Elise E.B. LaDouceur1, Sarah C. Wood2, Damien Laudier3, and Elemir Simko2 1 Joint Pathology Center, Silver Spring, MD, USA 2 Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada 3 Laudier Histology, New York, NY, USA Insects are the most diverse group of animals in the world. They are members of the phylum Arthropoda and are identified by three pairs of jointed legs, three body segments, an exoskeleton, one pair of antennae, and usually two pairs of wings. Several families have particular importance for human and environmental health, particularly Apidae (e.g., bees), Drosophilidae (e.g., Drosophila sp.), and Culicoidea (e.g., mosquitoes). Two‐thirds of all flowering plants depend on insects for pollination. Many insects are parasites and also serve as viral and bacterial disease vectors for humans. Additionally, insects may be the source of massive agricultural devastation, requiring intensive management for food production. The diversity of this taxon results from their adaptation to virtually every ecologic niche, including land, air, and fresh water. Qualities that are considered the most important to their success include resistance to dessication, flight, and holometabolous development. Holometabolous insects, which include beetles, butterflies, moths, bees, and ants, have immature stages that are markedly different from adult stages (e.g., caterpillar and butterfly). This allows for resource partitioning and decreased competition as the larvae occupy separate ecologic niches from the adults. The six largest orders of insects are Coleoptera, Lepidoptera, Hymenoptera, Diptera, Hemiptera, and Orthoptera. Coleoptera includes beetles, which have hardened wings. Lepidoptera includes butterflies and moths, which have two pairs of large wings covered with small scales. Hymenoptera includes bees, wasps, and ants, which have two pairs of thin, clear wings (some animals in this class only have wings during one life stage). Diptera includes flies and mosquitoes, which have two pairs of wings with the second pair being markedly reduced in size (and difficult to visualize). Hemiptera includes aphids and cicadas, which have short wings that form a triangle on the back. Orthoptera includes grasshoppers and crickets, which have four wings and jumping hindlimbs (Ruppert et al. 2004). Due to the marked diversity of insects, this chapter primarily uses honeybees for descriptions of anatomy and histology; comparisons to specific groups are provided when necessary. The external surface of insects is similar to other arthropods. It is overlain by a chitinous exoskeleton that is made up of hard plates connected by soft cuticles (or sutures). The exoskeleton functions as a protective barrier, prevents loss of water, and provides mechanical support, as skeletal muscle inserts directly to the underside of the exoskeleton. The body is divided into the three sections (or tagmata): head, thorax, and abdomen. The head bears at least two antennae in all species. Antennal morphology is varied, and is particularly complex in moths. Paired compound eyes and up to three unpaired ocelli are on the head. A number of mouthparts form a preoral cavity, which naturally leads to the oral cavity. The anterior mouthpart is a sclerotinized plate called a labrum, which is sometimes referred to as the “upper lip,” and can be raised or lowered by muscular attachments and a soft joint connecting it to the head. The posterior mouthpart is the hypopharynx, which is a soft, appendage‐like extension of the head that may function as a tongue. Posterior to the hypopharynx are paired maxillae, which are toothed and have sensory palps. The lateral mouthparts include the mandibles. There is some variation in oral anatomy, but the general plan described above is appropriate for most orders. The thorax consists of three segments, each bearing a pair of legs (forelegs, middle legs, and hindlegs). These thoracic segments are the prothorax (anterior), mesothorax (middle), and metathorax (posterior). The subphylum name “Hexopoda” means six legs. Each leg is divided into six, articulating articles. From proximal to distal, these are the coxa, trochanter, femur, tibia, tarsus, and pretarsus. The thorax in many species bears two pairs of wings, one pair on the mesothorax (forewings) and one on the metathorax (hindwings). The abdomen is typically composed of 9–11 