The Anatomy of Bees

Honey Bees, Queen Bees, Solitary Bees and all in Between

Bees have three body segments: the head, thorax and abdomen. The head contains the eyes, the mouthparts, antennae and brain, the thorax contains the muscles: insect legs, insect wings and their respiratory system. Everything else is in the abdomen – the honey stomach, the digestive system (and in bees & wasps: the stinger). These features are general and are largely unchanged between insect species.

This article provides an overview of the three parts of honey bee anatomy using the European honey bee, Apis mellifera as a model and briefly describes how these features could have evolved and outliers in bee behaviour. Readers interested in detailed anatomical investigations are directed to their respective article below

Comparing Drone Bees with Worker Bees and Queen Bees

First, a comparison of the three honey bee castes on the basis of their physical features:

The Bee Head – Sense, Secretion and Slurping

The honey bee head contains the sensory centres (brain, eyes and antenna), the royal jelly-producing hypopharyngeal glands¹ (secretions). The mouthparts are two structures: a proboscis (for sucking liquids) and mandibles for chewing (beeswax or the legs of an unfortunate male bee in late autumn) and carrying objects (hive wastes, dead larvae).

The honey bee’s brain gives them a capacity for learning and memory (such as the location of a suitable foraging area), as well as communication. The waggle dance’s complexity and accuracy come from the coordination of many neurons in a way that other bees can understand. The brain features “mushroom bodies”, which process the sensory inputs from the antennae, and change in response to stimuli and behavior as the bee ages (Groh & Rossler, 2020).

Bees have five eyes: the two large compound eyes, which allow bees to detect ultraviolet light patterns and distinguish between colours (reviewed in Srinivasan, 2010). The compound eyes are larger in drone bees to give them a better sense of the virgin queen and her location during mating flights. The three other eyes are called ocelli, they are simple eyes on top of the head that detect the intensity of light from a given source.

The antennae are their primary sense organs, detecting odors, vibrations and tastes (de Brito Sanchez, 2011). As worker bees spend a lot of time sticking their heads into flowers, it is often muddled with pollen. To mitigate this, the forelegs of honey bees contain notches filled with stiff hairs that serve as antennae cleaners. As pollen accumulates on the bee’s body, its forelegs brush it away from the eyes and antennae, whilst the mid and hind legs pack that pollen into the pollen baskets (Amador et al., 2017).

Thorax: Breathing and Movement

The thorax is the locomotion center, vital for flight and movement, with the bulk of it being occupied by the flight muscles to control the rapid wing movements. The thorax contains two pairs of wings and three pairs of legs for a variety of activities like walking, cleaning, gripping and pollen pressing. Within the thorax are spiracles, hollow tubes connected to the trachea that facilitate breathing.

As with other insects, honey bees have four wings: the forewings(four-wings. Pun intended) and hindwings. During flight, they hook together and function as one, beating rapidly to provide the necessary lift and produce the bee’s characteristic buzz. For those who observe their hives on a hot day, you’ll see the worker bees rapidly beating their wings and fanning their hives, regulating temperature and carbon dioxide gas inside the hive (Peters et al., 2017).

The six legs have functions beyond motion and keeping bees upright. As described above, the forelegs contain notches that clean the antennae. The middle legs along with the hind legs serve as special pollen collecting tools, whereby they carry pollen grains in the pollen baskets (corbiculae) located on the larger hind leg. To pack it tightly, the legs serve as a “pollen press”, which ensures a large payload is delivered back to the hive.

The bees’ respiratory system starts within the first two pairs of spiracles being located in the thorax. Air enters through these spiracles and is transported around the body via the tracheae. One major consumer of air are the flight muscles, which as mentioned above, control the rapid wing movements and occupy a significant portion of the thorax.

Further reading on the bee abdomen is available in “Bee Thorax“.

The Abdomen: Digestive & Reproductive Organs

To an entomologist, the bee’s abdomen is full of complexities and organs that maintain the functions of the insect. To the layperson, it’s where the pain comes from. To the beekeeper observing the queen bee, it is where “babies come from”. The technical article will provides further information on the organs and organelles within the abdomen; an overview is provided below.

The stinger is present in the female bees and used for defense. When a bee stings, the stinger, along with venom sac often detaches, leading to the bee’s death (this is true in honey bees, not so in bumble bees or wasps due to difference in stinger structure; Wu et al., 2014). The queen’s stinger is only used to kill rival queens during the swarming period. Male drone bees do not have a stinger, so go ahead and pick one up. Give it some pats for being such a good boy.

