Putting Out the Feelers: Antennae

Introduction

We had a lovely warm day last week and the butterflies were busy. Leo and I watched a colourful one on a viburnum flower. This shrub is blooming early this year which is good for insects. I don’t usually see flowers  on it till November. As requested, I lifted Leo up as close to the red admiral as possible so he was at eye level watching her antennae wave over the blossom and her very long proboscis probe the tiny flowers for nectar. Then we saw her roll up her proboscis and tuck it carefully away before she squirted a few tiny drops of straw-coloured fluid out of her back end and flew off over the hedge.

Antennae

Watching her spindly moving antennae reminded me of honey bee antennae and their important and versatile role in feeling, tasting, smelling, hearing, navigating, communicating and monitoring carbon dioxide levels and humidity inside their hives. Communications in a colony depend upon these sensory organs working well and it was disturbing to discover that oxalic acid crystals reduce their sensitive sensilla numbers.

Anatomy in a Nutshell

Let’s look more closely at these delicate organs.

Honey bees have two antennae which have three main segments each—scape, pedicel and flagellum (Latin for little whip or lash). They have two joints not unlike those found in human anatomy. The basal scape is attached to the head by pretty cool looking ball and socket joints which allow the whole antennae to rotate in any direction. Textbooks and beekeeping literature often refer to the subdivisions of the flagellum as segments but this is not correct. Snodgrass¹ explains that they are not true segments because the are not articulated on each other, nor do they have any muscles. Rather they are called annuli which comes from the Latin word annulus which means little ring, and the flagellum consists of a ring-like structure of fibrous tissue in which important sensory receptors are housed. Sometimes these subdivisions are referred to as flagellomeres. Female workers and queens have 10 annuli while drones have 11, and we will address the reason for the difference soon. The joint between the scape and the pedicel is hinged, just like our knee joints, allowing both pedicel and flagellum to move up and down relative to the scape.

The main part of a honey bee sense organ is a sensory cell on the outer chitinous covering of the bee called the ectoderm. Each sensory cell has a nerve process which extends inside the bee and connects to the central nervous system and the bee brain. When external stimuli are received and processed changes in bee behaviour occur depending upon the type of sensillum (also known as sensilla) involved. Sensillum refers to a receptor sense organ that receives external environmental stimuli and there are at least seven of these organs on the honey bee antennae each specialised for particular sensory functions. They are complex, varied and differently shaped too with some hair-like trichoid sensilla (large and small), coeloconic sensilla with small flask-shaped depressions in the cuticle covered in pegs, and oval plate-shaped placoid sensilla that have rows of pores between them. All of these are examples of olfactory receptors² that detect odours.

If you are looking for more detailed descriptions of how these different sensilla function and their numbers, don’t be tempted to ask Chat GPT or other AI sources because the chances are they are inaccurate. The former certainly is. Jackie Elliott has painstakingly put together an excellent and seriously useful website and I can recommend it to you, especially if you are studying for exams. https://www.scientificbeekeeping.co.uk/Antennae.html. It has good photographs too. There are for example around 3000 trichoid sensilla on the worker antennae and there are five different types of these. The drone has a longer flagellum and its antennae is twice the surface area of a female honey bee antenna because he needs to be able to locate queens some distance away in the drone congregation area by smell. There are about 16,000 placoid plate receptors on a drone antenna compared with only 2,700 in a worker so you can see that he has superior powers when it comes to perfume.

Inside the pedicel there is a special cylinder-shaped structure called the organ of Johnson (named after the discoverer) that is really the bee’s ear. This is because the organ of Johnson picks up vibrations from within the flagellum, so watch out next time you dig near a bee hive or hammer in a post; the bees don’t like it. I know because my vegetable garden is close to one of the colonies. The pedicel is a bit like speedometer and it can also determine air speed from the vibrations created by air passing over the antennae during bee flight which is pretty incredible.

I hope you have enjoyed this quick tour of honey bee antennae.

Reference:

¹ Snodgrass, R.E., 1956, Anatomy Of The Honey Bee, Cornell University Press.

2 Goodman, L. 2003, Form and Function in the Honey Bee, IBRA.