able to track the bees for about twenty-five yards.”4 The kids didn’t seem to mind; in fact, they found it a great adventure. To Lenny their buzzing place study seemed part game and part scientific investigation, and he recalled how, insofar as they were playing, “my father was like a boy amongst other boys.”5

  The mysteries were many: How did the bees recognize the buzzing places? And, since only new bumblebee queens overwinter, how did the bees of successive generations return to much the same buzzing places as the previous years’ bees? Darwin kept field notes, recording his and the kids’ observations over the course of three years. He noticed that the bumblebees seemed to prefer to fly along hedges and paths, and to buzz around the bases of trees, concluding that the same routes and the buzzing places must have some kind of attraction for them. He was at a loss to identify anything special about the buzzing places, though. He experimented with changing the appearance of some buzzing places by removing vegetation and sprinkling flour on the spot, yet still the bees knew where they were.

  This suggested that the bees did not locate the buzzing places visually. Darwin and his young field assistants confirmed this with another experiment, this time by hiding a buzzing place with a net. Still, the bees stopped there: “Hoop-net . . . did not prevent bees coming . . . so not guided by vision,” he jotted in his notes.6

  Sketch of the routes of male bumblebees and their buzzing places at Down House, 1854–1861. Adapted from Freeman (1968), p. 180 by Leslie C. Costa.

  What was going on? Darwin never did figure it out. The bees were using markers that naturalists only really cottoned onto in the twentieth century: pheromones, airborne chemical cues secreted from specialized glands. He didn’t realize how close he was to putting his finger on it, though. In his field notes from July 1856 Darwin wondered: “How on earth do bees coming separately out of nest discover same place, is it like dogs at corner-stones?” Dogs, of course, mark their territory with urine—using scent to say something to other dogs. Later study showed that the bees, too, use chemical cues, marking their buzzing places with secretions from their mandibular glands. The bees are not being territorial, however. The fact that the patrolling and buzzing bees were all males might have been a clue to Darwin that this had something to do with mate-finding. The amorous males scent-mark prominent objects or sites (to them, anyway) along set paths they patrol, hoping to attract new queens. Different species establish routes at different heights. Darwin and his children were fortunate that the common garden bumblebee is a low flier or they may have missed the buzzing places altogether. No matter that they never solved the mystery. Lenny (then Major Leonard, sometime military man, MP, and economist) recalled how deeply he and his siblings were impressed by their father’s keenness to understand the phenomenon. “The fact,” he mused, “that it never even for a moment occurred to him to try to hide his entire ignorance of the meaning of the habit which we were watching was probably not without beneficial educative effects.”7

  It’s not clear why Darwin decided to conclude his investigation of bumblebee buzzing places, except that about the same time, in early 1858, his attention was increasingly consumed with another sort of buzzing place: bee hives, and the waxen cells built by bees.

  The Most Wonderful of All Known Instincts

  In early 1858 Darwin was poring over experiments, diagrams, and calculations relating to the cells construc
ted by honeybees and related bees and wasps. It was his latest hobbyhorse, and he was utterly consumed. Why honeybees? Insofar as honeybees were thought to construct their cells and comb with the accuracy of an architect equipped with the latest precision tools, they had long been celebrated as the height of instinctive intelligence in animals. They thus posed a special challenge, because to many they defied naturalistic explanation. These insects were already animals with a powerful mystique and great symbolic significance. They were exemplars of perfect monarchy or commonwealth to political philosophers, models of industriousness in folklore, an irresistible mascot for communities emphasizing the collective spirit over the individual. Beyond that they were hailed as living proof of divine design.

