Last month, I discussed the external anatomy of the honey bee. This month, I will turn my attention to the internal anatomy of the honey bee. I wanted to include high quality photographs of each organ discussed in this article so that the internal anatomy could be illustrated appropriately. However, such photographs are hard to acquire and nearly everything in a bee is creamy white or clear in appearance. Thus, the internal anatomy of a bee lacks visual stimulation, though the various parts are functional marvels. Rather than including color photographs that might be hard to understand, instead, I defaulted to using schematic drawings of some basic internal anatomy. I do not include images of everything I discuss. Instead, I would like to invite you to review the documents that I list in the Recommended Readings section of this article. All of the readings are fantastic references for helping you to understand and visualize bee internal anatomy in general. Some of the sources are even available for free online. As a final note, I list all of the anatomical features that I am discussing in bold font so that you can know exactly what I am trying to define.
Honey bees have an open circulatory system. This simply means that hemolymph (bee blood) does not pump through veins but rather freely circulates in the bee’s body cavity. Hemolymph does not transport oxygen but rather transports nutrients and hormones to the various body tissues. Furthermore, hemolymph picks up waste products generated in the body and transports them to the excretory organs. Hemolymph also serves as a reservoir of food and can aid in heat transfer within the bee.
How does the hemolymph get around to the various organs? Bees specifically and insects in general have a single vessel that runs from their abdomens, through their thoraxes and into their heads. This vessel is arranged dorsally, meaning that it runs down their back. The part of the vessel occurring in the abdomen is called the dorsal heart while the part in the thorax is called the dorsal aorta. The dorsal heart, the part in the abdomen, has small holes in its sides. These holes are called ostia. The dorsal heart pulses, pulling hemolymph through the ostia, into the vessel, and pumping it through the dorsal aorta in the thorax and into the head. From the head, the hemolymph percolates through the thorax and back into the abdomen. It bathes the various internal organs in route back to the abdomen. Once in the abdomen, the hemolymph absorbs nutrients acquired during food digestion and reenters the dorsal heart to start the cycle again.
The digestive system is composed of three main sections, the foregut, midgut, and hindgut. Interestingly enough, the three sections of the digestive tract form separately during bee development. The foregut (first third of the digestive tract) and hindgut (last third of the digestive tract) form as invaginations from either end of the developing bee. Imagine, for example, holding a balloon between the pointer fingers of both of your hands, with the fingers being on opposite sides of the balloon. Imagine, now, pushing your fingers toward one another, pressing both of the sides of the balloon in toward the center. This is a good model for how the digestive tract of the bee forms. The foregut and hindgut develop as invaginations from both ends of the bee. As a result, the foregut and hindgut are lined with the same material that lines the outside of the bee’s body (cuticle), just like the two depressions on the balloon are lined by the external surface of the balloon. Practically speaking, this means that the foregut and hindgut are not sites for nutrient absorption in the bee since nutrients cannot traverse the cuticular lining of either section. In contrast, the midgut is not lined with cuticle, thus making nutrient absorption its primary function. Given that the foregut and hindgut are lined with cuticle, the lining of both are shed as a bee molts (sheds its exoskeleton) during larval development.
The foregut is composed of the mouth, esophagus, and crop (Figure 1) of the honey bee. Food enters the digestive tract through the mouth and travels down the esophagus and into the crop. The esophagus is simply a tube that runs from the mouth in the head, through the thorax, and into the crop in the abdomen. The crop, or honey stomach as it is called sometimes by beekeepers, is a spherically shaped organ in the abdomen that serves as a site for food storage, as a storage place for nectar bees collect from flowers and fly back to the hive, or as an initial site for the digestion of food in the bee. The crop can expand significantly when it is full of honey or nectar, so-much-so that the abdomen swells. The foregut and midgut are separated by a valve called the proventriculus which is located at the end of the crop. This valve can grind and pulverize food particles (such as pollen) and filter pollen out of the crop contents. Food passes through the proventricular valve and into the bee’s midgut or ventriculus (Figure 1).
The midgut is the primary site of enzymatic digestion of food and absorption of nutrients. It is not lined by cuticle but rather is lined by the peritrophic membrane. This membrane likely protects the digestive cells (the cells that line the internal surface of the midgut) while allowing absorption of the nutrients straight into the hemolymph. Because the midgut is somewhat permeable, being so because of its function as the site of nutrient absorption, this is where many viruses and other bee pathogens enter the hemolymph. This is true especially for the Nosema pathogens (N. apis and N. ceranae) and some viruses.
Next along the digestive tract are the Malpighian tubules (Figure 1). These occur at the end of the midgut and are, essentially, spaghetti-like extensions of the tract that float freely in the bee’s body cavity. The Malpighian tubules extract waste products from the hemolymph. They produce uric acid granules and help with osmoregulation (water management) within the bee.
The hindgut, or final section of the digestive system, is composed of the ileum (Figure 1) and rectum (Figure 1). The ileum, sometimes called the small intestines, is a short tube that connects the midgut to the rectum. The rectum is important for the absorption of water, salt, and other beneficial substances prior to waste excretion. There are small areas on the rectum called rectal pads. These sections reabsorb >90% of the water that was used by the Malpighian tubules to collect waste. The latter is an important function. Bees, like most insects, try to retain as much moisture as possible from the food they eat. Thus, they do not excrete nitrogenous wastes in a urine-equivalent as humans do. Instead, they reabsorb much of their water and tend to defecate moderately liquid to dry feces. Uric acid solids and other unused foodstuffs, such as the shells of pollen grains, are excreted as relatively solid feces.
The glandular system of a honey bee has four basic functions: (1) internal (within the body) and external (outside the body) communication, (2) food processing, (3) defense and (4) wax production. The glandular system includes a number of glands located throughout the bee’s body. These glands are organs composed of clusters of cells that produce and secrete various products. Those glands that secrete products inside of the body to produce a change inside of the body are called endocrine glands while those that secrete chemicals through ducts to the outside of the body to produce a change in other organisms outside of the body are called exocrine glands. The chemicals released by …