Introduction

Insects generate a spectacular variety of visual signals, from multicolored wing patterns of butterflies, through metallic-shiny beetles to highly contrasting warning coloration of stinging insects and their defenseless mimics. Section 25.1 explains what colors are and the subsequent sections describe how insect colors result from a variety of physical structures (Section 25.2) and pigments (Section 25.3). Often, several pigments are present together, and the observed color depends on the relative abundance and positions of the pigments, as well as control signals generating color patterns during development (Section 25.4). The position of color-producing molecules relative to other structures is also important, and this may change, resulting in changes in coloration (Section 25.5). The many biological functions of color in insect signaling are covered in Section 25.6. Table 25.1 lists the sources of color in some insect groups. A small selection of insects also exhibits fluorescence or luminescence (Section 25.7).

The nature of color

Color is not an inherent property of objects; it is a perceptual attribute that depends on illumination, the spectral reflectance of an object and its surroundings, as well as the spectral receptor types and further neural processing in the animal in question. Thus the same object might appear differently colored to different viewing organisms. A red poppy, for example, is red to human observers, but appears as a UV-reflecting object to a bee pollinator, which does not have a red receptor and, like all insects studied to date, sees UV-A light between 300 nm and 400 nm. For reasons of simplicity, the color terminology in this chapter specifies what a human observer will perceive under daylight conditions. Information about UV is provided separately where available.

Introduction

The responses of insects to temperature are of increasing interest to a wide range of research fields. This is at least partly a consequence of the need to accurately forecast the effects of climate change on the abundance and distribution of insect pests of agriculture and vectors of human and animal disease, and also the need to predict the impacts of climate change on biodiversity and ecosystem function. Insect systems function optimally within a limited range of temperatures. For many insects, enzyme activity, tissue function and the behavior of the whole insect is optimal at a relatively high temperature, often in the range 30–40°C (see Figs. 3.15, 10.20). This chapter considers the factors that determine an insect's body temperature (Section 19.1), how body temperature is regulated (Section 19.2) and how insect performance varies as a function of temperature (Section 19.3). We next examine how behavior and survival are affected by temperature extremes (Sections 19.4–19.7), the mechanisms and processes affecting performance and survival at whole-animal, tissue, cell and nervous system levels (Sections 19.8–19.9), and finally, some of the large-scale patterns identified in insect thermal biology (Section 19.10). Two key terms that appear in the chapter are defined as follows:

1. Ectothermal body temperature depends on heat acquired from the environment;

2. endothermal body temperature depends on heat produced by the animal's own metabolism.

Heat gain

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. However, 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. Consequently, their activity produces a significant amount of heat and the thoracic temperatures even of quite small insects are elevated above ambient temperature during flight.

Introduction

The cells of all animals, including insects, are bathed in an extracellular fluid (ECF); and most cells of the animal exchange solutes with the ECF, not the external environment. In most animals, including insects, solute exchange between cells and ECF is facilitated by the bulk flow of ECF, powered by one-to-many pumps (including hearts). The physiological system mediating bulk flow is known as the circulatory system, the topic of this chapter. Most insects have one major pump, the dorsal vessel, and multiple accessory pumps, with the circulating portion of the ECF known as the blood or hemolymph. The insect circulatory system is open, meaning that the hemolymph flows freely around insect organs, in contrast to the closed circulatory system of vertebrates in which blood is retained within vessels. The structural organization of the insect circulatory system and the determinants of blood flow are described in Sections 5.1 and 5.2, respectively, and the composition of hemolymph is considered in Section 5.3.

Hemolymph has four key functions. It is the vehicle for transport of hormones and nutrients between tissues, and a site for storage of some nutrients and water; these two functions are considered in Section 5.3. It is also a crucially important component of the insect immune system. The humoral (non-cellular) immune function is addressed in Section 5.3 and the cellular function in Section 5.4. Other insect organs also contribute to insect immunity, notably the gut (Chapter 3), fat body (Chapter 6) and Malpighian tubules (Chapter 18).

