The Antennae and their Sensilla
Scientific Beekeeping
Chemical Communication in the Honey Bee Society.
Electron Microscopy Images from:
A comparative study of the antennal sensilla in corbiculate bees. de Carmo et al 2013
In the western honey bee there are a pair of antennae which arise from the space between the compound eyes just above the clypeus. They arise anteriorly low down on the face.

These antennae make impressive sensory organs, providing the senses of touch, smell, taste and, through vibration, hearing. Some specialised sensilla can pick up environmental status such as CO2, humidity and temperature. They also give positional cues in relation to the flagellum. The proximal segment is the scape which articulates with the head. There is an elbow-like "joint" along the antenna between the scape and the pedicel . The pedicel extends distally into the flagellum which has 10 segments called flagellomeres, (11 in the drone).

The scape is attached to the head by a rim of flexible intersegmental cuticle in a ball and socket joint (like the human hip). The function of the scape is to control the movement of the antenna through two sets of muscles attached to the inside of the head and scape and another pair attached to the outside of the scape and pedicel. Additionally it provides a channel for the nerves. Within the interior of the pedicel lining is the Organ of Johnston (OofJ) which runs the whole length of the pedicel reaching the distal end of the scape and proximal end of the flagellum. The OofJ consists of many scolopale sensory cells. It is able to detect movement of the flagellum. Such mechanical stress initiates a nerve impulse. Similarly the dorsal hair plate between the pedicel and scape picks up the squashing of the hairs as the pedicel becomes aligned with the scape again detecting movement. The flagellum can not only be made to move in a controlled way, as described above, but may also be the subject of wind deflection whilst both stationary and in flight. This allows the bee to alter its flight to accomodate current wind condiions.

On the flagellum there are different receptors for the different senses called sensillum, pleural sensilla. Functionally, sensilla are categorized as mechano - chemo - thermo - or hygro-receptors. Mechanoreceptors can provide audible signals through the action of sound waves on the air, causing changes in air pressure and vibrations. The antennal tip plays a key role in the perception of mechanosensory and gustatory information. In social insects, antennal sensilla are crucial for mating, foraging, recognition and communication between colony members through the sharing of pheromones.

Which sensilla are found on the antennae and in which location depends upon both sex and caste. The functions of the various sensilla are difficult to establish. The informatiion given here is from the latest available research but may in some cases be difficult to prove. Electrophysiology is used to test the sensilla response to stimuli.

Olfactory subsystems in the honeybee: sensory supply and sex specificity. Kropf et al, 2014
Trichoid Sensilla are hair like in shape. they number about 3,000 on the worker antenna.

Just as they have different shapes they also have different functions. Some are olfactory in function others are mechanoreceptive.

The A type trichoid sensilla is said to be concerned with olfaction, the sense of smell by Slifer and Sekhon (1961).

Martin and Lindauer (1966) suggested that type B was used to detect wax smoothness.

The second photo illustrates two types of Trichoid sensillum found on the antennae of Apis mellifera. They are both of the thinner type.

The paper by Groh et al is interesting because it discusses the right-handedness of bees.

Trichoid Sensilla are usually found in a surface depression. They are cateregorised by thickness, length and shape.
Placoid sensilla are oval plate like discs and have a large surface area on the antenna. There are c2,700 on the worker bee antenna and c 1600 on the queen's. In the drone they are very plentiful, between 16,000 and 17,000.

They are concerned with olfaction, the sense of smell, vital to the drone in finding a virgin queen.

Placoid sensilla have revealed responses to a broad range of odorants (Getz and Akers 1993). This might be attributable to the finding that these sensilla house many individual olfactory receptor neurons.
Trichoid sensilla (TR). Bar 1 µm
Basiconic sensilla (BA). Bar = 20 µm
Placoid sensilla (PL). Bar = 20 µm
Basiconic sensila are cone or peg like in shape and detect taste and many have a porous tip. They have pores on the lateral surfaces.

The base is not sunken as found with many trichoid sensilla . These gustatory sensilla are mostly located on the distal segment of the antennae.

They respond with varying sensitivity to sugars, salts, and possibly amino acids, proteins, and water.

In many reports they are not found to be present on the drone antennae.

They may be also concerned with olfaction according to some sources.

They can be categorised into several subtypes by diameter.
The coeloconic sensillum consists of a pit with a hidden peg in it. It is wider than the sensillum ampullaceum.

This peg sitting in the pit is covered in pores.

