Introduction

Hormones are chemical signals that are produced by the insect and circulate in the blood to regulate long-term physiological, developmental and behavioral activities. These signals complement those of the nervous system, which provides short-term coordination. The activities of the two systems are closely linked and sometimes not clearly distinguishable. General aspects of hormones are discussed in this chapter, including their chemical structures (Section 21.1); endocrine organs that secrete hormones (Section 21.2); the means by which hormones are transported in the hemolymph (Section 21.3); regulation of hormone titers (Section 21.4); and the mode of action of hormones on their target tissues (Section 21.5). Specific actions of hormones regulating particular functions are considered in other chapters; notable examples include molting and metamorphosis in Section 15.4, yolk synthesis in Section 13.2.4, embryonic cuticles in Section 14.2.10, diuresis in Section 18.3.3, mobilization of fuel for flight in Section 9.6.2, polyphenism in Section 15.5 and diapause in Section 15.6.

Chemical structure of hormones

Apart from molting hormones (polyhydroxylated steroids) and juvenile hormones (sesquiterpenes), most known insect hormones are polypeptides. Some biogenic amines are also known to function as hormones (see Section 20.2.3).

Introduction

Insects and other arthropods are built up on a segmental plan, and their characteristic feature is a hard, jointed exoskeleton. The cuticle, which forms the exoskeleton, is continuous over the whole of the outside of the body and consists of a series of hard plates, the sclerites, joined to each other by flexible membranes, which are also cuticular. Sometimes the sclerites are articulated together so as to give precise movement of one on the next. Each segment of the body primitively has a dorsal sclerite, the tergum, joined to a ventral sclerite, the sternum, by lateral membranous areas, the pleura. Arising from the sternopleural region on each side is a jointed appendage.

In insects, the segments are grouped into three units, the head, thorax and abdomen, in which the various basic parts of the segments may be lost or greatly modified. Typical walking legs are only retained on the three thoracic segments. In the head, the appendages are modified for sensory and feeding purposes and in the abdomen they are lost, except that some may be modified as the genitalia and in Apterygota some pregenital appendages are retained. This chapter introduces the structures of the head (Section 1.1), neck (Section 1.2) and antennae (Section 1.3). Chapter 2 concerns the mouthparts and feeding.

Preface

Reginald Chapman's The Insects: Structure and Function has been the preeminent textbook for insect physiologists for the past 43 years (since the moon landing, in fact). For generations of students, teachers and researchers The Insects has provided the conceptual framework explaining how insects work. Without this book, the lives of entomologists worldwide would have been substantially more difficult. Nevertheless, the most recent (fourth) edition of this remarkable book was published in 1998, and a great deal has happened since then. Sadly, Reg died in 2003 and there was no reasonable prospect of any other person taking on the next revision single-handed. We have decided to take a different approach: to invite a team of eminent insect physiologists to bring their expertise to the collective enterprise of writing the fifth edition of The Insects.

Our aim has been to protect the identity of The Insects by working with Reg's original text. Certain areas have needed more revision than others, and some sections have been shrunk to accommodate advances in others. Our sole major deviation from the style of previous editions has been to remove all citations to primary literature from the main text. These in-text citations had accreted across successive revisions, and were somewhat patchy in coverage throughout the book. With the availability of online literature search engines today, students and researchers alike are better served by a short list of key references at the end of each chapter to provide a lead-in to the literature.

Introduction

Insects are prodigious users of chemical signals and cues, which play diverse and fundamental roles in the transfer of information both within and between species. Indeed, it is likely that no other group of animals makes such sophisticated use of chemical signaling in their biology. This chapter begins by defining the different classes of signals (Section 27.1), before describing the nature of intraspecific chemical signals (pheromones) (Section 27.2), the information content of such pheromones (Section 27.3), their biosynthesis (Section 27.4) and the mechanisms regulating their production (Section 27.5), as well as their sensory perception by conspecifics (Section 27.6). In the next section (Section 27.7) interspecific signals are discussed (allelochemicals), followed by their mechanisms of production and release (Section 27.8). Section 27.9 concerns defensive compounds, and the chapter ends with chemical mimicry (Section 27.10).

