Wild Side: Face to face with bees

Interactions with flowers have driven the evolution of a wide variety of bee faces.

Male Colletes validus, showing facial geometry that has evolved to fit blueberry flowers. — Matt Pelikan

You probably think about bees from time to time. Their role as pollinators has come increasingly into the public eye in recent years, while apparent declines in bee numbers and diversity have focused public attention on these important insects.

But do you ever think about bee faces? No, I didn’t think so. And yet the anatomy of the face is where a great deal of bee diversity plays out. Scientific keys used for bee identification, and the formal descriptions that literally define the line between one species and another, are virtually obsessed with the facial anatomy of these insects.

More surprising, and in some ways more important, the ecological diversity of bees often finds its clearest anatomical expression in bee faces. While some bees happily feed on virtually any flower that’s available, a significant percentage of bees — and the vast majority of bees that are of conservation concern — exhibit more or less clear preferences for some types of flowers over others. These preferences can be extreme, with some monolectic bees relying entirely on just a single genus or species of flower.

It’s easy to see why bee interactions with flowers have driven the evolution of a wide variety of bee faces. Bees feed on flowers, with both bee sexes sipping nectar as fuel, and with females of most bee species transporting flower pollen to provision nests. Flowers vary in shape. So the front ends of bees, which are the parts that interact most intensively with flowers, have quite reasonably evolved to facilitate foraging, fitting readily into particular flower types.

I recently encountered a shining example of this principle. Colletes validus, also known as the blueberry cellophane bee, lives up to its billing by showing a strong association with blueberry flowers. The adult flight period of this medium-size bee starts in late April, just as the first blueberry flowers are opening, and adults become rare or absent by late May, the females wrapping up nest provisioning just as the final blueberries drop the last of their blossoms. I was able to watch a male Colletes validus forage on highbush blueberry blossoms outside my office building, and was able to collect him as a specimen to document the presence of the species and allow close examination.

The feeding efficiency of this bee was impressive. It perched for only a few seconds on each blueberry flower, hugging it like a giant coffee mug, before moving on to another flower (often the one right next door). And immediately upon landing, the bee inserted its face fully into the deep, bell-shaped blueberry blossom. You’d think the bee’s face had been engineered specifically to fit perfectly into a blueberry flower — and indeed, that’s precisely the case.

My first impression on viewing this bee under a dissecting microscope was to think of a stale old joke: An anteater walks into a bar, and the bartender says, “What’s with the long face?” C. validus can’t rival an anteater’s elongated schnoz! But the face of a blueberry cellophane bee compares to that of a short-faced bee about as the snout of a collie dog compares to that of a boxer. The clypeus, a plate of the exoskeleton that covers the lower face, extends far below the bottoms of the bee’s eyes. And the so-called malar area — the section of the face between the bottom of the eye and the base of the mandible — is vast compared with other bees I’ve studied. The tongue of C. validus is not notably long; for some reason, in evolving to feed on bell-shaped blueberry flowers, this bee opted for an elongated face rather than a longer tongue, and it works perfectly.

Facial structure isn’t the only way bees optimize their interactions with flowers. Bee tongues, for instance, vary widely in length and shape. Roughly analogous to our own tongue, a bee tongue is a sort of tubular proboscis which, when not in use, folds neatly into a slot on the underside of the bee’s head. When unfurled, the tongue allows its owner to tap into the nectar resource of a flower. Unsurprisingly, many bees that prefer tubular flowers have developed long tongues, while many generalist bees have short tongues that are adequate for feeding from a wide array of shallow flowers, such as apple blossoms or the flowers of milkweeds.

Behavior also plays a role in optimizing foraging. Carpenter bees, with relatively short tongues and broad, cumbersome faces, often access flower nectar by using their robust mandibles to chop holes in the bases of flowers, creating a side entrance for their tongues. And tiny bees in the genus Lasioglossum often crawl all the way into blueberry flowers, disappearing from sight as they feed fully enclosed within the flower.

But facial anatomy really matters. If you ask Colletes validus, “What’s with the long face?” it will demonstrate by burying its face neatly in the nearest blueberry flower.