Scientific Beekeeping
The sensilla are specialised sensory nerve cells (neurons) that feed back information to the brain on the environment the honey bee finds herself in. They have always been classified according to their appearance rather than function since when they were first described their function was not always clear. They are described by appearance here and there sensory function attributed where known. There still remains some disagreement about this. It is probable that many sensilla have more than one function. They can sense taste (gustatory sensilla), smell (olfactory sensilla), movement (mechanosensitive sensilla), humidity (hygrosensitive sensilla) and temperature (thermosensitive). There may be other elements of the environment thay can detect such as pH. It is said that carbon dioxide levels can be identified. The gustatory and olfactory sensilla are sometimes descibed in one group vis. chemosensitive. At a basic level they are essential in helping bees find food and mates. The gustatory sensilla are also essential in keeping the pheromones dispersed throughout the hive.

The olfactory senses of the honey bee were exposed as remarkable in the paper from Robertson et al 2006. A total of 170 odorant receptor genes were annotated in the bee. This probably explains the bees' olfactory abilities including perception of several pheromone blends, kin recognition signals and diverse floral odours. In contrast only 12 gustatory receptor genes were identified. We still have a lot to understand in this area.

Sensilla are found in the antennae, mouth, feet, legs, wings, near the ovipositor and throughout the surface of the exoskeleton. A sensillum trichodeum occurs in the pre-imago which does not survive to the adult state on the mandible, it breaks off at pupal eclosion.

There are eight types of sensilla generally recognised: trichodea, placodea, coeloconica, coelocapitula (AKA campaniform), basiconica, ampullacea, chaetica and scolopidia. The trichoid sensilla are divided further by their length, thickness and shape. In some texts the ampullacea sensilla are considered small coeloconica sensilla.

The eyes and ocelli are sense organs and are covered elsewhere.

There are alarming differences in the scientific papers about the functions of some of the sensilla. It may be that some types of sensilla have different functions when found in different parts of a honey bee's anatomy. References are given that will allow the reader to draw their own conclussions. Clearly some sensilla have more than one function.

The paper by Awal et al 2014 gives a clear indication of present thinking with good references.

All sensilla have a basic similarity in their anatomy. They form in the epidermis and grow through the cuticle out on to the surface of the bee's exoskeleton. At the bottom of the epidermis is a basement membrane. This is perforated by the axon of the nerve. The Sensilla scolopidia alone do not follow this pattern.

There are three cells from which the sensilla derive: The large trichogen cell secretes the long, tapering hair / seta or peg. The smaller tormogen cell forms the circular chitinous socket around the base of the hair. The thecogen support cell that makes the thecogen dendritic cap - a cuticle-like matrix around the tip of the dendrite and which encloses the main body of the neuron.

This diagram shows a typical sensillum trichodea. Many of these sensilla are mechanosensitive, in other words sensitive to touch through deflection. When the seta (hair) is moved, the stimulus is amplified in the sensory dendrite of the sensory nerve cell beneath the surface of the cuticle.

There are several varieties of trichoid sensilla, described according to their thickness and length.

In some cases these sensilla have an olfactory action.
Sensilla Trichodea
Adapted from

The Sensillum coeloconica have hygro-and thermoreceptive properties. The cuticular apparatus of the sensillum is a mushroomshaped protrusion, devoid of pores, set in a narrow cylindrical pit positioned centrally within a cuticular, shallow depression. There may be a pore at the tip. There may be three or four receptor cells. Three receptor cells have unbranched sensory cilia, containing densely packed microtubules, which extend distally into the cuticular apparatus and completely fill its cavity.

There are three dendrites one for "moist", one for "dry" and one for 'temperature".

Coeloconic literally means peg in a hole.
Sensillum Coeloconica
Sensilla Coelocapitula ( Campaniform )
The sensilla coelocapitula is a sunken peg in a hollow cavity. It has long been believed that these sensilla measure stress in the cuticle.

Coelocapitular sensilla are flattened oval discs that usually serve as flex receptors in the exoskeleton. They respond whenever mechanical stress causes the exoskeleton to bend. Coelocapitular sensilla are found throughout the body - especially on the legs, the mouth, base of the antennae, near the base of the wings, and along sutures where two sclerites of the exoskeleton meet.

Confusingly they are now often described as as hygroreceptors and thermoreceptors. This is mentioned as a posibility.
Sensilla Basiconica (Pegs)
These sensilla are only rarely found on drones.

Sensilla basiconica have many pores over the surface of the peg . Gustatory receptors such as these have dendrites of sensory neurons which branch profusely within these pores and may respond to very low concentrations of detectable compounds. Some receptors respond to a wide range of substances while others are highly specific.

In some literature it is said to have a secondary olfactory action.
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 greek terminology others latin hence the different names and word endings. 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 sensillum =Sensilla campaniformia = = Campaniformea sensilla = Campaniform sensilla Sensilla trichodea = Tricoid sensilla
Sensilla chaetica
Sensilla scolopidia = Sensilla scolopophora= scolpophore = Scolopophous sensillum
Sensilla Placodea
J-Y Kim
The chemoreceptor superfamily in the honey bee, Apis mellifera: Expansion of the odorant, but not gustatory, receptor family, Robertson et al, 2006
The tarsal taste of honey bees: behavioral and electrophysiological analyses, de Brito Sanchez et al, 2014
The Sensilla chaetica grow out from a socket and are bristle like with tapering tip ending in an apical pore. They are similar to trichoid sensilla but have thicker cuticular shafts and are stiffer and not freely moveable.

