The skeleton of the thoracic segments is modified to give efficient support for the legs and wings, and the musculature is adapted to produce the movements of these appendages.

SEGMENTATION

In larval holometabolous insects the cuticle is soft and flexible, or only partially sclerotized, and the longitudinal muscles are attached to the intersegmental folds (Fig. 7.1a). This represents a primitive condition comparable with that occurring in the annelids. Insects with this arrangement move as a result of successive changes in the shapes of the thoracic and abdominal segments (section 8.4.3), these changes of shape being permitted by the flexible cuticle. When the cuticle is sclerotized, sclerites in the intersegmental folds which have the longitudinal muscles attached to them are usually fused with the segmental sclerites behind. The large sclerite on the dorsal surface of a segment is called the tergum, or, in the thorax, the notum. Anteriorly it incorporates the intersegmental region, the original fold being marked by the antecostal sulcus where the cuticle is inflected. The narrow rim in front of the sulcus is called the acrotergite (Figs. 7.1b, 7.2). An acrotergite never occurs at the front of the prothorax because the anterior part of this segment forms part of the neck and the muscles from the head pass directly to the acrotergite of the mesothorax.

An area at the back of each segment remains membranous, forming a new intersegmental membrane. This does not correspond with the original intersegmental groove and so a secondary segmentation is superimposed on the first, but neither corresponds precisely with the ‘parasegments’ defined by molecular studies (see section 14.2.5).

ANATOMY OF THE INTERNAL REPRODUCTIVE ORGANS

The female reproductive system consists of a pair of ovaries, which connect with a pair of lateral oviducts. These join to form a median oviduct opening posteriorly into a genital chamber. Sometimes the genital chamber forms a tube, the vagina, and this is often developed to form a bursa copulatrix for reception of the penis. Opening from the genital chamber or the vagina is a spermatheca for storing sperm, and, frequently, a pair of accessory glands is also present (Fig. 13.1).

Review: Martoja, 1977

Ovary The two ovaries lie in the abdomen above or lateral to the gut. Each consists of a number of egg-tubes, or ovarioles, comparable with the testis follicles in the male. The oocytes develop in the ovarioles. The ovaries of Collembola are probably not homologous with those of insects, but are sac-like with a lateral germarium and no ovarioles.

The number of ovarioles in an ovary varies in relation to size and life style of the insect as well as its taxonomic position. In general, larger species within a group have more ovarioles than small ones; thus small grasshoppers commonly have only four ovarioles in each ovary, while larger ones may have more than 100. Similarly, in the higher Diptera, Drosophila has 10–30 ovarioles in each ovary, whereas Calliphora has about 100. By contrast, most Lepidoptera have four ovarioles on each side irrespective of their size.

The activities of the cell are carried out most efficiently within a narrow range of conditions. It is therefore important that the environment within the cell and in the animal in general should be kept as near optimal as possible, a process known as homeostasis. This involves the maintenance of a constant level of salts and water and osmotic pressure in the hemolymph, the elimination of toxic nitrogenous wastes derived from protein and purine metabolism, and the elimination of other toxic compounds which may be absorbed from the environment. The excretory system is primarily responsible for homeostasis, often following metabolic modification of toxic compounds to chemicals more readily excreted or which can be safely stored. Some molecules entering the body from the environment may be too large or too toxic to be dealt with by the excretory system and various other tissues may be involved in metabolizing them to less toxic or more readily excretable substances. Some insects sequester toxic compounds, isolating them from the major metabolic pathways.

EXCRETORY SYSTEM

Excretion involves the production of a fluid urine that carries potentially toxic materials from the body. This process is carried out in two stages: the relatively unselective removal of substances from the hemolymph, forming the primary urine, and the selective modification of this primary urine by the resorption of useful compounds or the addition of others that may be in excess in the body. In insects, the primary urine is produced by the Malpighian tubules.

Hormones are chemicals produced by an organism which circulate in the blood to regulate its long term physiological, developmental and behavioral activities. They complement the nervous system, which provides shortterm coordination, and the activities of the two systems are closely linked.

