Every few weeks a photo of a fly lands in my inbox, always accompanied by the same question: “What kind of bee is this?” The answer is simple. If your insect has short, stubby, barely visible antennae, it is not a bee.
On the contrary, a bee antenna is long, graceful, mobile, and insanely cute. But beyond that, the antennae are a bee’s major data collection tools, containing receptors for touch, taste, and smell. Antennae can also detect temperature, humidity, and carbon dioxide, along with gravity and wind speed.1 Much of what a bee “knows” arrives through those two slender filaments.
The word antenna is derived from the Latin antemna. On Roman sailing ships, an antemna was a type of horizontal mast-mounted spar designed to spread square-rigged sails. With a little imagination, perhaps you too can envision your bees with rigging. Sail ho!
Centralized data collection
The importance and complexity of antennae is not limited to honey bees. Bees of all types have similar antennae that collect the information needed for survival. But eusocial bees2—those that live in organized groups—need more information than solitary bees, so their data collection is more complex. Not only do they need details about food sources, weather conditions, and mates, but they also need to communicate with other colony members. Social bees have receptors that can perceive queen pheromones, behavioral pheromones, and chemicals which can affect the social structure of the entire colony.3
Basic antenna structure
The basic structure of antennae is the same for all bees. The base of each antennae sits in a bowl-like depression in the bee’s head, sometimes called the antennal socket. Four muscles extend from the base of the antenna into the bee’s head to control antennal movement.
The antenna itself is divided into three main parts. The first part, which rises from the antennal socket is called the scape. It is the longest single segment of the antenna. Attached to the distal end of the scape is the pedicel, a much shorter segment that has a rounded appearance. These two segments together are responsible for the way the antenna moves. The pedicel fits within the end of the scape to form an elbow-like joint that allows easy rotation in many directions. If you look at a bee whose antenna is flexed, the bend you see occurs between the scape and the pedicel.
The third major section of the antenna is called the flagellum. The flagellum is especially interesting to bee taxonomists because it can often be used to determine bee sex. The flagellum is divided into sub-segments called flagomeres. In nearly all species, including the honey bee, females have 10 flagomeres and males have 11. In some species, such as the long-horned bees, the male’s sub-segments are much longer than the female’s, giving him a noticeably longer antenna.
The sensory receptors
The outer surface of the flagellum is covered with different types of receptors, each type having a special purpose. The receptors can be identified by their shape and are often described as plates, pits, pegs, and hairs. In general, peg organs are chemoreceptors used for smelling, sensory hairs are mechanoreceptors used for tactile functions, and plate organs are both chemo- and photoreceptors. Estimates vary, but each worker antenna has roughly 3,000 chemoreceptors, whereas a queen’s antenna has only about 1,600. But drones, whose job is to find virgins queens in midair, have an estimated 300,000 chemoreceptors.1
The distribution of sensors on the antennae is very specific. For example, the honey bee uses a tuft of sensory hairs at the very tip of the flagellum to determine surface texture. Temperature sensors are found on the last six sub-segments, and the majority of olfactory sensors, called pore plates, are distributed over the last eight sub-segments of the worker’s antennae.4 The gustatory sensors, far fewer in number, are thread-like chemoreceptors with a pore at the end that can perceive sugar concentrations as low as one or two percent.3 Other sensors, not as well understood, can detect pheromones, humidity, carbon dioxide, gravity, and shape.
The inside of the antenna contains a nerve that leads from the receptors to the antennal lobe of the brain. It also contains two additional muscles that are independent of those found in the scape, and which pull the flagellum up and down. The nerve and antennal muscles receive oxygen through small tracheal tubes and hemolymph via the bee’s circulatory system. The antenna are so important that they have auxiliary hemolymph pumps, one at the base of each antenna, which help pump bee blood through the organ.
The Johnston’s organ
An additional receptor, known as the Johnston’s organ, is found inside the pedicel at the base of the flagellum. This sensor is able to detect vibrations and slight changes in antennal position. For example, in flight, bending of the antenna due to airflow helps the bee to determine her speed through the air.5
In addition, the Johnston’s organ is responsible for the bee’s ability to “read” dance language even in the dark. It works like this. The dancing bee causes sound waves to travel through the air. These waves deflect and vibrate the flagellum of a “listening” bee. The vibrations are transmitted from the flagellum to the pedicel where they bounce into a tight membrane that separates the pedicel from the scape. You can think of this intersegmental membrane as being similar to an eardrum. When it vibrates, it stimulates sensory cells in the pedicel which then send nerve impulses to the bee’s brain.6
Since the Johnston’s organ detects sound vibrations, it is sometimes called the “bee’s ear.” But unlike a typical ear, the Johnston’s organ is able to detect extremely slight deflections, such as those caused by both magnetic and electric fields.7 For example, Greggers demonstrated that electric fields are created when bees move, and these fields cause small but perceivable antennal movements in a receiving bee.8
How bees use their antennae
Bee antennae are in constant motion—touching, reaching, bending, and seeking. With so many receptors of different types, a seemingly endless stream of environmental data flows into the bee’s brain. What does the bee do with it all?
The ability to “hear,” taste, smell, and feel are essential to every part of the bee’s life. For example, the bee’s ability to detect surface texture helps a forager find her way into a flower. As we know, many flowers have visual patterns on their petals known as nectar guides that help a bee see her way to the nectaries. But in addition to visual clues, many flowers have tactile clues as well. The surface of some flower petals is texturally unique in the area leading to the nectary, a difference bees can detect with the tips of their antennae.9
The bees’ sense of smell is especially attuned to odors associated with food. This heightened sensitivity allows workers to find flowers and other potential food sources, including neighboring hives and hummingbird feeders. Since a bee has two antennae, she can compare the relative strengths of the two signals to guide her to the source. She uses this comparative analysis to fly in the direction that’s slightly stronger, continually adjusting her flight as she follows the odor stream.
