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 antennae 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 ….