Figure 4.1 Comparative anatomy of the vertebrate equine (a) with the invertebrate honey bee (b). Despite their marked physical differences, these two groups of animals share similar functional body systems. Note the separate colors for the various body systems: GI System – Brown = Foregut, Light Green = Midgut, Dark Green = Hindgut; Blue = Respiratory System; Red = Cardiovascular System; Yellow = Nervous System.

      Source: © Lauren D. Sawchyn, DVM, CMI. Chapter: Physiology of the honey bee, authored by Rolfe M. Radcliffe and illustrated by Lauren D. Sawchyn.

      Digestive System and Metabolism

      Insects are able to feed on a great variety of organic material in nature and their digestive systems are modified to reflect their specific diet (Wigglesworth 1972). The more complex vertebrate systems have also evolved to complement their diet, whether it be monogastrics, ruminants or birds (Molnar and Gair 2015). The digestive function of the insects is similar to vertebrates both having an alimentary canal composed of foregut, midgut, and hindgut segments (Ellis 2015; Ritter 2014). The digestive system of the honey bee serves three main functions: the intake and absorption of nutrients, the elimination of waste, and a means of transport and storage of nectar and honey (Snodgrass 1956). Importantly, the three honey bee castes – drone, worker, and queen – display many physiologic, morphologic, and behavioral differences (Snodgrass 1956; Wigglesworth 1972; Hrassnigg and Crailsheim 2005).

      The foregut of insects is composed of the mouth, esophagus, and crop, or honey stomach as it is known in the honeybee. The mouthparts of insects are adapted to fit their needs such as chewing solid materials like foliage, wood, or other creatures, as in many beetles or, as with the honeybee, to collect fluids such as the nectar of flowers (Wigglesworth 1972). The esophagus simply functions to transport food from the mouth through the head and thorax to the crop located in the abdomen, while the crop is the organ for storing nectar or water. The proventriculus functionally separates the crop and the ventriculus, and in the worker bee it regulates the food transfer between these two organs, retaining nectar in the crop for delivery to the hive (Snodgrass 1956). The proventriculus is comprised of four folds that act to separate the pollen from the nectar and control movement into the ventriculus. Because the proventriculus coordinates the nutrition between each honey bee and that of the hive, it represents the connection of the individual and social metabolic cycles of the honey bee colony (Ritter 2014).

The midgut or ventriculus of insects is the primary site for the digestion and absorption of nutrients. The ventriculus is the largest part of the alimentary tract and is the true stomach of the insect similar to the stomach of monogastric animals or the abomasum of the ruminant. The epithelium of the ventriculus functions to secrete digestive enzymes, absorb food materials, and excrete redundant products, such as calcium (Snodgrass 1956). In addition, cylindrical food envelopes, known as peritrophic membranes, are present the entire length of the ventricular epithelium; this layer is present in many insects and may improve digestion, act as a barrier to pathogens, and protect the epithelium from coarse pollen granules similar to the mucous boundary protecting the intestinal tract of vertebrates (Snodgrass 1956; Wigglesworth 1972; Vidal‐Naquet 2015). However, because the midgut region of the honey bee is semi‐permeable, this is also where many pathogens gain entrance into the insect, such as Nosema species and several viruses. The ventriculus opens into the anterior intestine – the beginning of the hindgut of insects, and this pyloric juncture is where the Malpighian tubules enter the intestinal lumen. These tubules are the excretory organs of the honey bee similar to the kidneys of vertebrates where waste products from the hemolymph are removed including nitrogen, urates, phosphates, and calcium (Wigglesworth 1972; Ritter 2014; Snodgrass 1956).

      The hindgut of insects includes the anterior or small intestine and the posterior intestine or rectum; like that of the cecum and colon of vertebrates, its primary function is the absorption of water. The rectal glands, or more appropriately termed rectal pads, are thought to be the location of water conservation in terrestrial insects (Snodgrass 1956; Wigglesworth 1972). Importantly, in the honey bee the rectum also serves to store waste products during the extended time spent inside the hive during the winter months in temperate climates. In fact, in the over‐winter worker bee, almost the entire abdomen may be distended with feces.

      Both vertebrates and invertebrates utilize carbohydrates, proteins, and fats as energy for growth and work activities (Wigglesworth 1972). Carbohydrates are the most readily available energy source of insects, and the honey bee is dependent upon glucose and other sugars to maintain an almost constant supply of energy for flight and the various hive activities (Wigglesworth 1972). Blood glucose alone may be able to maintain flight in honey bees for 15 minutes or a distance of approximately 5.5 kms, while extended forays for hours are possible with a full honey crop (Wigglesworth 1972). Proteins are necessary as the building blocks of insect muscles, glands, and other tissues, while fat is the primary form of energy storage.

      Unique to insects, the fat body is a large organ distributed throughout the body cavity and involved in metabolism and sustaining food reserves; this organ functions similar to the liver in vertebrate energy storage (Arrese and Soulages 2010; Wigglesworth 1972). Importantly, the fat body has many other functions including maintaining body homeostasis and supporting immunity. The insect fat body produces hemolymph proteins, circulating metabolites, antimicrobial peptides, assists nitrogen removal, collects toxic compounds and its function may be affected by disease and poor management (Arrese and Soulages 2010; Ritter 2014). In the honey bee the fat body stores fat, protein, and glycogen to be used for energy reserves during winter rest, summer colony growth, times of high energy demand such as flight or during times of food shortages (Snodgrass 1956; Wigglesworth 1972; Ritter 2014). Winter worker bees have a longer lifespan partly because of a less active lifestyle, reduced metabolic rate and increased fat bodies compared with summer worker bees.

      Circulatory and Respiratory Systems

      In contrast to vertebrates, insects have an open compared to a closed circulation system with a single blood vessel and simple heart (Wigglesworth 1972; Ritter 2014). This system fills most of the body of insects outside of the other organs and tissues (Snodgrass 1956). Blood flow is produced by the action of a pulsating dorsal vessel that has a heart‐like function in the abdomen, directing the hemolymph forward through the aorta toward the brain; the blood returns via the body cavity to the abdomen to repeat the cycle (Wigglesworth 1972; Ritter 2014; Snodgrass