Introduction

Cells operate 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 (“osmoregulation”), 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, and for the excretion of toxic compounds – often following metabolic modification to chemicals more readily excreted or which can be safely stored. The success of insects in exploiting a wide range of habitats can be attributed – at least in part – to the robustness of their osmoregulatory systems. Although osmoregulation and excretion are distinct functions, they overlap both in the tissues and the ions used, and so they are considered together in this chapter. Section 18.1 covers the structure of the excretory system; Sections 18.2–18.4 cover the production of primary urine by the Malpighian tubules, its modification in the lower tubules and hindgut and its endocrine control. In Section 18.5, the handling of nitrogenous waste is discussed, followed by other roles of the excretory system in detoxification (Section 18.6) and non-excretory roles (Section 18.7). Excretory functions of cells outside the gut (such as nephrocytes) are covered in Section 18.8, and water regulation at an organismal level in Section 18.9.

Excretory system

Excretion involves the production of urine that removes 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 reabsorption of useful compounds or the addition of others that may be in excess in the body. This “excretory cycle” is the primary task of the hindgut, comprising the Malpighian tubules, ileum and rectum (Fig. 18.1). In insects, the primary urine is produced by the Malpighian tubules. Its selective modification usually occurs in the rectum, but may also take place in the Malpighian tubules or in the ileum. In aquatic insects, epidermal cells outside the gut may also be involved in maintaining the insect's ionic balance.

Introduction

Embryogenesis is the process by which a single egg develops into a multicellular individual. Many of our most important discoveries in understanding the embryonic development of animals, including humans, derive from studies that were first conducted in insects. Some of these discoveries are also now being applied in the fields of medicine and agriculture. As discussed in Chapters 12 and 13, all future offspring produced by insects derive from specialized progenitor cells called germ cells, which migrate to the gonads, where they differentiate into gametes. The gametes produced by females are called oocytes (eggs), while the gametes produced by males are called spermatozoa (sperm). Most insects begin their embryonic development when genetic material from an egg and sperm fuse through the process of fertilization to form a zygote. The zygote then divides mitotically to produce all of the different cells that comprise the body of the nymph (exopterygote/hemimetabolous species) or larva (endopterygote/holometabolous species), which will hatch from the egg. Embryogenesis proceeds through a similar series of steps in most insect species, but there are also a number of variations that in some cases are associated with unique life histories. In this chapter we first summarize key morphological and functional features of insect eggs (Section 14.1). Next we discuss the process of embryogenesis, including some of the molecular mechanisms that control axis formation and nutrient acquisition (Sections 14.2 and 14.3). We then consider sex determination (Section 14.4) and end the chapter by discussing parthenogenesis (Section 14.5), pedogenesis (Section 14.6) and other unique forms of embryonic development.

The egg

Most insects produce large eggs relative to their own size. This is due to a majority of insects packaging their eggs with large amounts of yolk, which serves as the source of nutrients for growth and development of the embryo. In general, the eggs of Endopterygota contain less yolk and are smaller than those of Exopterygota. To some extent this may reflect differences associated with ovariole type (Section 13.2.1). For example, in two locust (Orthoptera) species, which have panoistic ovarioles, each egg weighs about 0.5% of female weight; among insects with telotrophic ovarioles, the egg of Trialeurodes vaporarium (Hemiptera) is over 1% of the female weight and that of Callosobruchus maculata (Coleoptera) 0.6%. By contrast, among insects with polytrophic ovarioles, comparable figures for Apis mellifera (Hymenoptera) and Grammia geneura (Lepidoptera) are 0.07% and 0.11%, respectively.

The ancestor of the Arthropoda was in all probability a segmented worm-like marine creature that lived in oceans during the late Precambrian. By the early to mid-Cambrian (540–520 million years ago) the early arthropods had already evolved into a range of clearly recognizable groups with distinct body plans. Arthropods are characterized by a number of features: the possession of a periodically molted, chitinous cuticle that acts as a rigid exoskeleton for the internal attachment of striated muscles; segmental paired legs; and the aggregation and/or fusing of body segments into discrete functional units, of which the most universal is the head. Besides the head there may be a trunk, as in the Myriapoda, or a separate thorax and abdomen as in the Crustacea and Hexapoda.

