Depression of metabolic rate has been recorded for virtually all major animal phyla in response to environmental stress. The extent of depression is usually measured as the ratio of the depressed metabolic rate to the normal resting metabolic rate. Metabolic rate is sometimes only depressed to approx. 80% of the resting value (i.e. a depression of approx. 20% of resting); it is more commonly 5–40% of resting (i.e. a depression of approx. 60–95% of resting); extreme depression is to 1% or less of resting, or even to an unmeasurably low metabolic rate (i.e. a depression of approx. 99–100% of resting). We have examined the resting and depressed metabolic rate of animals as a function of their body mass, corrected to a common temperature. This allometric approach allows ready comparison of the absolute level of both resting and depressed metabolic rate for various animals, and suggests three general patterns of metabolic depression.

Firstly, metabolic depression to approx. 0.05–0.4 of rest is a common and remarkably consistent pattern for various non-cryptobiotic animals (e.g. molluscs, earthworms, crustaceans, fishes, amphibians, reptiles). This extent of metabolic depression is typical for dormant animals with ‘intrinsic’ depression, i.e. reduction of metabolic rate in anticipation of adverse environmental conditions but without substantial changes to their ionic or osmotic status, or state of body water. Some of these types of animal are able to survive anoxia for limited periods, and their anaerobic metabolic depression is also to approx. 0.05–0.4 of resting. Metabolic depression to much less than 0.2 of resting is apparent for some ‘resting’, ‘over-wintering’ or diapaused eggs of these animals, but this can be due to early developmental arrest so that the egg has a low ‘metabolic mass’ of developed tissue (compared to the overall mass of the egg) with no metabolic depression, rather than having metabolic depression of the entire cell mass. A profound decrease in metabolic rate occurs in hibernating (or aestivating) mammals and birds during torpor, e.g. to less than 0.01 of pre-torpor metabolic rate, but there is often no intrinsic metabolic depression in addition to that reduction in metabolic rate due to readjustment of thermoregulatory control and a decrease in body temperature with a concommitant Q10 effect. There may be a modest intrinsic metabolic depression for some species in shallow torpor (to approx. 0.86) and a more substantial metabolic depression for deep torpor (approx. 0.6), but any energy saving accruing from this intrinsic depression is small compared to the substantial savings accrued from the readjustment of thermoregulation and the Q10 effect.

Secondly, a more extreme pattern of metabolic depression (to <0.05 of rest) is evident for cryptobiotic animals. For these animals there is a profound change in their internal environment – for anoxybiotic animals there is an absence of oxygen and for osmobiotic, anhydrobiotic or cryobiotic animals there is an alteration of the ionic/osmotic balance or state of body water. Some normally aerobic animals can tolerate anoxia for considerable periods, and their duration of tolerance is inversely related to their magnitude of metabolic depression; anaerobic metabolic rate can be less than 0.005 of resting. The metabolic rate of anhydrobiotic animals is often so low as to be unmeasurable, if not zero. Thus, anhydrobiosis is the ultimate strategy for eggs or other stages of the life cycle to survive extended periods of environmental stress.

Thirdly, a pattern of absence of metabolism when normally hydrated (as opposed to anhydrobiotic or cryobiotic) is apparently unique to diapaused eggs of the brine-shrimp (Artemia spp., an anostracan crustacean) during anoxia. The apparent complete metabolic depression of anoxic yet hydrated cysts (and extreme metabolic depression of normoxic, hypoxic, or osmobiotic, yet hydrated cysts), is an obvious exception to the above patterns.

In searching for biochemical mechanisms for metabolic depression, it is clear that there are five general characteristics at the molecular level of cells which have a depressed metabolism; a decrease in pH, the presence of latent mRNA, a change in protein phosphorylation state, the maintenance of one particular energy-utilizing process (ion pumping), and the down-regulation of another (protein synthesis). Oxygen sensing is now the focus of intense investigation and obviously plays an important role in many aspects of cell biology. Recent studies show that oxygen sensing is involved in metabolic depression and research is now being directed towards characterising the proteins and mechanisms that comprise this response. As more data accumulate, oxygen sensing as a mechanism will probably become the sixth general characteristic of depressed cells.