  Rev. William Kirby, vicar and entomologist of Kirby and Spence fame (the authoritative textbook of entomology Darwin knew backward and forward), described honeybees as “those Heaven-instructed mathematicians, who before any geometer could calculate under what form a cell would occupy the least space without diminishing its capacity, and before any chemist existed to discover how wax might be elaborated from vegetable sweets, instructed by the Fountain of Wisdom, had built their hexagonal cells of that pure material.”8 A similar thought came from the pen of Henry Lord Brougham, barrister, politician, and intellectual, who wrote Dissertations on Subjects of Science Connected with Natural Theology in 1839. He made an argument about bees reminiscent of Paley’s watch and watchmaker: the kind of “disciplined labor” seen in bees is only found in people under the direction of a “superintendent with a design.” The bees, too, must therefore be guided by such a superintendent. Darwin made many marginal notes in his copy of Dissertations, scribbling on one page that bees’ cells were “very wonderful—it is as wonderful in the mind as certain adaptations in the body—the eye for instance, if my theory explains one it may explain other.” Indeed, to natural theologians the constructions of bees were the behavioral equivalent of the eye. Darwin well knew that William Paley had written that the “examination of the eye [is] a cure for atheism,” quoting Saint Sturm, a medieval missionary. If he could not explain such wonderful structures as the eye or honeybee comb in light of natural selection, Darwin knew his theory would be seen as fatally flawed.

  Honeybees (Apis mellifera) and their relatives are remarkable in their abilities, including in ways that Darwin and his contemporaries never knew. These insects exhibit one of the most complex forms of social behavior found in nature, each colony essentially a large family headed by the queen. Only she reproduces, her myriad offspring functionally sterile and laboring to assist their mother to rear more siblings. The family is mainly female: males are transient and are produced infrequently. The life of the honeybee revolves around the honeycomb, each consisting of two layers of cells arranged back-to-back, opening on opposite sides. Fashioned largely of wax secreted by special glands on the underside of the female bees’ abdomen, honeycomb is both nursery and larder: some cells are used to rear brood while others become honeypots. An average hive can easily consist of 100,000 cells or more, often distributed among multiple combs. A hive of that size can take well over 2 pounds (1 kilogram) of wax to build, providing space enough to rear thousands of bees and store 44 pounds (20 kilograms) or more of honey (a substance that, as entomologists like to point out, may be considered the world’s first stored-food product). Honey is the colony’s winter food supply, made from flower nectar modified by the bees’ salivary secretions and evaporation—but it’s best not to think about eating bee spit as you enjoy a dollop with toast or tea. Protein-rich pollen is mainly used to rear the brood, fashioned into “bee bread” by mixing it with honey.

  It is difficult to overestimate the powerful allure of these bees and the irresistibility of seeing them through a human lens, like engineers or architects setting out to solve a building problem. But the bees have no abstract concept of a design or a problem, let alone an ability to cogitate on solutions.

  A Recondite Problem

  In the lead-up to the events of 1858, Darwin was busy as a bee. Home improvements were on the agenda in 1857, building a new dining room with a bedroom above to accommodate the needs of a growing family. Worries continued, too, about Etty’s health. She was periodically whisked off to the coast where her fretful parents hoped the sea air would do her some good. In research, Darwin oversaw his Lawn Plot and Weed Garden Experiments that year, observed the diversity sheltered within heathland enclosures, and computed (and frantically recomputed) his botanical arithmetic. But of course there was more—he also experimented with crossing chicken breeds to study their coloration patterns, documented the striping patterns of horses, and continued experiments with floating dried plants in saltwater. And through it all, he chipped steadily away at his big species book, Natural Selection—six chapters were completed in 1857, with the last of these, “Hybridism,” finished on December 29th. The family had no sooner rung in the new year when Darwin had already jumped into the next chapter, on the “Mental powers and instincts of animals.” That was when he became infatuated with bees.

  In his earliest notebook jottings on honeybees Darwin only remarked on how bees must have an instinct for measurement, given the cells’ geometrical properties. That instinct now became an urgent issue for him. All through 1857 he had collected random bits of information on cells built by different kinds of bee, but with the initiation of his chapter on instinct in 1858 he started to think more deeply about cells and comb—perfect networks of waxen amber-hued hexagons. These insects were said to “have practically solved a recondite problem,” he later wrote. How did bees achieve that regularity, coordinate their building, and come to use hexagonal cells, a shape that happened to be the best possible one to maximize cell number per unit area? Hexagons are optimal for cell-packing, as geometricians since classical antiquity appreciated.9 The problem for Darwin and his contemporaries was, how did the bees “know” this, if that’s the right word? The natural theologians claimed that they were divinely inspired.