Based on the ubiquity of ɑ-chitin in arthropod cuticles, similarities in musculature and tendon systems and recent molecular data, the overwhelming consensus of opinion is that this very large taxon is monophyletic. However, the relationships within the Arthropoda have been the subject of much controversy for more than 100 years. Recent molecular and genetic data confirm that the Hexapoda (comprising the Insecta and three other non-insect hexapod classes) are monophyletic, but that Crustacea are not. The monophyletic Hexapoda and paraphyletic Crustacea are now thought to form a single superclade called the Pancrustacea (Fig. 1). The mandibles of these two groups have similar origins, and the development of the nervous system is similar, as is the structure and wiring of the compound eyes.

Introduction

Light is perceived by insects through a number of different receptors. Most adult insects and larval hemimetabolous insects normally have a pair of compound eyes, whose structure (Section 22.1) and function in form and motion vision (Section 22.2) are described below in turn. Section 22.3 covers the molecular and physiological function of photoreceptors and mechanism of regulating light sensitivity before explaining the processes of color vision and polarization vision. Adult insects also typically have three single-lens eyes, called ocelli, whose optics and function are described in Section 22.4. Larval holometabolous insects have one or more single-lens eyes, known as stemmata, on the sides of the head (Section 22.5). Some insects also possess epidermal light receptors, and, in some cases, light is known to have a direct effect on cells in the brain (Section 22.6). Magnetic sensitivity aids orientation in at least some insects, and has known interactions with light sensitivity (Section 22.7).

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 (Hemiptera). This is also true of cave-dwelling species. Among termites (Isoptera), 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 degenerate during the permanently subterranean reproductive life. Among Apterygota, compound eyes are lacking in some Thysanura, but Lepismatidae have 12 ommatidia on each side. Well-developed 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 no compound eyes.

Introduction

Wings are the defining character of pterygotes, and were a key innovation in the evolutionary history of insects. Flight is important for a vast range of activities, and the detailed structure and form of the wings primarily reflects their adaptation for flight. The wings are connected to the thorax via the most complex joints of any animal, and the thoracic musculature provides control of the wings as well as high power output. By varying their detailed wingbeat kinematics, many insects are able to achieve excellent agility and maneuverability. The aerodynamics of insects differ greatly from the aerodynamics of fixed-wing aircraft, and it is these differences that explain the ability of insects to lift their weight with comparatively small wings. Even so, flight is the most energetically demanding of an insect's activities, and the power for flight may be provided by oxidation of several different substrates. Flight is only useful if it is stable and controlled, and a range of different sensory systems is used to provide the necessary feedback.

This chapter is divided into seven sections. Sections 9.1 and 9.2 describe the structure and form of the wings. Section 9.3 considers how the movements of the wings are generated, and is followed by Section 9.4 on wing kinematics. Section 9.5 reviews the aerodynamic mechanisms of insect flight, and is followed by Section 9.6 on power. Section 9.7 describes the sensory systems that are used by different insects in flight control.

Introduction

Stimulation by chemicals involves the senses of smell (olfaction) and taste (gustation). Olfaction implies the ability to detect compounds in the gaseous state. Insects have a range of receptors sensitive to odors. Insects have taste receptors on many parts of the body, often using them for purposes unrelated to feeding, and they 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” as used for mammals. The functional distinction between olfaction and contact chemoreception is usually clear, although olfactory receptors can respond to substances in solution and contact chemoreceptors can respond to high concentrations of some odors. The molecular receptors that provide the “lock and key” mechanism allowing detection of both forms of chemicals also show some overlap between the two chemosensory modalities. Processing within the central nervous system is, however, quite different for olfaction and contact chemoreception.

This chapter is divided into six sections. Section 24.1 describes the external structure of the chemosensory sense organs (chemosensilla). Section 24.2 deals with the cellular components within these structures. The distribution and numbers of chemosensilla are addressed in Section 24.3. Section 24.4 investigates the function of chemosensilla. The way function fits with insect behavior is examined in Section 24.5. The final section, Section 24.6, deals with neural projection of the chemosensory organs to the central nervous system.