The majority of studies on these general characteristics of metabolically depressed cells come from members of the most common group of animals that depress metabolism, those non-cryptobiotic animals that remain hydrated and depress to 0.05–0.4 of rest. These biochemical investigations are becoming more molecular and sophisticated, and directed towards defined processes, but as yet no complete mechanism has been delineated. The consistency of the molecular data within this group of animals suggests similar metabolic strategies and mechanisms associated with metabolic depression.

The biochemical ‘adaptations’ of anhydrobiotic organisms would seem to be related more to surviving the dramatic reduction in cell water content and its physico-chemical state, than to molecular mechanisms for lowering metabolic rate. Metabolic depression would seem to be an almost inevitable consequence of their altered hydration state.

The unique case of profound metabolic depression of hydrated Artemia spp. cysts under a variety of conditions could reflect unique mechanisms at the molecular level. However, the available data are not consistent with this possibility (with the exception of a uniquely large decrease in ATP concentration of depressed, hydrated Artemia spp. cysts) and the question remains: how do cells of anoxic and hydrated Artemia spp. differ from anoxic goldfish or turtle cells, enabling them so much more completely to depress their metabolism?

Gammarus spp. (Crustacea: Amphipoda) are widespread throughout a diverse range of marine, freshwater and estuarine/brackish habitats, often dominating benthic macroinvertebrate communities in terms of both numbers and/or biomass. Gammarus spp. are the dominant macroinvertebrate prey items of many fish, whether as a seasonal food source or a year-round staple. Selective predation by fish on Gammarus spp. is often linked to parasitism and the body size of the prey. Gammarus spp. populations are under increasing threat from both pollution and replacement/displacement by introduced species. Loss of populations and species invasions/replacements could have significant impacts on native predator species if the predator(s) cannot successfully adapt their feeding patterns to cope with non-indigenous Gammarus prey species. Despite this, many fish predation studies do not identify Gammarus prey to species level. This lack of precision could be important, as Gammarus spp. exhibit wide variations in physiochemical tolerances, habitat requirements, abilities to invade and susceptibility to replacement. Although rarely acknowledged, the impacts of non-piscean predators (particularly macroinvertebrates) on Gammarus prey species may frequently be stronger than those exerted by fish. A major aim of this review is to ascertain the current importance of Gammarus as a prey species, such that the implications of changes in Gammarus spp. populations can be more accurately assessed by interested groups such as ecologists and fisheries managers. We also review the dynamics of Gammarus spp. as prey to a diverse array of mammals, birds, amphibians, insects, flatworms, other crustaceans such as crabs and crayfish and, perhaps most importantly, other Gammarus spp.

The taxonomic distribution and evolution of viviparity in Diptera is critically reviewed. The phenomenon ranges from ovoviviparity (eggs deposited at an advanced stage of embryonic development; larva emerges immediately after deposition), through viviparity (larva hatches inside female before deposition) to pupiparity (offspring deposited as pupa). Some Diptera are known to be facultatively viviparous, which is hypothesized to be a step towards the evolution of obligate viviparity. Obligate viviparity is found to comprise unilarviparity (single large larva in maternal uterus) which evolved many times independently, the rare oligolarviparity (more than one but not more than 12 larvae) and multilarviparity (large numbers of developing eggs or larvae in uterus) which is typical for the two largest clades of viviparous Diptera. Unilarviparity is either lecithotrophic (developing larva nourished by yolk of egg) or pseudo-placental (larva nourished by glandular secretions of mother). Viviparity has clearly evolved on many separate occasions in Diptera. It is recorded in 22 families, and this review identifies at least 61 independent origins of viviparity. Six families appear to have viviparity in their ground-plan. Some families have a single evolution of viviparity, others multiple evolutions. Guimaraes' model for the evolution of viviparity in Diptera is tested against phylogenetic information and the adaptive significance of viviparity is reviewed in detail. Possible correlations with life-history parameters (coprophily, parasitism, breeding in ephemeral plant parts, malacophagy and adult feeding habits – especially haematophagy) are analysed critically, as are potential advantages (shorter larval life, less investment in yolk by mother, protection of vulnerable stages, better access to breeding substrates, predation on competitors). Morphological constraints, adaptations and exaptations are reviewed, including the provision of an incubation space for the egg(s), the positioning of the egg(s) in the uterus, and maternal glands. The main morphological adaptations include greater egg size, reduction of egg respiratory filaments, thinning of chorion, modified larval respiratory system and mouthparts, and instar skipping. Female morphology and behaviour is particularly strongly modified for viviparity. The terminalia are shortened, the vagina is more muscular and tracheated, and the ovaries of unilarviparous species have a reduced number of ovarioles with alternate ovulation. Many of the final conclusions are tentative, and a plea is made for more detailed morphological and experimental study of many of the viviparous species. Viviparity in Diptera provides a fascinating example of multiple parallel evolution, and a fertile field for future research.

