Honey Bee Stomach

The Abdomen’s Role in Digestion, Reproduction and Navigation

In the anatomy of bees, the abdomen is the “soft posterior” section, containing the striped pattern that gives it its characteristic appearance. Within it are the wax glands, for beeswax production, and the Nasonov glands, for communication, amongst others. The bee abdomen is also used for navigation, digestion and reproduction. In the female bees, it is tapered at the end and made up of six segments (seven in the male drones to accommodate the larger reproductive organs, the testes and endophallus).

An anthropomorphic view of worker bees states “they have two stomachs- the ‘honey stomach’ and their ‘true stomach'” – not so. What a beekeeper calls the “honey stomach” is part of her foregut (the “honey crop”) and is primary a foraged nectar storage organ, not a gastric stomach that digests food. Actual digestion and absorption occur in the midgut (“ventriculus”).

A non-Technical Summary of the Honey Bee Abdomen

For readers in a hurry, here is an overview:

Organs of the Bee Abdomen: Digestive Tract

The four organ systems described here are:

• Digestive tract,
• Respiratory system,
• The reproductive organs and
• The sting apparatus

The Honey Stomach (“Crop”) Stores Collected Nectar

The bulk of the bee’s digestive system is performed in the ventriculus (“true stomach”), whilst the honey crop stores nectar for later regurgitation and processing.

Structurally, the honey stomach is a muscular, expandable pouch holds nearly its own weight in nectar¹. To prevent this nectar from entering the ventriculus, a one-way valve is present at the honey stomach entrance known as the proventriculus (literally “before ventriculus”). This controls the passage of nectar in the worker’s body and also filters particulates found in the nectar (mostly pollen) to prevent its adulteration (Bailey, 1952).

Before the nectar enters the crop, it begins an enzymatic breakdown. In the saliva of bees is the enzyme invertase, which begins processing the sucrose in nectar to fructose and glucose (Nicolson and Human, 2008), the two main sugars in honey. All the while, the foraging bee is flying back to her hive to conduct further refinement via trophallaxis, whereby the nectar is regurgitated between worker bees. The passing of nectar loads reduces the water content and includes detoxifying molecules (glucose oxidase, generation of hydrogen peroxide), which aim to discourage microbial growth in the soon-to-be-stored honey (Reviewed in Berenbaum & Calla, 2020).

The Ventriculus is the Primary Digestive Organ

The actual digestion and absorption of nutrients takes place in the ventriculus (also called “midgut”). It contains digestive enzymes that break down food particles, and allow for nutrient absorption, akin to the “true stomach” in your and me. The 2012 review by Engel & colleagues showed that the bacterial communities within honey bees had different degrees of carbohydrate-degrading enzymes, with fructosidases (enzymes that break down fructose-based molecules) being the most prevalent. Such findings provides support for why honey bees favour high fructose-corn syrup feeds, despite its downsides on colony health.

Briefly: The Gut Microbiota’s Role in Digestion

Main article: How Bees Digest Pollen

In short, the microbiota “pre-digests” tough components and produces beneficial metabolites, acting as an extended digestive system for the adult bee.

In A. mellifera, there is a core group of 5 bacterial phylotypes each with their own role in disease resistance, immune function and larval development. Species like Gilliamella apicola and Snodgrassella alvi help degrade complex carbohydrates and detoxify plant compounds (“polyphenols”). The former G. apicola, breaks down pectin, a major component of the pollen inner layer, aiding nutrient release (Zheng et al., 2016).

The work of Brown and colleagues showed that honey bee workers with an intact, “natural” gut microbiome gain more weight and live longer when fed pollen, whereas bees lacking their normal gut bacteria (in this case, due to antibiotics) cannot fully utilise the pollen nutrients (Brown et al., 2022). 

Waste Excretion by the Hindgut & Malpighian Tubules

The remaining undigested food particles travel to the hindgut in preparation for excretion. In this structure, additional water and minerals are reabsorbed, and the waste begins to solidify. Bees (and other insects) do not have kidneys, rather thin, filament-like structures called “Malpighian Tubules”, which branch off at the junction between the midgut and hindgut (they also filter wastes from the bee blood, [haemolymph] Dow 2009).

The final segment of the hindgut is the rectum, where feces are stored until they’re expelled. The rectum absorbs the final amounts of water, making the feces more solid (Dimou & Thrasyvoulou, 2009). For beekeepers in climates cold enough for winter clusters, the bees take “cleansing flights” on the first days of spring, whereby they void large amounts of accumulated poop and painting the immediate surroundings a cheerful yellow!

Bees are hygienic creatures and do not defecate inside the hive. Worker bees take daily flights to remove waste outside, and should the beekeeper find excrement within the hive, then it is likely a symptom of honey bee disease.