Introduction

Insects can produce sounds that spread through air or water, and vibrations transmitted through the substrate on which they are resting. Sounds and vibrations may result from the insect's normal activities, such as the sounds produced by wing vibration in flight or by the mandibles of an insect chewing. In addition, numerous species produce specific acoustic and vibrational signals for intra- and interspecific communication that are generated independently of other activities by specialized structures and behaviors. Acoustic and vibrational communication signals can mediate information about the localization, species, size, physiological state and genetic quality of the sender and serve important functions for reproduction, competition and coordination of activities in social insects.

This chapter is divided into five sections. Section 26.1 introduces the nature and transmission of acoustic and vibrational signals. Section 26.2 describes the behavioral significance of acoustic and vibrational signals and Section 26.3 explains the mechanisms that insects use to generate these signals. An overview of acoustic and vibrational patterns and their information content is provided in Section 26.4. Section 26.5 describes the neural regulation of sound production in a few well-investigated insect species. To make use of acoustic and vibrational communication signals, insects have evolved various types of hearing organs and central nervous auditory processing strategies, which are considered in Chapter 23.

Introduction

Postembryonic development allows some of the most remarkable examples of the diversity and ecological success of insect. The distinction between juvenile and adult phenotypes can be extreme, allowing different life-stages to occupy very different ecological niches. Changes in the rate of development throughout the lifecourse can allow insects to wait out periods of inclement conditions by entering diapause, and some insects express alternative phenotypes under particular environmental conditions, allowing multiple phenotypes to be packed within the same genome. In Section 15.1 the process of hatching from the egg is considered, followed by aspects of larval development (Section 15.2), metamorphosis (Section 15.3) and control of postembryological development through to adulthood (Section 15.4). Section 15.5 then provides some examples of environmentally determined polyphenisms. The chapter ends with a final section on delayed development during diapause (Section 15.6).

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 extraembryonic 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.

To Instructors and Students

THIS BOOK IS a concise introduction to Bayesian statistics and econometrics. It can be used as a supplement to a frequentist course by instructors who wish to introduce the Bayesian viewpoint or as a text in a course on Bayesian econometrics supplemented by readings in the current literature.

While the student should have had some exposure to standard probability theory and statistics, the book does not make extensive use of statistical theory. Indeed, because of its reliance on simulation techniques, it requires less background in statistics and probability than most books that take a frequentist approach. It is, however, strongly recommended that the student become familiar with the forms and properties of the standard probability distributions collected in Appendix A.

Since the advent of Markov chain Monte Carlo (MCMC) methods in the early 1990s, Bayesian methods have been extended to a large and growing number of applications. This book limits itself to explaining in detail a few important applications. Its main goal is to provide examples of MCMC algorithms to enable students and researchers to design algorithms for the models that arise in their own research. More attention is paid to the design of algorithms for the models than to the specification and interpretation of the models themselves because I assume that the student has been exposed to these models in other statistics and econometrics classes.

THE ANALYSIS OF time series data has generated a vast literature from both frequentist and Bayesian viewpoints. This chapter considers a few standard models to illustrate how they can be analyzed with MCMC methods. Section 11.6 provides references to more detailed explanations and additional models.

Autoregressive Models

This section is concerned with models of the general form

where and t = 1, …, T. The disturbance ∊t is said to be autoregressive of order p, denoted by ∊t ∼ AR(p). This model is a way to capture the possibility that disturbances in a particular time period continue to affect y in later time periods, a property that characterizes many time series in economics and other areas.

Assume that the stochastic process defining ∊t is second-order stationary, which means that the means E(∊t) and all covariances E(∈s∊t) of the process are finite and independent of t and s, although the covariances may depend on ∣t – s∣. Because the variance is the special case of the covariance when t = s, it is finite and independent of time.

The stationarity property imposes restrictions on the ϕs. To state these, I define the lag operator L. It operates on time-subscripted variables as Lzt = zt−1, which implies that for integer values of r. The polynomial in the lag operator

allows you to relate ∊t and ut as Φp(L)∊t = ut.