The senses located in the bee’s antennae

While the bee amazes us with its highly social behaviour, its anatomy and multifunctional organs are truly awe-inspiring.

The bee is equipped with two antennae attached to the upper part of its head, composed of three main segments: the scape, the pedicel, and the flagellum. The antennae are in constant motion thanks to four muscles located in the head capsule that enable rotation of the scape; the latter contains two groups of muscles responsible for the movements of the flagellum and the pedicel. The motor neurons involved in activating these muscles (nine for the scape, six for the flagellum) have their cell bodies located in the dorsal lobe, which constitutes the motor and mechanosensory system of the antennae. The antennae thus serve as the bee’s central organs for perceiving its environment and, through thousands of sensilla, transmit an enormous amount of information linking it to the external world. The antennae therefore support the senses of smell, taste, touch, hearing, and the perception of electric fields.

Olfaction

The very numerous placoid or porous sensilla are specialised in odour detection and are located on the flagellum, or whip, which is composed of 10 articulated segments (articles) in queens and workers and 11 in males. On the last eight mobile segments are porous plates comprising seven different sensory structures involved in olfaction. Per antenna, the following numbers are observed:

1,600–3,000 porous plates in the queen;

2,400–6,000 porous plates in the worker;

> 30,000 porous plates in the drone.

Basiconic sensilla, which are fewer in number (150 per antenna), are also sensitive to odours and externally display a hair-like termination whose wall is perforated to allow odorant molecules to access the sensillar fluid, from which the nerve impulse is transmitted to the bee’s brain. The bee uses its antennae to orient itself towards an odour source by comparing the olfactory information from both antennae. Certain odours are attractive, such as floral scents or those emitted by the queen. Males are strongly attracted to queens during mating flights thanks to molecules emitted at very low concentrations.

Other odours are repellent, such as smoke from the smoker; others trigger aggressiveness in workers, for example molecules produced by guard bees when danger is detected or when the beekeeper approaches.

Taste

The sense of taste is ensured by gustatory receptors located on the 3rd and 10th segments of the flagellum and responding to sweet substances. Bees recognise the four basic tastes (sweet, salty, sour, bitter), although they are not perceived in the same way as in humans. Sensitivity thresholds also differ: the threshold for hydrochloric acid is lower and that for acetic acid higher in bees than in humans.

In addition, the sense of taste is also present on receptors located on the tarsi of the forelegs and on the proboscis.

Touch

The sense of touch, which enables deliberate exploration of the physical environment, is mediated by trichoid sensilla bearing a small, thick-walled hair on the flagellum, which the bee uses somewhat by “feeling around,” similar to a blind person’s cane. The antennae of a worker are heavily engaged, constantly palpating and touching surrounding objects. For example, they are capable of recognising the size of the cells in the comb. Bees also perform many activities that integrate the sense of touch. They possess grasping abilities: with their legs, they collect pollen and propolis. With their mandibles, they can knead wax and propolis, grasp a dead individual and remove it from the hive, bite, and so on.

In addition to the antennae, a set of sensory receptors is distributed over the entire body of the bee. These are mechanoreceptive sensilla equipped with a bristle capable of slight movement at its base. Sensory cells located at the base of these bristles record even the slightest movement and inform the bee about its physical environment.

Some of these bristles are grouped to provide specific information to the bee. Thus, sensory bristles located on the eye between the ommatidia are responsible for perceiving air currents and produce information used by foragers during flight. Sensory fields located at the articulation between the neck and the petiole allow the bee to orient itself with respect to gravity.

Hearing

The bee is all the more capable of hearing and perceiving vibrations when these are transmitted through the substrate on which it is located (notably the combs). Thus, a mother queen detects the calls of young queens that are about to emerge from their cells.

Three organs are involved in hearing:

The first, located in the forelegs, consists of a thin internal membrane that vibrates when sounds or vibrations are reflected by the combs of the hive. This organ is also sensitive to the auditory frequency corresponding to the queens’ “song.”

The other two organs are located in the antennae, one just at their base and the other in their final third. These consist of campaniform sensilla sensitive to vibrations, as well as two groups of trichoid sensilla (a total of approximately 8,500 on both antennae).

Considered as a whole, the antenna can resonate with sounds of a frequency of up to 20 hertz, such as those used in the waggle dance. The terminal part of the antenna, by contrast, perceives frequencies ranging from 250 to 300 hertz. However, using tape recorders, it has been shown that bees are also sensitive to sounds between 600 and 2,000 hertz. Other trembling sounds detected by bees, produced by wing friction of workers and thoracic muscles, correspond to frequencies of 180 to 190 hertz. In queenless colonies, this frequency range shifts from 175 to 240 hertz.

Other types of sensilla also exhibit specific shapes, but their function is not always clearly understood. The bee takes great care of its antennae, cleaning them very regularly with the brush located on the basitarsus of the foreleg.
		Detail of the antennal brush or tarsal notch at the level of the basitarsus of the foreleg.
The antenna contains additional sensory receptors: on the pedicel, an articulated intermediate structure between the scape and the flagellum, lies Johnston’s organ, whose highly sensitive cells inform the bee about even the slightest movements of the flagellum. This sensor is particularly useful for recording information transmitted by the movements of other workers within the colony. It also enables bees to assess flight speed via the curvature of the antenna.

Physico-chemical sensitivity

The antennae also allow the bee to perceive its physical environment, such as temperature, humidity, and carbon dioxide concentration.

Electrosensitivity

During flight or certain movements, the bee accumulates positive electrical charges on its body and wings. As the wax covering the exoskeleton is an insulating material, these charges dissipate only slowly, even once the insect has landed. As a result, the bee continuously emits an electric field during flight and in the moments following landing.

This electric field allows certain particles, such as pollen, to adhere to the bodies of pollinators when they land on a flower. The electric field of the flower is modified by contact with an insect, which can be perceived by other insects.

Bees are capable of detecting the shape and intensity of electric fields. They can therefore identify nearby flowering plants, which modify both factors through their mere presence. British researchers from the University of Bristol demonstrated that pollinators detect these electric fields, which are distorted in specific ways by each plant depending on its shape and size.

The antennae of bees are highly sensitive to electric fields, which induce curvature of the flagellum. This repulsion of the antennae by positive electric fields (with flexion of the flagellum and stimulation of the pedicel and its Johnston’s organ) complements communication mechanisms that enable bees to communicate in complete darkness and, for example, to identify the waggle dance of a forager indicating the location of a food source.

Beyond their primary function as sensory organs, antennae also provide information on the physiological state of bees, for example a state of relaxation characterised by three types of antennal behaviour: immobile antennae, antennae contracting approximately every minute, or antennae producing broad pendulum-like movements.

Finally, the antennae of males, richly endowed with sensilla, release sexual pheromones, which are particularly useful during the reproductive period in mating flights.

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