The marvel of bee mandibles

Dogs. Cats. Mice. Your next-door neighbor. What do they all have in common? One answer is a moveable lower jaw. In fact, lots of creatures have one movable jaw—known as a mandible—and one stationary jaw or maxilla. But your bees, along with many other invertebrates, have two movable jaws, both known as mandibles. Not only do they both move, but they swing in and out instead of up and down. Cool, right?

In bees, the mandibles attach to the head at each end of the labrum. The labrum is a short, wide flap that functions much like an upper lip and protects the mouthparts when they are not in use. Each mandible is controlled by two muscles, the abductors and the adductors, which pull the mandibles in and out. At rest, the mandibles fold over the front of the labrum where they are barely visible.¹

An All-Purpose Tool

Never underestimate the value of a bee’s mandibles. Before emergence until the end of life, bee mandibles are in constant use. The first thing a honey bee worker does with her mandibles is chew her way out of the cell where she developed from a fertilized egg. As she emerges, she perforates the capping until it opens like a manhole cover, and a nurse bee working in the area uses her mandibles to set aside the cap for later use.

Once free, the newly emerged worker goes back down the hole, head first, to prepare her brood cell for the next bee. Using her mandibles, she scrapes debris from the cell walls and polishes the waxen surface, assuring it is clean and smooth for the next sister. In the meantime, slightly older bees mend damaged comb or build new ones by using their mandibles to manipulate the wax.

Stabilizing the Proboscis

Among other things, bee mandibles stabilize the proboscis. The proboscis is a long and flexible multi-piece tongue that could be easily damaged, so the mandibles act like a portal to protect the tongue and support it while in use. When a bee is not using its proboscis, it is folded into a compartment inside the head. There, the mandibles close over the front of the mouth, one over the other like a pair of crossed arms.

A study is now underway to determine which mandible folds over which. After looking at 50 members in each of 28 different species divided by sex, preliminary data show that it’s roughly 50/50. In other words, half the bees fold the right one in first and put the left one on top, while the rest do the opposite.² The count for honey bee workers was exactly 50/50—the perfect trivia question for your next bee club meeting.

Feeding the Young

One of the early jobs of a newly emerged worker is feeding the young larvae, and worker mandibles are perfectly designed for this task. The inner surface of each worker mandible is depressed into a spoon shape, and the depression is connected to each end of the mandible by a set of channels. By repeatedly opening and closing her mandibles, the brood food is squeezed from the worker’s glands and transported through the channel, filling the depressions behind the mandibles like a reservoir.¹

When she is ready to feed larvae, the nurse bee places her mandibles in the brood cell and allows the food to flow from the reservoir into the cell. The worker parcels out the exact amount needed and then moves to the next, wasting nothing. She continues feeding until she must once again refill the reservoirs.

Jaws Fit for a Queen

The queen bee has mandibles, too, but hers are different. A queen’s mandibles are specially designed to cut through the tough layers of wax that form the cell where she matured from egg to queen. Unlike a worker cell with its flaky cap, a queen is sealed within thick and sturdy layers of wax. The queen must literally saw her way out of the cell, making a neat circular cut at the bottom end.³

But does the queen put away her mandibles once she’s free? No way. After emergence a new queen will search the brood nest looking for other virgin queens that may be still be resting inside their sealed chambers. When she finds one, she uses her mandibles to slice an opening in the side of the cell just big enough for her to back in and sting the unfortunate occupant. She will repeat this act again and again until she’s the last queen standing.

Drones, too, have mandibles, but since drones are not big on manual—or rather mandibular—labor, their jaws are very small. In fact, their mandibles are so small that drones often need help emerging from their natal cells, but with so many sisters close at hand, big mandibles are not necessary.

Beyond the Nursery

As you can see, mandibles play a vital role in the lives of young bees. But that’s just the beginning, especially for workers. Mandibles will play a role in almost everything a worker bee does for the rest of her life.