General aspects of hormones are discussed in this chapter. The specific actions of hormones regulating particular functions are considered in other chapters:

molting and metamorphosis – Section 15.4.

yolk synthesis – Section 13.2.4.2.

embryonic cuticles – Section 14.2.10.

diuresis – Section 18.3.3.

mobilization of fuel for flight – Section 9.12.2.

polyphenism – Section 15.5.

diapause – Section 15.6.5.

General review of insect hormones: Nijhout, 1994

CHEMICAL STRUCTURE OF HORMONES

Apart from molting hormone and juvenile hormone, most known insect hormones are peptides. Some biogenic amines are also known to function as hormones.

Molting hormones

Molting hormones are ecdysteroids (Fig. 21.1) which, in immature insects, are produced by the prothoracic glands. In most insects these glands secrete ecdysone, but some larval Lepidoptera are also known to secrete 3-dehydroecdysone which is converted to ecdysone by enzymes in the hemolymph (Fescemeyer et al., 1995). Ecdysone is a prohormone; it is converted to the active hormone, 20-hydroxyecdysone, in the fat body or epidermis by a cytochrome P-450 enzyme. In the honeybee Apis, and in Heteroptera, the principal ecdysteroid is makisterone A.

Insects cannot synthesize steroids. Consequently, sterols, usually cholesterol or a closely related structure, are essential dietary constituents.

Stimulation by chemicals involves the senses of smell (olfaction) and taste (gustation). Olfaction implies the ability to detect compounds in the gaseous state. Taste, in humans, refers to detection of compounds in solution, or in a liquid state, by receptors in the oral cavity. Insects have comparable receptors on many parts of the body, often using them for purposes unrelated to feeding, and they also have the ability to detect chemicals on dry surfaces as well as in solution. For these reasons, it is usual to refer to ‘contact chemoreception’ in insects rather than ‘taste’. The distinction between olfaction and contact chemoreception is usually clear, although olfactory receptors can respond to substances in solution and contact chemoreceptors respond to high concentrations of some odors. Processing within the central nervous system is, however, quite different. The axons from all olfactory receptors terminate in the antennal lobes in the brain (section 20.4.2) whereas axons from contact chemoreceptors terminate within the ganglion of the body segment on which the receptors occur.

Reviews: Frazier, 1992 – chemoreceptor physiology; Morita & Shiraishi, 1985 – chemoreceptor physiology; Payne, Birch & Kennedy, 1986 – olfaction

OLFACTION

Structure of receptors

Olfactory sensilla comprise cuticular and cellular components comparable, in a general sense, to those of cuticular mechanoreceptors and contact chemoreceptors (Fig. 24.1, and compare Figs. 23.1, 24.8).

Review: Zacharuk, 1985

Cuticular components The cuticle of olfactory receptors is characterized by the presence of numerous small pores which permit the entry of chemicals (Figs. 24.1, 24.2).

THE EGG

Insect eggs are typically large relative to the size of the females that produce them because they contain a great deal of yolk. It is generally believed that the eggs of Endopterygota contain less yolk and are smaller than those of Exopterygota (Anderson, 1972b). To some extent, this may reflect the types of ovariole they possess. For example, in two locust species which have panoistic ovarioles, each egg weighs about 0.5% of female weight; amongst insects with telotrophic ovarioles, the egg of Trialeurodes vaporarium is over 1% of the female weight, and that of Callosobruchus maculata (Coleoptera) 0.6%. By contrast, amongst insects with polytrophic ovarioles, comparable figures for Apis mellifera and Grammia geneura (Lepidoptera) are 0.07% and 0.11%, respectively.

Egg size is affected by factors other than the type of ovariole, however. Amongst Lepidoptera from temperate regions, species overwintering in the egg stage have larger eggs than species that overwinter in some other stage, and species feeding on woody plants have bigger eggs than those feeding on herbaceous plants (Reavey, 1992). Individual females of at least some butterflies lay smaller eggs as they grow older, and females of the cornborer moth, Ostrinia, lay smaller eggs if they do not receive adequate nutrition (Leahy & Andow, 1994). The parasitic Hymenoptera which lay their eggs in their hosts have very small eggs and this is also true amongst the Tachinidae (Diptera).