Bonus: The Bee Abdomen as a Pollen Carrier

Female Megachilidae (leafcutter) bees carry pollen on their abdomen instead of their legs (Muller & Bansac provide an example using Megachile pilicrus; Muller & Bansac, 2003 ). These solitary bees would harvest pollen from Asteraceae flowers, and then brush the pollen from their legs into the abdomen.

Yellow-faced bees from the genus Hylaeus lack the hairs (scopa) that distinguish bees from other insects. They ingest the pollen from the flowers and transport it in their stomach, as other bees would with nectar (Muller, 2023). The pollen is regurgitated at the nest and added to the stockpile of larvae food.

Glands of the Honey Bee Abdomen

Three glands will be covered: the wax glands for comb building, the Nasonov gland for communication, the sting gland (yes, it has a gland).

Wax Glands Produce Fresh Wax for Comb Building

Wax glands are located on the underside of the worker bee’s abdomen (specifically, the ventral side of the last four abdominal segments). Wax-building duties are assigned to the younger worker bees at six days of age, peaking at 14 days and waning after that (Hepburn et al., 1991). When wax is newly produced, it comes out as a thin flake that hardens when exposed to air. As bees walk on the structure, it darkens into yellow and then brown in older comb, due to the accumulation of dirt.

Wax is a complex series of hydrocarbons that is energetically costly to produce (a process involving acetic acids and large secretory cells termed “oenocytes”). As the saying goes “one pound of wax needs eight pounds of honey”. Bees require energy to activate the adipocytes (fat cells) for wax production (Hepburn et al., 1991). The fatty acids within the cells are converted to wax esters and secreted through the cuticle as thin flakes, whereby the the hind legs of the worker bees remove it from the abdomen and transport it to the mouthparts for manipulation. Honey bees require a high colony temperature of 32-36°C for optimal wax-building conditions. Combining this temperature with the chewing process makes wax a pliable, workable substance for use.

Nasonov Gland is for Communication During Foraging and Swarming

The Nasonov gland is found at the tip of the abdomen, and produces a pheromone by which bees communicate, such as in foraging and swarming. The gland is exposed between the grooves of the abdominal segments (tergites) when bees find the entrance of their hive, when foraging for nectar or pollen, or when locating a water source. In swarming, it is exposed as they fly (to lead other bees in the right direction), atop the now-swarmed colony (to attract bees returning from scouting² for a natural beehive), or to lead an unfortunate queen bee back to the cluster, should she get lost (Pickett et al., 1980).

The Nasonov Gland Triggers Undertaking Behaviour in Apis cerana

The Nasonov pheromone is used to remove dead bees from Apis cerana hives (it also has a different chemical composition [no volatile terpenes] compared to Apis mellifera; Klett et al., 2021). When a worker bee dies, it releases odorous fatty acids via the Nasonov gland, which triggers undertaking behaviour in other worker bees, thus removing the corpse before it becomes a vector of disease. Bees with higher hygienic traits were more sensitive to the chemical imbalances (i.e, more fatty acids released from a deceased hive mate), and thus performed these removals faster than others (Klett et al., 2021).

The Sting Gland Produces Venom

Every beekeeper has felt it, many bystanders have feared it, and all worker bees have it. The stinger is a fatal hive defence tool as the attacking bee dies upon stinging vertebrates. When the stinger deatches from the bee and sticks into the victim, it continues to inject venom into that tissue (Schmidt, 2014). Its effectiveness comes from the ability to pierce the highly elastic skin of vertebrates without easily dislodging (achieved by having a barbed structure), and being able to pump venom autonomously.

The honey bee stinger is a modified ovipositor that evolved from a structure used for laying eggs (Smith 1970). The queen’s stinger is smooth and can sting multiple times without harming her (it stings other queens during the queen cell hatching frenzy).

Bee Stinger Composition

Readers interested in a visual description are pointed to the electron microscopy & CT scans by Ramirez-Esquivel & Ravi (2023).

1. Lancet: The sharp & pointy part of the stinger; this is what you feel and see on your skin. The barbed nature in worker bees means their hooks face backwards to ensure that once it’s in, it stays in.
2. Venom Sac: Attached to the stinger and stores the venom produced by the glands. When a bee stings, muscles around the venom sac contract, pumping venom through the stinger and into the target.
3. Valve Chamber: a connector between the venom sac and the stinger to ensure the flow of venom into the sting site.
4. Muscles & Nerves: coordinate the sting action – controlling the penetration and pumping of venom (lancet protractor and retractor muscles). They are autonomous and will continue pumping venom even when detached from the bee body.
   1. Nouvian & colleagues (2016) showed that after stinging, a bee continues defending the hive for up to 4 days despite losing part of her lower digestive tract.
   2. The stinger detaches from the bee as the three sets of the muscles that attach to the abdominal interior are weak (atrophied) in bees. Combined with the elasticity of mammal tissue, the worker bee lacks the strength to pull the stinger out (without extreme effort), so it tears away from her body.