Many cladistic analyses of animal phylogeny have been published by authors arguing that their results are well supported. Comparison of these analyses indicates that there can be as yet no general consensus about the evolution of the animal phyla. We show that the various cladistic studies published to date differ significantly in methods of character selection, character coding, scoring and weighting, ground-pattern reconstructions, and taxa selection. These methodological differences are seldom made explicit, which hinders comparison of different studies and makes it impossible to assess a particular phylogeny outside its own scope. The effects of these methodological differences must be considered before we can hope to reach a morphological reference framework needed for effective comparison and combination with the evidence obtained from molecular and developmental genetic studies.

One way to build larger, more comprehensive phylogenies is to combine the vast amount of phylogenetic information already available. We review the two main strategies for accomplishing this (combining raw data versus combining trees), but employ a relatively new variant of the latter: supertree construction. The utility of one supertree technique, matrix representation using parsimony analysis (MRP), is demonstrated by deriving a complete phylogeny for all 271 extant species of the Carnivora from 177 literature sources. Beyond providing a ‘consensus’ estimate of carnivore phylogeny, the tree also indicates taxa for which the relationships remain controversial (e.g. the red panda; within canids, felids, and hyaenids) or have not been studied in any great detail (e.g. herpestids, viverrids, and intrageneric relationships in the procyonids). Times of divergence throughout the tree were also estimated from 74 literature sources based on both fossil and molecular data. We use the phylogeny to show that some lineages within the Mustelinae and Canidae contain significantly more species than expected for their age, illustrating the tree's utility for studies of macroevolution. It will also provide a useful foundation for comparative and conservational studies involving the carnivores.