segments. These can be divided into the pregenital (1–7), genital (8 and 9), and postgenital (10 and 11) segments. The gonopore is usually on segment 8 and 9 in females and males, respectively. Some species have appendages, such as copulatory appendages and stingers, extending from the abdomen (Ruppert et al. 2004). Most insects that are processed for histology can be treated with organs in situ. Lateral sections of smaller insects typically provide orientation of organs and are excellent for morphology studies (Figure 12.1). Transverse sections are frequently superior for diagnostic pathology as more tissue can be examined per animal with multiple transverse sections of each body segment (Figures 12.2–12.4). Resin‐embedded sections may be superior to paraffin embedded as they maintain the architecture of the chitinous exoskeleton without the need for softening solutions. If dissection of an insect is desired, excellent references are available (Dade 2009; Löw et al. 2016). Dissection typically requires a dissecting microscope, as well as a board or plate for pinning the insect. See Table 12.1 for a list of organs that may be identified histologically in insects. As in all arthropods, the exoskeleton encloses the body cavity, called a hemocoel, which contains hemolymph that bathes the viscera and muscle (Figure 12.5). The hemocoel is lined by the basal lamina of the epidermis and gut. It is divided by a perforated horizontal partition (dorsal diaphragm) into a dorsal region (pericardial sinus, which surrounds the heart) and a ventral region (perivisceral sinus, which surrounds the viscera, including the gut, gonads, Malpighian tubules, etc.). Additionally, a ventral diaphragm separates the larger perivisceral sinus from a perineural sinus, which surrounds the ventral nerve cord on the ventral‐most aspect of the hemocoel. A ventral diaphragm is absent in Diptera and Coleoptera. The ventral diaphragm generally consists of connective tissue attached to a neuronal cord and muscles (Harrison and Locke 1990). Perforation of both diaphragms allows hemolymph to flow between sinuses (Ruppert et al. 2004). Figure 12.1 Sagittal section of a honeybee (Apis mellifera). Scale bar, 1.0 mm. HE. An, antenna; B, brain; CE, compound eye; CL, cardiac lumen; MT, Malpighian tubules; SM, skeletal muscle; SI, small intestine; V, ventriculus. Figures 12.2–12.4 Transverse sections of the head of a honeybee (Apis mellifera). 20×. PAS. B, brain; CE, compound eye; Es, esophagus; HG, hypopharyngeal glands; MG, mandibular gland; Mo, mouth; OC, ocellus; SM, skeletal muscle. Table 12.1 Organs for histologic evaluation in Insecta.a a Alternative names for organs are provided parenthetically, in italics. Figure 12.5 Body wall of a gypsy moth caterpillar (Lymantria dispar). The body wall is composed of a cuticle subtended by the epidermis. The body wall encloses the hemocoelom, which contains hemolymph. 200×. HE. BW, body wall; F, fat body; H, hemocytes; HC, hemocoelom; SM, skeletal muscle; T, tracheae. The exoskeleton, or cuticle, is composed of alpha chitin and protein. It is secreted by the subjacent epidermis and undergoes molting (or ecdysis) for growth, addition of limbs, etc. An extension of the exoskeleton is continuous with a cuticular lining of multiple structures that extend deep into the body, including the foregut, hindgut, fat body, and tracheal system (Figure 12.6). The external cuticle is divided into plates called sclerites that are connected by articular membranes. There are multiple layers of the exoskeleton, specifically a thin, outer epicuticle, a slightly thicker middle exocuticle, and a thick, inner endocuticle (Figure 12.7). The epicuticle is a water‐resistant or water‐proof layer that either prevents dessication (in terrestrial insects) or restricts osmostic influx (in aquatic insects). In terrestrial insects, the epicuticle includes wax produced by glandular cells in the epidermis that secrete wax through ducts in the endocuticle and exocuticle. The cuticle is thickest during intermolt periods. Insects deposit cuticle after feeding, resulting in a thickened cuticle. At molting, the new cuticle that is deposited is substantially thinner than that during intermolt (Harrison and Locke 1990). Figure 12.6 Sagittal section of a gypsy moth caterpillar (Lymantria dispar). The body wall encloses the hemocoelom, which contains hemolymph and internal viscera. 