Dufour Glands Label Eggs as “Safe”

The function of Dufour glands in Hymenoptera is contested. These gland is located in the sting apparatus and in other insects, secret chemicals that protect the newly-laid egg. In bees however, Abdalla and Cruz-Landim (2001) suggest that it is used for chemical signaling. The worker bees would mark eggs laid by the queen with their Dufour gland to prevent other workers from eating them. This strategy is effective in preventing eggs laid by workers from hatching, but a question arises in determining how would this be in colonies with two laying queens?

Comparing the Reproductive Organs between the Three Castes

The queen’s reproductive system is developed for high egg laying (fecundity) and longevity, with queens living up to five years and laying over a thousand eggs per day at optimal conditions. Workers have underdeveloped reproductive systems and if they were to lay eggs, they would hatch into drones. A laying worker is a sign of something wrong with the hive. Drones have a spermathetca that detaches from the body after mating, leading to their death…by snuu snuu.

Reproduction deserves its own post, and a general overview is provided here.

Queens Lay Fertilized Eggs and Live a Long Life

The queen has fully developed ovaries with around 150-180 tubules in each ovary. These ovarioles produce eggs that are fertilized by the spermatheca; a spherical organ located in the queen’s abdomen. This structure stores the sperm accumulated during the mating flight, and when an egg is fertilized, it develops into a worker.

Commercially-reared Queens have a Different Gut Microbe Composition

A bonus: Powell & colleagues (2018) and Tarpy & colleagues (2015) found that queens isolated after hatching (as in artificial queen rearing) from the colony and attendant nurse bees had larger and more diverse gut microbial communities. The long term implications of this are unknown, and the authors suggest that early contact with nursing bees is key in developing the queen’s immune system. In wild colonies, the microbiome of mature queens was similar to that of the workers form the same colony.

Perhaps the long term effects of this alternation in is unknown because beekeeper re-queen ever 1-2 years?

Worker Bees Do Not Lay Eggs Under Normal Conditions

Unlike queens, workers have rudimentary ovaries with fewer ovarioles as they do not lay eggs. However, in the absence of a queen (Hoover et al., 2003) or brood pheromones (Pankiw 2004), some workers will be triggered to lay unfertilized eggs, as they do not posses spermatheca nor mate with drones.

The Drone Bee Abdomen is Largely Testes

Drones have two large testes within their abdomen, taking up a significant portion of their internal structure to produce sperm and seminal fluid for mating with queens. Their abdomen is bulkier and rounded at the tip rather the elongated and pointy. Each testis is made up of numerous tubules (testioles), where spermatozoa are produced (spermatogenesis) prior to moving into the seminal vesicles for storage (Yaniz et al., 2020). Accessory glands produce seminal fluid containing proteins, enzymes, and other substances that maintain sperm viability and mobility for a successful fertilization process.

Despite this, they fly fast, far faster than queens to catch them during the mating flights³. The endophallus facilitates the transfer of sperm to the queen during her nuptial flight. Drones die post-mating due to the rupturing of their abdomen from the high internal pressure of ejaculation (Metz & Tarpy 2022). One way to know if the queen successfully mated is to check for the “Mating sign”, or the remains of the drone endophallus around her abdomen.

The Iron Granules in the Honey Bee Abdomen Contribute to Navigation

The waggle dance is a well studied form of honey bee communication, which combines olfactory cues, visual landmarks and the sun to provide a compass. As early as 1978, honey bees were reported to orient with the earth’s magnetic field, and this was recently shown to be due to the iron granules within the bee abdomen (Liang et al., 2016). The granules contain magnetoreceptors, which enable bees to align themselves with respect to the Earth’s magnetic field and thus help them find their way to the hive or flower patch. Follow your stomach, indeed!

The Honey Bee Abdomen: More Than a Stomach

The abdomen is more complex than the overview provided; there are more glands and nuances to its functions. The interested reader is directed to the respective scientific articles on the topic, or to the subsequent posts on the bee thorax and bee head.

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Footnotes

1. Remember those fun facts that a bee can carry half its weight in nectar? Well, the honey crop has a volume of 50 milligrams. The average weight of a worker bee is 100 milligrams, so when the honey crop is full, the bee is carrying 50 mg nectar as well as the 100 mg of its own weight, hence half its weight! ↩︎
2. One school of thought for capturing swarms is to bait hives using lemongrass oil, as it contains a citral, one of the components of the Nasonov pheromone. I much rather use old brood comb, as the smell of broodrearing (pollen remains, defecations, shed cuticles etc….) is far more effective. ↩︎
3. Drone speed varies greatly between Apis species (Koeniger et al., 2005), flying up to 3.7 meters/second, or 13.3 kilometres/hour. Not bad for a ~22mm insect! ↩︎