Three closely related 4-hydroxy-3(2H)-furanones have been found in a range of highly cooked foodstuffs where they are important flavour compounds with aroma threshold values as low as 20 μg kg−1 water (approximately 0.14 μmol l−1). The compounds are formed mainly as a result of the operation of the Maillard reactions between sugars and amino acids during heating but one compound, 5-(or 2)-ethyl-2-(or 5)-methyl-4-hydroxy-3(2H)-furanone, appears in practice to be produced by yeast, probably from a Maillard intermediate, during the fermentation stages in the production of soy sauce and beer. The compounds are also important in the flavour of strawberry, raspberry, pineapple and tomato but the route of biosynthesis is unknown. Two 3-hydroxy-2(5H)-furanones, emoxyfuranone and sotolon, which are produced spontaneously from amino acids such as threonine and 4-hydroxy-L-leucine are major contributors to meaty and spicy/nutty flavours in foods. The biosynthesis of 5-(1,2-dihydroxyethyl)-3,4-dihydroxy- 2(5H)-furanone (ascorbic acid, vitamin C) and 5-hydroxymethyl-3,4-dihydroxy-2(5H)-furanone (erythro-ascorbic acid) from sugars in plants and yeast, respectively, has been characterized to the enzymic level. After treatment with chlorine, humic waters contain a range of chloro-furanones, some of which, particularly 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), are powerful mutagens. The furanones which occur in foods are also mutagenic to bacteria and cause DNA damage in laboratory tests. However, these compounds are, in practice, very effective anti-carcinogenic agents in the diets of animals which are being treated with known cancer-inducing compounds such as benzo[α]pyrene or azoxymethane. Two of the food- derived furanones have antioxidant activity comparable to that of ascorbic acid. A biological function has been discovered for some of the furanones besides vitamin C. 5-Methyl-4-hydroxy-3(2H)-furanone is a male pheromone in the cockroach Eurycolis florionda (Walker) and the 2,5-dimethyl derivative deters fungal growth on strawberries and is an important component of the attractive aroma of the fruit. The red seaweed Delisea pulchra (Greville) Montagne produces a range of brominated furanones which prevent colonisation of the plant by bacteria by interfering with the acylated homoserine lactone (AHL) signalling system used by the bacteria for quorum sensing. In addition, these compounds can deter grazing by marine herbivores. It is proposed here that the evolved biological function of a number of furanones is to act as inter-organism signal molecules in several different systems. This has resulted in two coincidental effects which are important for humans. Firstly, the easily oxidized nature of the furanones in general, which is likely to be an important property in their functioning as signal molecules, results in both mutagenic and anti-carcinogenic activity. The balance of these two effects from compounds in the diet has yet to be fully established. Secondly, and more specifically, the 4-hydroxy-3(2H)-furanones associated with fruit aromas act to attract animals to the fruit, which ensures seed dispersal. In the case of humans, the coincidental synthesis of some of these compounds in foods during preparation results in these foods appearing particularly attractive through the transferred operation of the original signalling mechanisms.

Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitude of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5–20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended to reveal the underlying behavioural mechanism and the factors that cause the daily displacements. The ultimate analysis deals with the adaptive significance of DVM and the driving forces that were responsible for the selection of the traits essential to the behavioural mechanism. The freshwater cladoceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defined as a light-oriented swimming towards (positive phototaxis) or away (negative phototaxis) from a light source, is considered the most important mechanism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis which is caused by changes in light intensity. Both types of reaction are superimposed on normal swimming. This swimming of Daphnia spp. consists of alternating upwards and downwards displacements over small distances. An internal oscillator seems to be at the base of these alternations. Primary phototaxis is the result of a dominance of either the upwards or the downwards oscillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drift. Adverse environmental factors may produce a persistent primary phototaxis. Rare clones of D. magna have been found that show also persistent positive or negative primary phototaxis and interbreeding of the two types produces intermediate progeny: thus a genetic component seems to be involved. Also secondary phototaxis is superimposed on normal swimming: a continuous increase in light intensity amplifies the downwards oscillator phase and decreases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relation between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rate of change causing a response if continued for an infinitely long time. DVM in a lake takes place during a period of 1·5–2 h when light changes are higher than the rheobase threshold. Accelerations in the rate of relative increase in light intensity strongly enhance downwards swimming in Daphnia spp. and this enhancement increases with increasing fish kairomone and food concentration. This phenomenon may represent a ‘decision-making mechanism’ to realize the adaptive goal of DVM: at high fish predator densities, thus high kairomone concentrations, and sufficiently high food concentrations, DVM is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light–dark boundaries or contrasts. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48·9° with the normal (Snell's window). Contrasts are fixed by the compound eye and appropriate turning of the body axis orients the daphnid in an upwards or an obliquely downwards direction. A predisposition for a positively or negatively phototactic orientation seems to be the result of a disturbed balance of the two oscillators governing normal swimming.

Some investigators have tried to study DVM at a laboratory scale during a 24 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropriate light intensity changes, vertical distributions looking like DVM were obtained. The results can be explained by phototactic reactions and the artificial nature of the compressed environmental factors but do not compare with DVM in the field.

A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Using the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated comparable to those in a lake.