Insect eggs have a wide variety of forms. Commonly, as in Orthoptera and many Hymenoptera, they are sausage shaped (Fig. 14.10), but they may be conical, as in Pieris, or rounded, as in many moths and Heteroptera.

STRUCTURE

Basic muscle structure

Each muscle is made up of a number of fibers, which are long, usually multinucleate cells running the whole length of the muscle. The characteristic feature of muscle fibers is the presence of myofibrils (fibrils) which are embedded in the cytoplasm (sarcoplasm) and extend continuously from one end of the fiber to the other. The fibrils in their turn are composed of molecular filaments consisting mainly of two proteins: myosin and actin. These filaments are much shorter than the whole muscle and they are arranged in units called sarcomeres. Each fibril comprises a large number of sarcomeres stacked end to end (Fig. 10.1a,b).

The thick (myosin) filaments are stouter and are made up of numerous myosin molecules. These are elongate structures with two globular ‘heads’ at one end, and, in each sarcomere (see below), all the molecules in one half are aligned in one direction, while all those in the opposite half are aligned in the opposite direction (Fig. 10.1c). The myosin molecules are probably arranged round a core of another protein, paramyosin, with their heads arranged in a helix.

The thick filaments are each surrounded by a number of thin (actin) filaments which consist of two chains of actin molecules twisted round each other. The actin filaments are oriented in opposite directions at either end of the sarcomere and those of adjacent sarcomeres slightly overlap each other and are held by an amorphous material.

Hormones are concerned with regulation within the organism. Other chemical substances are concerned with communication between organisms, both intra- and interspecifically. Chemicals with an intraspecific function are called pheromones; those with an interspecific function are often called allelochemicals. Some chemicals can serve both functions, even in one insect (Blum, 1996). Most pheromones and allelochemicals are produced by ectodermal glands secreting to the outside of the body. The basic structure of these exocrine glands is similar irrespective of whether they produce pheromones or allelochemicals, but their positions on the body and the chemicals they produce exhibit extraordinary diversity. Glands producing chemicals for interspecific communications usually have reservoirs while most pheromone glands do not. Pheromones used for marking, however, are usually stored in a reservoir. Many chemicals used for defense are stored within the body and are not secreted.

PHEROMONES

A pheromone is a secretion which, if passed to another individual of the same species, causes it to respond, physiologically or behaviorally, in a particular manner. Pheromones are thus concerned with signalling between individuals of the same species and are therefore often important in sexual behavior and in regulating the behavior and physiology of social and subsocial species. It is probably true that pheromones are used in communication by at least some species in all the insect orders.

Many pheromones are perceived as scents by olfactory receptors and affect the recipient via the central nervous system. In other cases, pheromones are ingested by the recipient.

OCCURRENCE AND STRUCTURE OF WINGS

Fully developed and functional wings occur only in adult insects although the developing wings are present in larvae. In hemimetabolous larvae they are visible as external pads (section 15.3.1), but they develop internally in holometabolous species (section 15.3.2.2). The Ephemeroptera are exceptional in having two fullywinged stages. The final larval stage molts to a subimago, which resembles the adult except for having fringed and slightly translucent wings and rather shorter legs. It is able to make a short flight, after which it molts and the adult stage emerges. In the course of this molt the cuticle of the wings is shed with the rest of the cuticle.

The fully developed wings of all insects appear as thin, rigid flaps arising dorsolaterally from between the pleura and nota of the meso- and meta-thoracic segments. Each wing consists of a thin membrane supported by a system of veins. The membrane is formed by two layers of integument closely apposed, while the veins are formed where the two layers remain separate and the cuticle may be thicker and more heavily sclerotized (Fig. 9.1). Within each of the major veins is a nerve and a trachea, and, since the cavities of the veins are connected with the hemocoel, hemolymph can flow into the wings (section 5.1.2.1).

Basic structure of the wing

The structure of the wing is determined primarily by the need to optimize the production of favorable aerodynamic forces during flight.