Defining chemical signals

Chemical signals and cues have been collectively called semiochemicals, derived from the Greek word “semeon” for signal. However, it has been suggested that the term “infochemical” may be more appropriate, based on the argument that nomenclature should be based on a “cost–benefit analysis” rather than the actual source of the signal. While there has not been complete acceptance of either term and both are used in the current literature, we will use infochemical when referring to “a chemical substance, which in a natural context, is implicated in the transfer of information during an interaction between two individuals that results in a behavioral and/or physiological response in one or both.” In this chapter the term “signal” is applied to an infochemical produced by an emitter which has been shaped by evolution to transmit a specific message to the intended receiver. An example of this would be the release of a sex pheromone for the specific purpose of attracting a mate. The term “cue” is used to describe an infochemical that conveys information to a receiver, but was not shaped by natural selection for this purpose – that is, it is exploited by receivers, often to the detriment of the emitter. For example, the sex pheromone emitted by an insect to attract a mate may also be exploited as a kairomone by a predator or parasitoid.

THE PREVIOUS CHAPTER discussed methods that generate independent observations from standard probability distributions. But you still have the problem of what to do when faced with a nonstandard distribution such as the posterior distribution of parameters of the conditionally conjugate linear regression model. Although themethods previously described can, in principle, deal with nonstandard distributions, doing so presents major practical difficulties. In particular, they are not easy to implement in the multivariate case, and finding a suitable importance function for the importance sampling algorithm or a majorizing density for the AR algorithm may require a very large investment of time whenever a new nonstandard distribution is encountered.

These considerations impeded the progress of Bayesian statistics until the development of Markov chain Monte Carlo (MCMC) simulation, a method that became known and available to statisticians in the early 1990s. MCMC methods have proven extremely effective and have greatly increased the scope of Bayesian methods. Although a disadvantage of these methods is that they do not provide independent samples, they have the great advantage of flexibility: they can be implemented for a great variety of distributions without having to undertake an intensive analysis of the special features of the distribution. Note, however, that an analysis of the distribution may shed light on the best algorithm to use when more than one is available.

Because these methods rely on Markov chains, a type of stochastic process, this chapter presents some basic concepts of the theory, and the next chapter utilizes these concepts to explain MCMC methods.

Introduction

Nutrition concerns the chemicals required by an organism for growth, tissue maintenance, reproduction and the energy necessary to maintain these functions. Many of these chemicals are ingested with the food, but others are synthesized by the insect itself. In some insects, microorganisms contribute to the insect's nutritional requirements. Achieving optimal nutrition involves a complex interplay between feeding behavior and post-ingestive processing of food. Insects must eat appropriate amounts of suitable foods, but avoid ingesting harmful excesses of toxins and nutrients. In this chapter we begin by considering the nature of nutritional requirements (Section 4.1). In Section 4.2 we set out the ways in which insects maintain nutritional balance, both by adjusting feeding behavior and post-ingestive processing. Next, we discuss the consequences for growth, development, reproduction and lifespan of failing to maintain nutrient balance (Section 4.3). Finally, we detail the nutritional contributions made by symbiotic microorganisms (Section 4.4).

Required nutrients

How nutritional requirements are identified

Most insects have qualitatively similar nutritional requirements because their chemical compositions and metabolic capabilities are broadly uniform. Variation among insects arises from adaptations to particular diets or associations with microorganisms that provide specific nutrients.

Introduction

Muscles power all the movements, external and internal, in insects. All insect muscles are striated, like vertebrate cardiac and skeletal muscle. In their structure, protein content, contractility and regulation, insect muscles show high levels of homology to these vertebrate muscles. However, the varieties of movements and modes of locomotion in insects, particularly larval crawling, flight, feats of jumping and stridulation (or singing) have led to a wide range of muscle specializations in structure, function and regulation. The study of specialized insect muscles has allowed us to understand how these muscles cope with the demands made on them in achieving insect movement. However, insects have also been important as model organisms for understanding muscle more generally. Hence our knowledge of muscle contraction owes much to work particularly on the waterbug, Lethocerus, whereas the fruit fly, Drosophila, is providing new insights into muscle function, development and human diseases.