The most important adaptive significance of DVM is avoidance of visual predators such as juvenile fish. However, in the absence of fish kairomones, small-scale DVMs are often present, which were probably evolved for UV-protection, and are realized by not enhanced phototaxis. In addition, the ‘decision-making mechanism’ was probably evolved as based on the enhanced phototactic reaction to accelerations in the rate of relative changes in light intensity and the presence of fish kairomones.

Within the Aphidoidea, most species of Aphididae, as long as they are in small numbers and not carrying plant viruses, do little perceptible damage to their food plants. In species that cause toxicoses, it is usually assumed that some component of the saliva must be responsible. Paradoxically, however, the salivary enzymes of Aphididae are similar to those that already occur in plants – oxidases and enzymes that depolymerize polysaccharides – and the salivary enzymes are injected in very small amounts relative to their counterparts in the plant. Damage to plants triggers defensive, biochemical responses, and it is suggested that the injected enzymes serve mainly to divert or counter responses at the immediate interface of stylets and plant tissues. The saliva of Aphididae contains non-enzymic, reducing compounds which, in the presence of oxidases, can combine with and inactivate defensive phytochemicals – including those released in response to damage and transported in the phloem sieve tube sap on which Aphididae feed. Salivary and gut oxidases deactivate ingested phytochemicals by oxidative polymerization. Aphididae inject saliva into sieve tubes before sustained ingestion of sap, and this saliva has been presumed to condition the sieve tubes, but in what way remains unclear. It is suggested that there is a dynamic biochemical interaction between aphids and plants; that the interaction is usually well balanced for most of the Aphididae; hence, no outcome is readily observable. Where a significant imbalance occurs, however, either the aphid is unable to feed, i.e. the plant is resistant, and/or the aphid does not effectively counter a hypersensitive response. Not all plant responses are disadvantageous to aphids. Gall-forming Aphidoidea trigger and control abnormal growth in the plant to the insects' advantage, possibly by eliciting vigorous oxidation in selective meristematic tissues, thereby limiting supply of molecular oxygen and inhibiting oxygen-dependent growth-controls. Current problems and possible approaches for further research are reviewed.

The olfactory response of maggots (the larvae of cyclorrhaphous flies) and its neuroanatomical basis have been a subject for scientific investigation since the 17th century, preoccupying both fundamental and applied scientists. Despite its apparently arcane nature, the subject raises a series of major neurobiological problems, in particular, the relationship between the number of odours that can be detected and the apparently simple systems of detection and processing available to larvae. Molecular biological techniques in both neuroanatomy and cell biology have made it possible to begin to resolve some of these problems, if data from a wide range of studies are integrated. Four sectors of research on a large number of species are reviewed: the behaviour involved in the olfactory response, the wide range of odours that can be detected, the neuroanatomical basis of olfaction in cyclorrhaphous larvae and the number of receptors involved in detecting these odours. Finally, a neuroanatomical model of olfactory processing is presented, together with perspectives for future research, emphasising the importance of studying the ecology of the species under investigation.

Tunicates are primitive chordates that develop a transient ‘tail’ in the larval stage that is generally interpreted as a rudimentary version of the vertebrate trunk. Not all tunicates have tails, however. The groups that lack them, salps and pyrosomes, instead have a trunk-like reproductive stolon located approximately where the tail would otherwise be. In salps, files of blastozooids are formed along the sides of the stolon. The tail and caudal trunk in more advanced chordates could have evolved from a stolon of this type, an idea referred to here as the ‘stolon hypothesis’. This means the vertebrate body could be a composite structure, since there is the potential for each somite to incorporate elements originally derived from a complete functional zooid. If indeed this has occurred, it should be reflected in some fashion in gene expression patterns in the vertebrate trunk. Selected morphological and molecular data are reviewed to show that they provide some circumstantial support for the stolon hypothesis. The case would be stronger if it could be demonstrated that salps and/or pyrosomes are ancestral to other tunicates. The molecular phylogenies so far available generally support the idea of a pelagic ancestor, but offer only limited guidance as to which of the surviving pelagic groups most closely resembles it. The principal testable prediction of the stolon hypothesis is that head structures (or their homologues) should be duplicated in series in the trunk in advanced chordates, and vice versa, i.e. trunk structures should occur in the head. The distribution of both rhabdomeric photoreceptors and nephridia in amphioxus conform with this prediction. Equally striking is the involvement of the Pax2 gene in the development of both the inner ear and nephric ducts in vertebrates. The stolon hypothesis would explain this as a consequence of the common origin of otic capsules and excretory ducts from atrial rudiments: from the paired rudiments of the parent oozooid in the case of the otic capsule (these express Pax2 according to recent ascidian data), and from tubular rudiments in the stolon in the case of the excretory ducts.