20×. HE. BW, body wall; HC, hemocoelom; L, legs; MG, midgut. Figure 12.7 Body wall of a gypsy moth caterpillar (Lymantria dispar). Higher magnification of the previous figure revealing three layers of cuticle: outer epicuticle (Ep), middle exocuticle (Ex), and inner endocuticle (En). The epicuticle has numerous small setae. The epidermis (arrow) is cuboidal with basal nuclei. Beneath the basement membrane of the epidermis is the hemocoelom (HC). 400×. HE. The epidermis is a simple layer of polygonal cells on a basal lamina. In most regions, the epidermis is columnar, but it can be flattened. The epidermis is referred to in some texts as “hypodermis” as it is subjacent to the cuticle, which is secreted by the epidermis. The epidermis is separated from the subjacent hemolymph by only a basement membrane, which allows the epidermis to phagocytose materials directly from the hemolymph. The epidermis contains microtubules that are attached to pigment granules. Pigment can move rapidly in the microtubules to produce color changes. During intermolt, the epidermis is active, laying down cuticle and replicating. Dividing epidermal cells may fail to completely separate after replication, resulting in multinucleated and polyploid epidermal cells (Harrison and Locke 1990). Acinar glands are multifocally embedded in the epidermis. Cells comprising these glands are referred to as Class 3 cells. These cells have abundant variability in their organization. In all cases, however, the inner aspect of the cells connects with the outer cuticle via a duct that extends between epidermal cells and through the cuticle. The duct is surrounded by canal cells (which are modified epidermal cells) as it passes through the epidermis (Figure 12.8). The duct is lined by inner endocuticle throughout its length. It can be uniform in diameter throughout its length, or have multiple bulbous distensions. Figure 12.8 Body wall of a gypsy moth caterpillar (Lymantria dispar) with epidermal glands. Epidermal glands are present beneath the body wall and empty through ducts in the body wall. Aggregates of cells in the hemocoelom (HC) are hemocytes (arrowheads). 100×. HE. BW, body wall; EG, epidermal glands; F, fat body; HC, hemocoelom. Figure 12.9 Skeletal muscle of a honeybee (Apis mellifera). The skeletal muscle (SM) attaches directly to the underside of the sclerite (arrow). An arthrodial membrane (soft portion of the cuticle; AM) is adjacent to the site of attachment. T, tracheae. 400×. HE. In addition to distinct acinar glands, some epidermal cells are modified for glandular secretions. These cells secrete directly into the cuticle and do not have an associated duct. Glandular secretions are typically under hormonal control, and less commonly under nervous control (Harrison and Locke 1990). The interior surface of the head portion of the exoskeleton has an elaborate number of apodemes (which are invaginations of the exoskeleton to form inner projections), collectively called the tentorium, which serve as attachment sites for muscles. Skeletal muscles attach directly to the underside of sclerites. Skeletal muscles frequently attach to adjacent sclerites, passing over articular membranes, to allow for contraction and movement. Skeletal muscles are striated (Figure 12.9). Wings are very thin folds of the body wall. They are composed of a double sheet of epidermis and cuticle (Figure 12.10). There are multiple, arborized veins throughout the wings that are hollow structural elements containing tracheae, nerves, and hemolymph. Large flight muscles in the thorax facilitate wing movement. Flight muscles either insert directly on the wings (i.e., direct muscles) or do not have direct connection (i.e., indirect muscles). Flight muscles may be so large that they occupy the majority of the thorax in some species (Ruppert et al. 2004). Figure 12.10 Forewing of a beetle (unspecified species). The wing is composed of double sheets of epidermis and cuticle (arrows) enclosing hemolymph in the hemocoel (asterisk). 