They are concerned with, temperature and humidity (dry or moist).

Image Adapted from: Standard Methods for Apis mellifera anatomy and dissection The Coloss Beebook Volume 1
n.b. 1. Sensillum as the singular form of the noun and sensilla as the pleural has been used here. In Lesley Goodwin's excellent book he uses sensilla as the singular and sensillae as the pleural. Both are correct.
2. "Scape" is interchangable with the word "scapus" which is less commonly used.
3. When reading about sensilla some authors use the latin name and others the English language name.
Sensilla placodea = Sensilla plachodea =Placoid sensilla = poreplate
Sensilla ampullacea = Ampulloid sensilla
Sensilla coeloconica = Coeloconic sensilla
Sensilla basiconica = Basiconic sensilla
Sensilla coelocapitula = Coelocapitular sensilla, formerly known as campaniform sensilla =Sensilla campaniformia = Campaniformea sensilla = Campaniform sensilla Sensilla trichodea = Tricoid sensilla
Sensilla chaetica
Sensilla scolopidia= scolpophore = Sensilla Scolopophora
Electron-microscopy photos of worker sensilla on antennae
The coelocapitular sensilla appears as a button like knob.
The sensillum coelocapitulum only number about 45 - 60 on the tip of each worker antenna.

It has always been believed that they are stress receptors, in part beacuse of their distribution. It may be that they have two functions.

Confusingly they are now often described as as hygroreceptors and thermoreceptors. This is mentioned as a possibility.

The coelocapitular sensilla have a nonperforated, mushroom-shaped protrusion set in a narrow pit in the oval-shaped cuticular depression.
Coelocapitular or Campaniform sensilla. Bar 3 μm
Generalised image of a Antenna of the Honey Bee
This axial section of the pedicel from the paper "Vibration-processing interneurons in the honeybee brain." shows the internal arrangement in the antennae with the antennal nerve passing through the centre. Also illustrated is the arrangment of the scolopale cells that make up the OoJ.
Vibration-processing interneurons in the honeybee brain, Hiroyuki Ai, 2010
This work done in 2014 looked at several species of bee and noted where each type of sensilla was found on every segment of both castes and sexes. It included Apis mellifera and the table is copied on the right.
A Comparitive Study of the Antennal Sensilla in Corbiculate Bees, do Carmo et al, Journal of Apicultural Research, 2014
Coeloconic Sensilla Bar = 3 µm
Ampulloid sensilla. Bar 10μm
There are few of these sensilla and tend to be found on the eighth segment of the antenna of the worker Apis mellifera. They are discribed as having a small opening and inside there is a small peg with a sculptured surface.

It is postulated that it can detect carbon dioxide levels.

There is some controversy as to whether or not these are a different sort of sensilla to Coeloconic sensilla.

Sensilla ampullacea are different in that they are smaller than sensilla coeloconica.

Another paper came up with slightly different results. The reference is available to the left.
Comparative Studies on the Antennal Sense Organs of Queen and Worker Honey Bees, Apis mellifera L. Abdelmegeed, 2015
Selective blocking of contact chemosensilla in Apis mellifera, Groh et al 2001
Sensilla chaetica
These are stout bristles with their base inserted into a socket and are said to be sensitive to mechanical or gustative stimuli. They have an apical pore.

They are found at various places on the body of the honey bee. They are stiffer than a trichoid sensilla.
A general photo showing Placoid (Pl), Coeloconic (Co), Coelocapitular (Ca) Trichoid a (Ta), Trichoid b (Tb)
Electron-microscopy photos of Drone sensilla on antennae
Scape of male of Apis mellifera showing long and branched hairs (BH). Bar = 100 µm
Placoid sensilla (PL) and trichoid sensilla (TR) in Apis mellifera. Bar = 20 µm. C
Apical portion of antenna of male of Apis mellifera, showing a set of trichoid (TR), placoid (PL), coelocapitular (CA), basiconic (BA) and coeloconic (CO) sensilla. Bar = 20 µm
Tip of the Basiconic sensilla. Bar = 20 µm
Functional and Topographic Segregation of Glomeruli Revealed by Local Staining of Antennal Sensory Neurons in the Honeybee Apis mellifera, Nishino et al 2009
Nishino et al
Nishino et al
In Apis mellifera, drones have 18,600 olfactory placoid sensilla per antenna, each equipped with receptor neurons sensitive to the queen's sex pheromone