1972; Ritter 2014; Snodgrass 1956). Similar to vertebrates, hemolymph, or bee blood, contains many blood cells or hemocytes and functions to transport nutritional substances and waste products to and from the tissues, respectively; however, unlike vertebrates, insect hemolymph does not have hemoglobin and therefore cannot carry oxygen. In addition to their role in metabolism, insect hemocytes are also thought to have several other functions, including phagocytosis, resistance to microorganisms and parasites, blood coagulation and immunity (Wigglesworth 1972).
The vertebrate respiratory system is characterized by gills or lungs that act to exchange oxygen and carbon dioxide gases in concert with a complex circulatory system. A simpler method of gas exchange occurs in insects where the respiratory and circulatory systems are separate and blood has only a minor role in gas transport (Winston 1987). The respiratory system of most insects consists of a series finely branched trachea and tracheoles that allow for the direct diffusion of oxygen to their tissues and the removal of carbon dioxide (Wigglesworth 1972; Ritter 2014; Snodgrass 1956). Some insects, like the honey bee, also have saccular dilations of the trachea forming great air sacs. Air is brought into the insect through breathing apertures along the lateral thorax and abdomen known as spiracles (Snodgrass 1956). Ventilation of the air sacs and larger tracheal tree is thought to occur via specialized body wall movements that act to renew the tidal volume (Wigglesworth 1972). Most breathing insects have control over expiration alone, while inspiration is passive via the elasticity of the exoskeleton; however, honey bees have muscles that control both inspiration and expiration. The tracheal air sacs act largely as reservoirs for ventilation, and the respiratory movements of the abdomen in honey bees produce expansion and contraction of the air sacs similar to vertebrate lungs (Snodgrass 1956). Carbon dioxide is delivered via the hemolymph to the trachea for exhalation although elimination also occurs via direct diffusion through body tissues (Wigglesworth 1972; Ritter 2014).
Nervous System
Even though an insects' nervous system is relatively simple compared with a vertebrate's complex nervous system, it is well adapted to the environment (Vidal‐Naquet 2015). The central nervous system of the honey bee consists of a primitive brain and a ventral cord that controls sensory perception, movement, navigation, defense, etc… whereas the vertebrate nervous system is comprised of a well‐developed brain, medulla, and spinal cord. The brain of insects largely coordinates sensory perception of the environment (Vidal‐Naquet 2015) and is comprised of three sections: the protocerebrum enables vision and forms two large optic lobes linking the compound eyes, the deutocerebrum controls olfaction via the sense organs of the antennae, and the tritocerebrum facilitates taste through the labrum. The ventral cord section of the central nervous systems – comprised of seven ganglia throughout the thorax and abdomen – innervates insect mouthparts, all of the legs and wings and the sting apparatus (Vidal‐Naquet 2015). The peripheral nervous system supports the various sense organs supporting interactions among honey bees and perception of their environment.
Sense Organs
Sensory perception is quite advanced in insects, providing these animals a remarkable ability to sense and adjust to their environment. Insects, including honey bees, have several types of sense organs of their exoskeleton that may respond to pressure, odor, taste, sound, or light (Wigglesworth 1972; Snodgrass 1956). Seven different sense organs are described and even though they vary markedly in structure, they have in common a basic unit called the sensillum. The sensillum is composed of one or more sense cells connected to the central nervous system via a sensory axon, and a specific cuticular structure with accessory cells (Snodgrass 1956; Vidal‐Naquet 2015; Wigglesworth 1972). Some of these organs have a small hair, peg, or plate, or a group of sensilla connected to the sense cell or cells and provide many sensory functions for insects (Wigglesworth 1972; Snodgrass 1956). The antennae of honey bees (faced with several types of sense organs) are a major center of communication with many sensory roles, including odor and chemoreception, detection of movement and vibration, as well as the perception of sound, temperature, and humidity (Snodgrass 1956; Vidal‐Naquet 2015).
The Organ of Johnston, located on the pedicel of the antennae, is another sensory organ used in a variety of ways by insects including flight control, navigation, detection of gravitational and electromagnetic fields, mate identification, sound perception, and communication (Wigglesworth 1972). In many insects this organ indicates velocity and orientation in flight and other movements. In the honey bee the Johnston's Organ is also thought to be important for communication between foraging bees during the waggle dance (Tsujiuchi et al. 2007). This organ detects changes in the position of the antennae via mechanical stimulation – from abdomen waggling and wing vibration of a dancing bee – and together with other sensilla translates direction and distance communication to following forager bees during the waggle dance (Brockmann and Robinson 2007; Tsujiuchi et al. 2007).
In addition to the abundant sense organs of the antennae, mouth parts and other portions of the body, honey bees have two types of eyes – the paired compound eyes and three ocelli (Snodgrass 1956). The compound eyes are complex visual organs composed of thousands of hexagonal facets – known as ommatidia – that each function independently to receive, concentrate, and perceive light (Winston 1987). Specific groups of facets are specialized and work together for various functions including detecting light polarization, pattern recognition, color vision, and head movement (Winston 1987). The honey bee collects a mosaic of sensory input to the brain that is integrated to form an image; bees are good at identifying shapes and detecting movement, and may visualize shorter wavelengths (ultraviolet) of light compared with humans. Further, the compound eyes have sensory hairs near the facet junctions that perceive airflow and likely aid in navigation and orientation (Snodgrass 1956). The three ocelli or simple eyes of the honey bee likely do not form an image as with the compound eye, but rather are thought to be important for detecting variations of light intensity that may help diurnal navigation and orientation (Winston 1987).
Immune System
Social, compared with solitary organisms, are at an increased risk of disease because often, as with the honey bee colony, large numbers of individuals are living in a confined nest with stored resources; however, group living also imparts heightened infection control measures (Evans et al. 2006; Fefferman et al. 2007; Kurze et al. 2016). The social structure of the hive helps defend against disease in many ways (e.g. grooming, hygienic, and necrophoric behaviors, use of propolis with anti‐microbial properties, social fever in response to disease, as well as nest hygiene and defense among others) (Evans et al. 2006; Vidal‐Naquet 2015).
Individual bees also have physical properties that help prevent infection in addition to both cellular and humoral immunity for the recognition and removal of pathogens. Several morphologic characteristics of insects help combat infection (Evans et al. 2006). A layer of antimicrobial secretions covers the external surfaces of many types of insects and the intestinal tract with digestive enzymes is not friendly to pathogen survival, although the semipermeable midgut is documented as entrance site for several honey bee pathogens (Davidson 1973). In addition, the gastrointestinal tract of the adult worker honey bee is characterized by a core set of bacteria that are not only important in nutrition and metabolism, but also protective against pathogen infection (Raymann and Moran 2018). Next, the exoskeleton cuticle of the honey bee forms a physical barrier against pathogen invasion, as does the peritrophic membrane of the intestinal tract (Vidal‐Naquet 2015; DeGrandi‐Hoffman and Chen 2015). Further penetration of the honey bee by an infectious agent elicits an immune response at the level of the hemolymph and fat body. Such immune defenses of insects are similar to the innate immune system of vertebrates, both sharing many characteristics including the actions of phagocytosis, secretion of antimicrobial peptides, enzymatic degradation of pathogens, as well as similar architecture and orthologous components (Evans et al. 2006). Unlike vertebrates however, insects lack adaptive immunity and cannot produce antibodies; rather the honey bee immune response is characterized by non‐specific reactions against pathogens via both cellular