CIRCULATORY SYSTEM

Structure

Insects have an open blood system with the blood occupying the general body cavity, which is known as a hemocoel. Blood is circulated mainly by the activity of a contractile dorsal longitudinal vessel which opens into the hemocoel. The hemocoel is often divided into three major sinuses; a dorsal pericardial sinus, a perivisceral sinus, and a ventral perineural sinus (Fig. 5.1). The pericardial and perineural sinuses are separated from the visceral sinus by the dorsal and ventral diaphragms, respectively. In most insects, the visceral sinus occupies most of the body cavity, but in ichneumonids the perineural sinus is enlarged.

Reviews: Hoffman, 1976; Jones, 1977; Miller, 1985a

Dorsal vessel

The dorsal vessel runs along the dorsal midline, just below the terga, for almost the whole length of the body although in the thorax of adult Lepidoptera and at least some Hymenoptera, it loops down between the longitudinal flight muscles (see Fig. 19.3). It may be bound to the dorsal body wall or suspended from it by elastic filaments. Anteriorly it leaves the dorsal wall and is more closely associated with the alimentary canal, passing under the brain just above the esophagus. It is open anteriorly, ending abruptly in most insects, but as an open gutter in orthopteroids. Posteriorly, it is closed, except in larval mayflies (Ephemeroptera) where three vessels diverge to the caudal filaments from the end of the heart.

The insect fat body is the principal organ of intermediary metabolism. Most hemolymph proteins are synthesized in the fat body, and it also functions in the storage of proteins, lipids and carbohydrates. It consists of thin sheets or ribbons, usually only one or two cells thick, or of small nodules suspended in the hemocoel by connective tissue and tracheae. All the cells are consequently in immediate contact with the hemolymph, facilitating the exchange of metabolites. There is generally a peripheral, or parietal, fat body layer immediately beneath the body wall, and often a perivisceral layer surrounding the alimentary canal can also be distinguished (Fig. 6.1). The fat body is most conspicuous in the abdomen, but components extend into the thorax and head, and insinuate around the other tissues. Within a species, the arrangement is more or less constant, but there are considerable differences between insects in different orders. In hemimetabolous insects, the larval fat body persists in the adult without major changes, but in holometabolous insects the fat body is completely rebuilt at metamorphosis. In most cases this involves rebuilding a new structure from existing fat body cells following histolysis of the larval tissue, but in some adult Diptera it may be developed de novo (section 15.3.2.2).

Light is perceived by insects through a number of different receptors. Most adult insects and larval hemimetabolous insects have a pair of compound eyes and often three single-lens eyes, called ocelli. Larval holometabolous insects have one or more single-lens eyes, known as stemmata, on the sides of the head. Some insects are also known to possess epidermal light receptors, and, in some cases, light is known to have a direct effect on cells in the brain.

COMPOUND EYES

Occurrence

Compound eyes are so called because they are constructed from many similar units called ommatidia. They are present in most adult pterygote insects and the larvae of hemimetabolous insects, but are strongly reduced or absent in wingless parasitic groups, such as the Phthiraptera and Siphonaptera, and in female coccids. This is also true of cave-dwelling forms. Amongst termites, compound eyes are greatly reduced or absent from stages that are habitually subterranean, and, although present in winged reproductives, the sensory components of the eyes may degenerate. Amongst Apterygota, compound eyes are lacking in some Thysanura, but Lepismatidae have 12 ommatidia on each side. Fullydeveloped compound eyes are present in Archaeognatha. In the non-insect orders of Hexapoda, Collembola have up to eight widely spaced ommatidia, while Protura and Diplura have none.

Each compound eye may be composed of several thousand ommatidia. There are about 10000 in the eyes of dragonflies and drone honeybees, 5500 in worker honeybees and 800 in Drosophila.

Insect systems function optimally within a limited range of temperatures. For a majority of insects, enzyme activity, tissue functioning and the behavior of the whole insect is optimal at a relatively high temperature, often in the range 30–40 °C (see Figs. 3.22, 10.17b). This chapter considers the factors that determine an insect's temperature, how body temperature may be regulated, and how insects cope with extremes of temperature.

DEFINITIONS

Ectothermal – body temperature depends on heat acquired from the environment.

Endothermal – body temperature depends on heat produced by the animal's own metabolism.

Heterothermal – body temperature is sometimes determined by the organism's metabolism (endothermy); at other times it is governed by the environment (ectothermy).