They are gustatory and mechanosensory.

They are of different sizes, divided into long (Chatica I) and short (Chatica II) and may be found on the glossa, labial palps, galeae, antennae and tarsi of the worker honey bee.

They have only one pore opening at the tip. Taste receptors are abundant on the mouthparts, but are also be found on the antennae, tarsi, and genitalia (especially near the tip of the female's ovipositor) High concentrations of irritant compounds (e.g. ammonia, chlorine, acids, essential oils, etc.) simulate avoidance reactions and cleaning behavior. Insects can detect these compounds even when all known chemoreceptors have been covered or destroyed. The irritants evidently trigger a generalised response from other types of sensory neurons.
Sensilla Chaetica
Lim et al
Characterization and its implication of a novel taste receptor detecting nutrients in the honey bee, Apis mellifera, Lim et al 2019

Simplied anatomy of Sensilla Chaetica
There are four gustatory receptor neurons (red, green, purple and grey) and one mechanosensory neuron (black)
Approximately 100 taste sensillae are located on the tarsus and pretarsus. These are mostly chaetic sensilla, which are evenly distributed between the five subsegments of the tarsus, and which are densely concentrated on the terminal claw-bearing pretarsus. Chaetic sensilla share similarities with those found on the mouth parts, with a mechanosensory cell ending at their base and four cells with dendrites running to the tip of the shaft .

Among the insects, the honey bee genome encodes very few gustatory receptors (Gr). Based on bioinformatic identification of Gr genes in honey bees, A. mellifera has twelve GRs
The sensilla placodea are the main olfactory sense cell. There are a great many of them on the drone antennae.

These sensilla have many pores over the surface representing different nerve cells. This olfactory receptor is a thin-walled plate with numerous pores through which airborne molecules diffuse. Odorant molecules reach the dendrites of olfactory responsive neurons by diffusing through the extracellular fluid, (the sensillum lymph), filling the vacuole or lymph space.

Dendrites of sensory neurons branch profusely within these pores and may respond to very low concentrations of detectable compounds (e.g. pheromones). Some receptors respond to a wide range of substances while others are highly specific. Olfactory receptors are most abundant on the antennae, but may also be associated with the mouthparts or external genitalia.
Influence of Different Statuses of Honey Bee Queens, Apis mellifera L. on
the Ultrastructure of the Flagella on (3-Day Old) Workers, Awad et al ,2014
Adapted from Snodgrass, Cornell University
Erik Schneider
Sensilla Scolopidia
Sensillum scolopidium are specialised mechanoreceptors and usually have no surface cuticular presence. They are usually grouped together in chordotonal organs. In the bee to date have been found:
1) Organ of Johnston (in each antenna, which detects nearby airborne sound generated by the wings of sister bees during the waggle dance and converts "duration of airborne vibration during the dance" into profitable information "distance to the food source"),
2) Unnamed in the maxillary palp (two scolopidia), labial palp (12 scolopidia each)
3) On each leg four chordontal organs, one on the femur, two on the tibia (one of which is known as the subgenual organ) and one on the tarsus.

In the Organ of Johnston complex information is received which, in humans, we would call sound. In the other chordotonal organs simpler positional information is picked up.

Each chordotonal organ comprises one or more individual sensory units called scolopidia, and each scolopidium consists of three cells arranged in linear alignment: a sensory cell, a scolopale cell, and an attachment cell

A typical tympanal scolopidial organ, consists of three cell types.
A) The dendrite of a sensory neuron projects into a fluid-filled space (lumen) formed by the walls of an enveloping scolopale cell.
B) The distal tip of the dendrite inserts into the scolopale cap, an extracellular secretion of the scolopale cell.
C) The attachment cell connects the sensory neuron and scolopale cell to the tympanal membrane, either directly or indirectly via a tracheal air sac.

The hairs of the sensillum scolopidium react to movement of the heamolymph.
Chordotonal Organs of Insects, Field and Matheson, 1998
Field & Matheson
Hoy & Yack
Sensilla Ampullacea
The Sensilla ampullacea are covered very little in the literature. There are no diagrams to describe its anatomy. Since it often considered to be a small Sensillum coeloconica the diagram is repeated here.

They are said to be carbon dioxide receptors from research carried out by Lacher 1964.
Erik Schneider
Taste Perception in Honey Bees, Maria Gabriela de Brito Sanchez, 2011
Lacher, V. (1964). Elektrophysiologische Untersuchungen an einzelnen Rezeptoren für Geruch, Kohlendioxid, Luftfeuchtigkeit and Temperatur auf den Antennen der Arbeitsbiene und der Drohne Apis mellifera L. Z. Vgl. Physiol., 48:587-625.

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