Darwin on bumblebees: the quick and clever workmen

We could be tempted to think about insects as little 'robots', behaving blindly and rigidly responding to external cues following an internal instinctive program. However, in the last few decades, research on insect learning has taken off providing an astonishing amount of data on the behavioural plasticity and learning abilities of these animals. Much of the research has taken place using bumblebees as models as they can be easily kept in the laboratory and many individuals can be produced and used in experiments. Bumblebees, as social bees, have a rich behavioural repertoire. After emerging from their natal nests, queens explore their environment to find a suitable nesting site, feed themselves with nectar and pollen, build their nests, make cells, lay eggs and provision their offspring with more nectar and pollen. Foraging involves taking decisions as to which species of plant and which individual flowers to visit, the available flowers changing continuously as the season progresses or from site to site. In addition, not all nectar and pollen is easy to collect: in many nectar-rich flowers, nectar is located deep inside the flower (e.g. Honeysuckle, Nasturtiums) and pollen from different flowers has to be gathered in various ways. Recent studies illustrate the flexibility of individual bee learning, but also the less known aspect of social learning: bees not only learn on their own (from trial and error) but also from each other - and even from other species. Despite the flurry of recent research, many of the facts had been observed and hypothesized by Charles Darwin over a century earlier. Darwin never ceases to amaze me: he was a meticulous, keen observer, paying attention and being enthused by the littlest things; never ceasing to test hypothesis, clearly illuminated by his encyclopedic knowledge in all things related to plants and animals. He gathered observational and experimental data painstakingly, often in his own garden and sometimes even enlisting his own children.
He wrote on social learning:
"One day I saw for the first time several large humble-bees visiting my rows of the tall scarlet Kidney Bean; they were not sucking at the mouth of the flower, but cutting holes through the calyx, and thus extracting the nectar. I watched this with some attention, for though it is a common thing in many kinds of flowers to see humble-bees sucking through a hole already made, I have not very often seen them in the act of cutting. As these humble-bees had to cut a hole in almost every flower, it was clear that this was the first day on which they had visited my Kidney Beans. I had previously watched every day for some weeks, and often several times daily, the hive-bees, and had seen them always sucking at the mouth of the flower. And here comes the curious point: the very next day after the humble-bees had cut the holes, every single hive bee, without exception, [...] sucked through the cut hole; and so they continued to do for many following days. Now how did the hive-bees find out that the holes had been made? [...]  I am strongly inclined to believe that the hive-bees saw the humble-bees at work, and well understanding what they were at, rationally took immediate advantage of the shorter path thus made to the nectar." (Darwin, 1857)
And on the use of experience and memory:
"I observed also bees flying in a straight line from one clump of a yellow-flowered Oenothera to every other clump of the same plant in the garden, without turning an inch from their course to plants of Eschscholtzia and others with yellow flowers which lay only a foot or two on either side. In these cases the bees knew the position of each plant in the garden perfectly well, as we may infer by the directness of their flight; so that they were guided by experience and memory." (Darwin 1876)

A fragment of a description of social learning in bees by Darwin in his work "The effects of cross and self fertilisation in the vegetable kingdom" (1876)

A recently emerged Bombus lapidarius queen feeding on Agapanthus

A Bumblebee Bombus terrestris, worker gathering pollen from a poppy

Links
All of Darwin's work, books, articles and letters are now freely available and fully searchable online in The Complete Works of Darwin Online.
Social learning in insects, a recent summary