Poikilotherm – an animal whose body temperature is variable and dependent on ambient.

Homeotherm – an animal whose body temperature is constant as a result of endothermy.

BODY TEMPERATURE

Heat gain

Internal heat production

The body temperature of an insect is always a reflection of ambient conditions coupled with any heat that may be produced by metabolic activity. Because the mechanical efficiency of muscles is very low, any muscular activity produces heat, but, in insects, because of the small size of the muscles and the high rate of heat loss from the organism, the effects of muscular activity on body temperature are usually insignificant. The flight muscles, however, are relatively large and oscillate at high frequencies when generating the power needed for flight.

Many insects communicate using vibrational signals. These may be air-or water-borne sounds, which are sometimes audible to humans, or they may be transmitted through the substrate on which the insect is resting. Sometimes the vibrations are purely adventitious, resulting from the insect's normal activities, such as the sounds produced by wing vibration in flight or by the mandibles of an insect chewing, but they often have some special significance for the insect and, in most cases, are produced independently of other activities.

Vibrations may be produced in water and solids with little or no anatomical specialization. By contrast, insects using airborne vibrations to communicate use structures and behaviors that enhance sound radiation. The structure transmitting vibrations to the air is known as the sound radiator; it is often a part of the wing which is suspended from the surrounding cuticle in a way that facilitates its free vibration.

A sound radiator functions most efficiently in producing sounds with wavelengths similar to or less than its own dimensions. Consequently, because of their small size, insects are constrained towards producing sounds with short wavelengths (and high frequencies). However, high frequency sounds attenuate more rapidly than lower frequency, longer wavelength sounds so they are less good for distance communication. In practice, airborne sounds produced by different insects cover a very wide range of frequencies and are often ultrasonic to humans (Fig. 26.1).

ALIMENTARY CANAL

The alimentary canal of insects is divided into three main regions: the foregut, or stomodeum, which is ectodermal in origin; the midgut, or mesenteron, which is endodermal; and the hindgut, or proctodeum, which again is ectodermal (Fig. 3.1). The epithelium of all parts of the gut consists of a single layer of cells.

Since the foregut and hindgut are ectodermal in origin, the cells secrete cuticle which is continuous with that covering the outside of the body. The lining cuticle is known as the intima. It is shed and renewed at each molt. Although the midgut does not secrete cuticle, in most insects it does secrete a delicate peritrophic envelope around the food.

Usually the gut is a continuous tube running from the mouth to the anus, but in some insects that feed on a fluid diet containing little or no solid waste material the connection between the midgut and the hindgut is occluded. This is the case in some plant-sucking Heteroptera, where the occlusion is between different parts of the midgut (see Fig. 3.9), and in larval Neuroptera which digest their prey extra-orally. A similar modification occurs in the larvae of social Hymenoptera with the result that the larvae never foul the nest. In these insects a pellet of fecal matter is deposited at the larva–pupa molt.

HATCHING

Mechanisms of hatching

Most insects force their way out of the egg by exerting pressure against the inside of the shell. The insect increases its volume by swallowing the extra-embryonic fluid and in some cases by swallowing air which diffuses through the shell. Then, waves of muscular contraction pump hemolymph forwards so that the head and thoracic regions are pressed tightly against the inside of the shell. In grasshoppers, and perhaps in other insects, these muscular waves are interrupted periodically by a simultaneous contraction of the abdominal segments which causes a sudden increase in pressure in the anterior region. The dorsal membrane of the neck in grasshoppers has a pair of lobes, the cervical ampullae, which are inflated by the increase in hemolymph pressure (Fig. 15.1a). They serve to focus the pressure on a limited area of the shell. If the shell does not split, the ampullae are withdrawn and a further series of posterior to anterior waves of contraction follows ending with another sudden abdominal contraction. One of these sudden contractions ultimately ruptures the shell.

The position of the rupture generally depends on where the insect puts pressure on the chorion. In grasshoppers, the chorion is split transversely above the ampullae; in the water beetle, Agabus, the split is longitudinal, while in some species it is variable in position. The chorion of some species has a line of weakness along which it splits (section 14.1.1.3).