Bees have two stomachs (de Paula et al., 2022). The honey stomach (or honey crop) is a specialized organ to store collected nectar. Enzymes like glucose oxidase and invertase break down the sugars while the nectar is in the vessel in preparation for honey production. The second “true” stomach is part of the digestive system, whereby bees consume nectar and pollen for day to day sustenance. The gut is identified by two parts: the midgut where digestion takes place, and the hindgut where where wastes are expelled.

The queen has a fully developed reproductive system and after mating, lays fertilized eggs. Drones have testes and seminal vesicles for sperm production and storage. Worker bees have an underdeveloped reproductive system; in times of queenlessness, they are capable of laying unfertilized eggs that only produce drones (Peso et a., 2016).

The final major glands are the wax glands. Only the worker bees produce and manipulate wax for comb building and hive architecture. There are four wax glands (plates) on the underside of the worker bee’s body, producing almost transparent wax that darkens in colour as it is processed and manipulated by the bees (Hepburn et al., 2009).

Further reading on the bee abdomen is available in “Honey Bee Stomach“.

The Evolution of Honey Bee Anatomy

Evolution is an ongoing process. The honey bee colony as we see it today is the result of millions of years of adaptation from continuous environmental pressures (elaborated on in Evolution Terms). The ancestors of modern bees were likely predatory wasps, who adapted to a diet based on pollen and nectar in response to selective pressures.

To a wasp, switching to this vegetarian diet required new tools, as shown by the emergence of specialized structures for collecting pollen like the pollen baskets or body hair (gathering pollen is much easier when it sticks to your body). As flowering plants diversified, these proto-bees evolved alongside them. The surge in floral varieties meant that bees had to identify and access the nectar and pollen, so more specialized structures emerged, like compound eyes to identify suitable flowers and mouthparts for both chewing and sucking.

We do not know when the jump to complete sociality (eusociality) occurred, only that the division of labour within honey bee colonies is a result of evolutionary pressures to ensure the survival of the hive as as single organism (more accurately, the superorganism). Over time, genetic mutations and behaviours that enhanced colony efficiency and survived and were passed onto subsequent generations of honey bee colonies.

Bees that do not Bee-have like Honey Bees

The anatomical descriptions provided above are similar across the entire Class Insecta (indeed, even beetles have three pairs of legs, two pairs of wings and a three-segmented body). Between species, there might be small variations in morphology, such as longer proboscises to access deep flowers, while others might have specialized structures for nesting or carrying pollen. Below I present four examples of bees that look like honey bees, but with their own special characteristics.

1. Stingless Bees: These bees don’t sting, they bite, and in great numbers. They are prevalent in tropical regions and the social stingless species produce honey, albeit in smaller amounts than Apis mellifera (a few kilograms per year compared to the dozens of kilograms per honey bee hive).
2. Bumble Bees: Larger and hairier than honey bees, they live in smaller colonies and have a robust body that allows them to fly in cooler temperatures. They conduct “buzz pollination” whereby the bee grabs onto the flower and rapidly vibrates its flight muscles, causing the flower to release its pollen. Certain plants do not readily provide pollen, so buzz pollination is one such mechanism to collect it.
3. Solitary Bees: The vast majority of bee species are solitary and do not live in colonies. Examples include mason bees and leafcutter bees, who nest in small holes within trees or in the ground. The interested beekeeper or insect appreciator is encouraged to provide local resources that help native bees, who are largely solitary species.
4. Cuckoo Bees: Ah yes, the parasitic cuckoo bees. These bees are classified as “cleptoparasitic”, laying their eggs in the nests of other bees. The cuckoo larvae hatch prior to that of the host, whereby they consume that solitary bee’s nest stores and (sometimes) the host larva. Fortunately, there are no cuckoos of honey bee colonies.

Worker Bee, Honey Bee

The anatomy of bees seen today was produced by a process of constant evolution, and these morphologies are consistent throughout the Hymenopteran Order. An overview of the basic bee body functions and structures was presented here. For those interested in more a technical breakdown of each body segment, refer to the individual articles in the subsections above.

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Footnotes

1. Royal jelly is commonly associated with honey bees in the Apis genus, however some social stingless bees have their own version of “royal food” (Hartfelder & Engles, 1989). ↩︎