Since bees are extremely sensitive to sweetness, they can decide whether a certain nectar is worth collecting. Nectar with a very low sugar concentration could be a waste of time for honey bees to harvest, so after a quick taste with their antennae, a forager can reject the source and search for something better. Although bees also have taste receptors on their mouth parts and feet,7 the antennal taste receptors often make the initial contact.
Communicating in the dark
Antennation is a process in which bees use their antennae to communicate with each other. For example, the antennae are very active during trophallaxis, the transfer of food from one bee to another. Observers believe that bees, both givers and receivers, signal their readiness to begin food transfer by using their antennae.6 In addition, the antennae help them align their bodies for the transfer. Like nearly everything else a honey bee does, trophallaxis usually happens in total darkness, so bees must rely on tactile clues to get the job done.
Honey bees also use their tactile sense to build wax combs.10 Although the hive interior is dark, the bees can detect comb dimensions with their antennae. The shape of the cells, as well as the thickness and depth of the wax walls, are determined by using the antennae.
Because sensors on the antenna can detect small changes in carbon dioxide levels, bees can take corrective action. As carbon dioxide in the hive rises, bees may increase fanning to aid ventilation and lower the carbon dioxide concentration.4 The ability to detect carbon dioxide may also assist with colony defense. Sometimes spikes in carbon dioxide levels are due to the exhalation of a threatening creature, such as a beekeeper.
The antennal sensors also help maintain the colony as a cohesive unit. They allow the workers to monitor the queen by tracking her pheromones, they enable the bees to “hear” queen piping, and they allow the bees to “read” messages of danger, such as alarm pheromone. The antennae even perceive substances such as ethyl oleate, a primer pheromone11 that regulates the development of foragers. An increasing level of ethyl oleate is a signal from the active foragers that the work force is large enough. As a result, the rate of transition of nurse bees into foragers slows down and bees prolong their nursing phase.3 When the workforce drops, ethyl oleate levels decrease, and the rate of maturation from in-hive bees to foraging bees returns to normal. But if lots of foragers are lost, then bees will accelerate development and transition to foraging precociously.
Bees even use their antennae while washboarding. During this strange activity the bees stand on the exterior of the hive and rock back and forth, rubbing the surface with their mandibles. As they rock, their antennae are in constant motion, frequently examining the surface.10 Washboarding is a fascinating behavior we still don’t understand, though it’s usually done by young adult bees.
Keeping it all clean
Antennae are vital to bee life, so it is not surprising that bees have unique ways to keep them spit-shined and polished. Each of the forelegs of all bees are equipped with an antenna cleaner. The antenna cleaner is made of two parts: a notch in the basitarsus, which is outfitted with a ring of stiff hairs, and a corresponding spur on the tibia. According to Mattingly, “To use the antenna cleaner, the [bee] raises her foreleg over her antenna and then flexes her tarsus. This action allows the spur to close the notch and form a ring around the antenna.”9 As the bee pulls each antenna through the circular enclosure, the bristles remove debris such as pollen and dust which otherwise might interfere with the sensory organs of the antenna.
Some bee species tend to clean their antenna more frequently than others. Worker honey bees don’t clean after every flower, but they clean frequently after visiting blooms containing especially heavy or sticky pollen. Squash and hibiscus blossoms, for example, can trigger repeated antennal cleaning. Honey bee drones tend to clean their antennae before going in search of virgin queens. Often they can be seen on the landing board polishing their antennae in advance of the big event. On the other hand, species such as Halictus rubicundus clean their antenna constantly. In fact, after an afternoon of watching them, I wonder how they get anything else done.
A sensory bundle
Besides containing the equivalent of nose, fingers, ears, and taste buds, the antenna functions as a protractor, hygrometer, thermometer, speedometer, direction finder, and CO2 sensor. What could be more convenient? Now that I think about it, I wonder how those flies manage to find their way around from day to day.
Honey Bee Suite
Notes and references
- Sammataro D, Morse RA. 1998. The Beekeeper’s Handbook. Ithica NY. Cornell University Press.
- Eusocial bees are those that live in cooperative groups characterized by shared brood rearing, overlapping generations, and a division of labor. Both honey bees and bumble bees are considered eusocial.
- Southwick E. 2015. Physiology and Social Physiology of the Honey Bee. In JM Graham (ED.) The Hive and the Honey Bee (pp. 183-202 ). Hamilton IL. Dadant & Sons.
- Snodgrass RE, Erickson EH, Fahrbach SE. 2015. The Anatomy of the Honey Bee. In JM Graham (ED.) The Hive and the Honey Bee (pp. 183-202 ). Hamilton IL. Dadant & Sons.
- Winston ML. 1987. The Biology of the Honey Bee. Cambridge MA. Harvard University Press.
- Kevan PG. 2007. Bees: Biology & Management. Cambridge ON. Enviroquest Ltd.
- Caron DM, Connor LJ. 2013. Honey Bee Biology and Beekeeping. Kalamazoo MI. Wicwas Press.
- Greggers U, Koch G, Schmidt V, Dürr A, Floriou-Servou A, Piepenbrock D, Göpfert MC, Menzel R. 2013. Reception and learning of electric fields in bees. R. Soc. B 2013 280 20130528; DOI: 10.1098/rspb.2013.0528.
- Mattingly, RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland OR. Beargrass Press.
- Gary N. 2015. Activities and Behavior of Honey Bees. In JM Graham (ED.) The Hive and the Honey Bee (pp. 271-308). Hamilton IL. Dadant & Sons.
- A primer pheromone causes long-term changes in both physiology and behavior. In contrast, a releaser pheromone causes rapid changes in behavior.