This chapter covers insect muscle from its structure and molecular function to its integration into the controlled regulation of insect movements. Section 10.1 describes insect muscle ultrastructure with the location of specific proteins. Section 10.2 describes how insect muscles contract. Section 10.3 concerns muscle innervation, the activation of muscle contraction and its regulation. Section 10.4 deals with the energetics of muscle function and Section 10.5 links muscle control to locomotion in the whole organism. Muscle development is considered in Section 10.6, together with the role of the muscles in major lifecycle events.

Introduction

Gaseous exchange in insects occurs through a system of air-filled internal tubes, the tracheal system, the finer branches of which extend to all parts of the body and may become functionally intracellular in muscle fibers. Thus oxygen is carried in the gas phase directly to its sites of utilization. While the blood is not concerned with oxygen transport in most insects, some insects have now been shown to have hemocyanin, an oxygen-carrying pigment, in the blood. In terrestrial insects and some aquatic species, the tracheae open to the outside through segmental pores, the spiracles, which generally have some filter structures and a closing mechanism reducing water loss from the respiratory surfaces. Other aquatic species have no functional spiracles, and gaseous exchange with the water involves arrays of tracheae close beneath the surface of thin, permeable cuticle.

This chapter is divided into ten sections. Section 17.1 describes the tracheal system, its structure, distribution and development. Section 17.2 deals with the number, structure and distribution of the spiracles. Section 17.3 follows with cutaneous gas exchange; Section 17.4 treats respiratory pigments; and Section 17.5 describes gaseous exchange in terrestrial insects, considering diffusion and ventilation in resting and flying insects and control of ventilation. Section 17.6 addresses the gaseous exchange in aquatic insects, with oxygen uptake from the air and by gills. Section 17.7 gives attention to insects subject to occasional submersion. Section 17.8 refers to the gas exchange in endoparasitic insects. Section 17.9 is concerned with other functions of the tracheal system, and Section 17.10 with gas exchange in insect eggs.

THIS CHAPTER INTRODUCES several important concepts, provides a guide to the rest of the book, and offers some historical perspective and suggestions for further reading.

Econometrics

Econometrics is largely concerned with quantifying the relationship between one or more variables y, called the response variables or the dependent variables, and one or more variables x, called regressors, independent variables, or covariates. The response variable or variables may be continuous or discrete; the latter case includes binary, multinomial, and count data. For example, y might represent the quantities demanded of a set of goods, and x could include income and the prices of the goods; or y might represent investment in capital equipment, and x could include measures of expected sales, cash flows, and borrowing costs; or y might represent a decision to travel by public transportation rather than private, and x could include income, fares, and travel time under various alternatives.

In addition to the covariates, it is assumed that unobservable random variables affect y, so that y itself is a random variable. It is characterized either by a probability density function (p.d.f.) for continuous y or a probability mass function (p.m.f.) for discrete y. The p.d.f. or p.m.f. depends on the values of unknown parameters, denoted by θ. The notation y ∼ f(y∣θ, x) means that y has the p.d.f. or p.m.f. f(y∣θ, x), where the function depends on the parameters and covariates.

THE END OF the previous chapter mentions that simulation has greatly expanded the scope of Bayesian inference. This chapter reviews methods for generating independent samples from probability distributions. The methods discussed here form the basis for the newer methods discussed in Chapter 7 that are capable of dealing with a wide variety of distributions but do not generate independent samples.

All major statistics packages contain routines for generating random variables from such standard distributions as those summarized in Appendix A. The following examples are intended to illustrate methods of generating samples. I do not claim that the algorithms are the best that can be designed, and you should not study the methods in great detail. The goal for the chapter is to present the standard techniques of simulation and explain the kinds of questions that simulated samples can help answer.

Many of the applications discussed can be regarded as attempts to approximate a quantity such as E[g(X)] where X ∼ f(X), but the necessary integral, ∫ g(x)f(x)dx, cannot be computed analytically. This problem includes the computation of expected values (where g(X) = X) and other moments, as well as P(c1 ≤ X ≤ c2), for which you set g(X) = 1(c1 ≤ X ≤ c2).

Probability Integral Transformation Method

The most basic method of generating samples takes advantage of the ability of computers to generate values that can be regarded as drawn independently from a uniform distribution on (0,1), U(0, 1).