200×. Modified trichrome. After metamorphosis, the wings are initially filled with abundant hemolymph, similar to the remainder of the coelom. Once the wings are opened/spread for the first time, the heart beats in the reverse direction, resulting in hemolymph being sucked back into the abdomen (this process may take several hours). Concurrently, there is postecdysial diuresis, resulting in increased viscosity of the hemolymph, which causes negative hemolymph pressure and expansion of the respiratory tree, specifically the air sacs. Together, these actions cause almost complete loss of hemolymph from the wings, leading to direct apposition of the upper and lower wing laminae. The only hemolymph that remains in the wings is in the wing veins (Harrison and Locke 1990). There are air sacs occupying a large volume of the coelomic cavity, usually around the waist, in some insects. Smaller air sacs are present in the head, wing veins, and legs. In Lepidoptera, large air sacs occupy the metathorax and first abdominal segment. In some Diptera, large air sacs occupy the first abdominal segment. Air sacs function as an “aerocoel,” which partitions the hemocoel. As in all arthropods, the alimentary tract can be divided into the foregut, midgut, and hindgut. The foregut is generally responsible for storage and some mechanical digestion prior to enzymatic digestion and absorption in the midgut. Salivary glands surround the foregut and are composed of acini supported by abundant ducts (Figure 12.11). In worker bees, there are large glands in the head called hypopharyngeal glands that produce food (royal jelly) for bee larvae (Figures 12.12 and 12.2–12.4). The hindgut functions in fecal formation and resorption of water (Ruppert et al. 2004). The fat body is analogous to the vertebrate liver, but has a greater functional diversity (Harrison and Locke 1990). Figure 12.11 Transverse section of the head of a honeybee (Apis mellifera) at the level of the compound eyes (CE). The esophagus (E) and brain (B) are in the center of the head, and are surrounded laterally by the salivary glands (SG; inset). 20×. HE. Figure 12.12 Hypopharyngeal glands in a worker honeybee (Apis mellifera). 400×. HE. T, tracheoles. The foregut (also called stomodeum) can be divided into four parts: pharynx, esophagus, crop, and gizzard (proventriculus). The pharynx has muscles embedded on the frontal dorsal part of the head. The esophagus is thin and short. The crop is large and occupies much of the abdominal hemocoelom. In some Diptera, the crop forms a lateral diverticulum for storage (Harrison and Locke 1990). The gizzard (proventriculus) is a bulge at the termination of the crop and is bordered posteriorly by digestive ceca in some species (Löw et al. 2016; Ruppert et al. 2004). The termination of the proventriculus has soft cushions (also called pulvilli) that have short setae that act as a sieve to prevent larger particles from entering the midgut (Figures 12.13 and 12.14) (Löw et al. 2016). The foregut is derived from the ectoderm and has an inner cuticular lining subtended by a simple epithelium on a basement membrane (Figure 12.15). These are further surrounded by an inner longitudinal and outer circle muscle layer. In most of the foregut, the cuticle is nonsclerotized and flexible. In insects with biting mouthparts, the foregut cuticle is sclerotized in the proventriculus. The proventriculus may bear sclerotized (toothed) plates (or teeth) or spines that facilitate continued mechanical breakdown of food. Figure 12.13 Junction of the crop (C), proventriculus (P), and ventriculus (V) in a honeybee (Apis mellifera). The gizzard is lined by cushions (asterisk
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Arthropoda: Insecta
12.1 Introduction
12.2 Gross Anatomy
12.2.1 Dissection
12.3 Histology
12.3.1 Body Wall and Coelom
Organ system
Organs
Body wall/musculoskeletal
Exoskeleton (cuticle), skeletal muscle, epidermis (hypodermis)
Digestive
Alimentary canal
Foregut (stomodeum), midgut (mesodeum or ventriculus), hindgut (proctodeum)
Fat body
Excretory
Malpighian tubules
Circulatory
Heart, aorta, sinuses
Immune
Hemocytes
Respiratory
Tracheae, tracheoles
Nervous
Brain (supraesophageal ganglion), circumesophageal connectives, subesophageal ganglion, ventral nerve cord
Reproductive
Male
Testes, vas deferens, accessory sex glands
Female
Ovaries, oviducts, accessory sex glands
Special senses/organs
Compound eyes, ocelli (may be absent in adults)
12.3.1.1 Exoskeleton
12.3.1.2 Epidermis
12.3.1.3 Epidermal Glands
12.3.1.4 Connective Tissues
12.3.1.5 Wings
12.3.1.6 Air Sacs
12.3.2 Digestive System
12.3.2.1 Foregut
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