Comb building is an excellent example. Comb construction begins when young workers secrete wax from four pairs of glands under their abdomen. Although it oozes out in liquid form, the wax quickly hardens into small disks that look similar to the scales of a fish. Workers collect these flakes by stabbing them with spines on their legs and passing them forward to the mandibles where they are made pliable by chewing. The worker adds enzymes from her salivary glands, chewing and manipulating the wax much like a chipmunk working an acorn. Busy, busy.

When the wax is soft and malleable, it is ready to become part of a comb. The worker uses her mandibles to cut, form, measure, and polish each individual cell. In essence, her mandibles are cutters, rasps, calipers, and pliers—all the tools she needs to build a complex structure of interlocking hexagons, all angled up just enough to keep the nectar from running out. How clever is that?

Outside of their hive, honey bees use their jaws for manipulating flower parts and even for nectar robbing. Although honey bees are hesitant to pierce a flower petal, they are not above enlarging a crevice that someone else started, especially if it produces easy access to the nectar in an otherwise tricky flower. Honey bees can effectively use their mandibles to separate layers of petals, as well.

Resin collection and propolis production

Every aspect of propolis manufacture is highly dependent on honey bee mandibles. When she finds a source of plant resin, the worker harvests chunks of it with her mandibles, adds some salivary secretions, then works the resin until it is soft enough to be stored in her pollen baskets. When she finally gets back to her hive, another worker must use her mandibles to help the resin collector shed her load, biting at the forager’s legs until she offloads the loot.

Before the propolis can be used, it must be chewed even more. When it’s ready to spread, the workers once again use their mandibles to apply the propolis where it’s needed, smearing it into place.

The Bite of the Bee

Honey bees also use their mandibles as a weapon for biting a pest or predator. Several years ago, scientists studying the role of the ketone 2-Heptanone in honey bee colonies discovered that if a pest or predator is too small to sting, the honey bee simply chomps into the creature with its mandibles and injects a dose of 2-Heptanone.⁴ Apparently, the channels on the backside of the mandibles that are used to deliver brood food can also deliver 2-Heptanone from a pair of glands at the base of the mandibles.

The 2-Heptanone acts like the anesthetic Lidocaine. If the bee can pierce the cuticle of a pest like a wax moth, the chemical paralyzes the larva for a few minutes, just long enough for the workers to remove it from the hive. The researchers speculate that a strong biting response by honey bees may be related to the hygienic success of certain colonies.

Some honey bees seem more willing to bite than others. You can sometimes see guards on a landing board facing out with their mandibles spread wide. This threatening stance is reminiscent of a guard dog, teeth bared and mouth drooling, daring someone to make the wrong move. In fact, many beekeepers report being bitten by their honey bees.

Other Household Chores

Washboarding, too, is dependent on mandibles. During this activity honey bee workers stand on the surface of their hive and rock back and forth while scraping their mandibles against the surface as if they were polishing it. While it is unclear why honey bees do this, an examination of their mandibles after washboarding reveals an accumulation of debris that they removed from the surface.

During trophallaxis, the transfer of food from bee to bee, the offering bee spreads her mandibles apart to reveal a droplet of food. The receiver bee then opens her mandibles, extends her proboscis, and vacuums up the offering. In other words, the mandibles act like a slurpy gate, inviting the transfer and stabilizing the transaction.

Trash removal is another chore for the mandibles. Undertaker bees remove the dead, hauling them from the hive and dropping them outside, and trash collector bees can remove anything from dead animals to debris left behind by the beekeeper, such as cardboard strips, string, or paper plates.

Large-Jawed Bees

As cool as honey bee mandibles are, they don’t hold a candle to those of the bees in the Megachilidae family. In fact, the name Megachile means large-jawed or large lipped. Instead of secreting wax to build a nest, bees in this family collect materials from the environment and bring it home for building. Pebbles, petals, leaves, mud, sand, and fibers can be gathered and ferried home in a sturdy set of mandibles. Depending on the species, some of these mandibles have multiple teeth, and others have knife-sharp edges.⁵

Some bees, like those in the genus Dianthidium, use pebbles and mud to build a stone hut attached to a tree branch. Others like the woolcarder bees make the interior of their nests soft and cushy with balls of plant fiber.  Mason bees make inner partitions of mud or chewed leaves, and resin bees waterproof everything with glistening layers of plant exudates. But whatever the bees use is collected and manipulated with those enormous mandibles.