  Darwin sought to demystify the cell-building behavior of these remarkable insects, knocking them off of their apian pedestal (or at least bring them down a notch or two—he was a great admirer of bees, after all). He soon came to a twofold approach. First, he sought to show that teamwork was not necessary per se to produce beehive cells. Instead, the action of multiple independent individuals, each attuned to its own work, resulted in multiple hexagonal cells (a process reminiscent of Adam Smith’s “invisible hand”). Second, he had a hunch that by stepping back and looking at honey bees’ structures in relation to those of other bees, he would find less complex nests that would look like transitional steps along the evolutionary pathway to the honeybees’ perfect honeycomb (although not necessarily an actual evolutionary series). Darwin was sure that highly complex structures were not pinnacles of perfection, isolated and unreachable from the rest of nature, as natural theologians believed (and as their intellectual descendants, today’s intelligent design fans, still believe). A transitional series pointed to a stepwise process, moving from the simple to the more complex.

  When grooming a new “hobbyhorse” Darwin would read up on the subject, exhaustively scouring the literature and pumping anyone he knew for relevant information. He soon came across an 1835 encyclopedia article on bees by George Waterhouse, a naturalist and friend at the British Museum who had described Darwin’s mammal and insect specimens from the Beagle voyage. Waterhouse gave him the seed of an idea: he thought that hexagonal bees’ cells started out cylindrical, circular structures being easily made merely by pivoting around a fixed point. This is why circular or cylindrical structures are so common in the animal world, from birds’ nests to burrows to the leaf discs excised by leaf-cutter bees and ants. “We observe,” Waterhouse wrote, “that the works of almost all insects (perhaps we may say almost all animals) proceed in circles or segments of circles. The cells of almost all the various species of bees are of this construction.”10 And so, in working a mass of wax honey bees would “mos
t probably” fashion cylinders. “Supposing,” he continued, “the circumferences of three cylinders were to touch, the bees working in each of these cylinders would cut away the wax at the point where they meet.” Because each cylinder would tend to be surrounded by six others (the only number of equal-sized circles that could fit symmetrically around one of the same diameter), as wax is removed from the spaces where each pair meets and flat walls are built between them, the cylinders would become hexagons. This is the simplified version: there was much more detailed math to the honeycomb’s construction, and Waterhouse gave an elaborate discussion of how the rhombic bases of cells develop and how cells are fashioned in back-to-back layers into comb—far more detail than we need to get into here.

  George Waterhouse’s diagrams illustrating his hypothesis for the origin of hexagonal bees’ cells. (Left) The cells start out cylindrical (a shape easily built by pivoting around a fixed point as they build). The bees then excavate the empty spaces between touching cylinders and flatten the walls where cylinders meet (a, a, and a). (Right) As this proceeds with other surrounding cells, the flat walls surrounding each cell collectively take on a hexagonal shape. From Waterhouse (1835), figs. 7 and 16.

  Darwin was intrigued. He wrote to Waterhouse for more information, including details of cell building by wasps and solitary bees. He thought his friend was basically correct, but that there was more to it. How did cell-building by the honeybee’s relatives compare? What were the steps in cell construction? Did cells in the middle of the comb, surrounded by other cells, have the same shape as those on the edge? Was each cell built by just one bee? He had so much work to do, but could not get bees’ cells out of his mind. In one almost stream-of-consciousness letter written to Hooker in late February 1858, he dipped into topic after topic like one of the foraging bees he loved to watch in the garden. Does Hooker know what bees are pollinators in New Zealand? Will he kindly answer the enclosed questions on classification? Can he spare volumes 10, 11, and 12 of Candolle’s flora again? Are all the species of a genus classified together because they really are very similar, or because there are fewer differences between them and those of other genera? But he closed the letter with a comment revealing what was really on his mind: “Farewell, I have partly written this note to drive Bees-cells out of my head; for I am half mad on subject to try to make out some simple steps from which all the wondrous angles may result. Forgive your intolerable but affectionate friend.” A week later he wrote Hooker again, thanking him for the botanical books and other information, and declaring that lately he had been “beyond measure interested on the construction instinct of the Hive bee.”11