Leafcutting bees have the charming habit of sitting on the piece of leaf they are excising, much like the cartoon character who sits on the limb he is sawing from a tree. But leafcutters don’t fall. Instead, they wrap their legs around the piece as they cut. By the last snip, the disk looks like a carpet rolled into the cargo hold of an airplane—perfectly aligned to minimize wind resistance on the trip home.

The giant resin bee, Megachile pluto, uses her jaws to slash tree bark, causing resin to ooze through the wound. Other bees, such as the sculptured resin bee, Megachile sculpturalis, attack rival carpenter bees by coating them with resin that they prepare in their mandibles.

Other Applications

Carpenter bees use their mandibles like drills, boring perfectly cylindrical cavities into wood to construct nesting chambers called galleries. In fact, you can often find a substantial pile of sawdust beneath the hole where the carpenter is working.

Male bumble bees patrol their territories by flying in circles around the area they are claiming. But occasionally the bumble will stop, alight on a plant, and rapidly open and close his mandibles—an action that pumps scent from his mandibular glands that will he will use to mark his territory.

Some of the buzz pollinators use their mandibles to release pollen from flowers. These bees grab a pollen-laden anther between their mandibles, hang on tight, and furiously vibrate their flight muscles until the pollen explodes into a cloud. That said, some bees skip this step entirely and just head-butt into stubborn flowers.

One of the craziest things male bees do with their mandibles is sleep. Many species while away the night by clamping onto a stem or leaf and then falling asleep. Oftentimes, large groups of male bees cling onto one plant, each hanging by his jaws and covered with dew.

Stingless Biters

Honey bees are not the only bees that bite. Though they have no stingers, bees in the tribe Meliponini are far from defenseless and simply chomp on their enemies when provoked.

Closely-related bees in the genus Oxytrigona are called “fire bees” because they have mandibular glands that secrete formic acid. When they bite, they spit a little formic acid into the wound to make sure the offender learns and remembers. Very effective, I hear. Stingless bees also use their mandibles to distribute scent trails that lead back to rich sources of nectar.⁶

A Mandible for Every Purpose

As you can see, mandibles come in an array of designs depending on what a bee needs to accomplish in its short life. The next time you are out bee-peeking, try to correlate a bee’s lifestyle to its mandible design. And if you have nothing better to do, determine which one folds over which. It might make you famous.

Rusty
Honey Bee Suite

Acknowledgment

Thanks to Robert Noble of Brampton, Ontario for his lovely photos of bee mandibles at work. See more of his work at BobNoblePhoto.wordpress.com.

References

1. Snodgrass RE, Erickson EH, and Fahrbach SE. 2015. The Anatomy of the Honey Bee. In JM Graham (Ed) The Hive and the Honey Bee (pp 121-122). Hamilton, Illinois: Dadant & Sons, Inc.
2. Research currently underway by Sam Droege and Catherine Graham. USGS, Beltsville MD.
3. Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland, OR: Beargrass Press.
4. Papachristoforou A, Kagiava A, Papaefthimiou C, Termentzi A, Fokialakis N, Skaltsounis A-L, et al. (2012). The Bite of the Honeybee: 2-Heptanone Secreted from Honeybee Mandibles during a Bite Acts as a Local Anaesthetic in Insects and Mammals. PLoS ONE 7(10): e47432. https://doi.org/10.1371/journal.pone.0047432
5. Wilson JS and Carril OM. 2016. The Bees in Your Backyard. Princeton, NJ: Princeton University Press.
6. Wilson-Rich N. 2014. The Bee: A Natural History. Princeton, NJ: Princeton University Press.