Glycolysis, the process responsible for the conversion of monosaccharides to pyruvic acid, is a ubiquitous feature of cellular metabolism and was the first major biochemical pathway to be well characterized. Although the majority of glycolytic enzymes are common to all organisms, the past quarter of a century has revealed that glycolysis in higher plants possesses numerous distinctive features. Research in the nineteenth century established convincingly that plants carry out alcoholic fermentation under anaerobic conditions. In 1878, Wilhelm Pfeffer asserted that a non-oxygen-requiring ‘intramolecular respiration’ was involved in the aerobic respiration of plants. Between 1900 and 1950 it was demonstrated that plants metabolize sugar and starch by a glycolytic pathway broadly similar to that of yeasts and muscle tissue. In 1948, the first purification and characterization of a plant glycolytic enzyme, aldolase, was published by Paul Stumpf. By 1960 the presence of each of the 10 enzymes of glycolysis, presumed at the time to be located in the cytosol, had been confirmed in higher plants. Shortly after 1960 it was shown that the mechanism of glycolytic regulation in plants had features in common with that of animals and yeasts, especially as regards the important role played by the enzyme phosphofructokinase; but important regulatory properties peculiar to plants were soon demonstrated. In the last 30 years, higher-plant glycolysis has been found to exhibit a number of additional characteristics peculiar to plant systems. One conspicuous feature of plant glycolysis, discovered in the 1970s, is the presence of a complete or nearly complete sequence of glycolytic enzymes in plastids, distinct and spatially separated from the glycolytic enzymes located in the cytosol. Plastidic and cytosolic isoenzymes of glycolysis have been shown to differ in their kinetic and regulatory properties, suggesting that the two pathways are independently regulated. Since about 1980 it has become increasingly clear that the cytosolic glycolysis of plants may make use of several enzymes other than the conventional ones found in yeasts, muscle tissue and plant plastids: these enzymes include a pyrophosphate-dependent phosphofructokinase, a non-reversible and nonphosphorylating glyceraldehyde-3-phosphate dehydrogenase, a phosphoenolpyruvate phosphatase (vacuolar location) and a three-enzyme sequence able to produce pyruvate from phosphoenolpyruvate avoiding the pyruvate-kinase step. These non-conventional enzymes may catalyze glycolysis in the plant cytosol especially under conditions of metabolic stress. Experiments on transgenic plants possessing significantly elevated or reduced (reduced to virtually nil in some cases) levels of glycolytic enzymes are currently playing an important part in improving our understanding of the regulation of plant glycolysis; such experiments illustrate an impressive degree of flexibility in the pathway's operation. Plant cells are able to make use of enzymes bypassing or substituting for several of the conventional enzymic steps in the glycolytic pathway; the extent and conditions under which these bypasses operate are the subject of current research. The duplication of the glycolytic pathway in plants and the flexible nature of the pathway have possibly evolved in relation to the crucial biosynthetic role played by plant glycolysis beyond its function in energy generation; both functions must proceed if a plant is to survive under varying and often stressful environmental or nutritional conditions.

The relationships between macro-ecological patterns and physiological investigations in insects, especially those dealing with respiratory metabolism, are assessed in an attempt to encourage the development of the interaction between macroecology and physiological ecology. First, we demonstrate that although physiological ecology has been explicitly concerned with a number of issues relating to species boundaries, many questions remain unanswered. We argue that there are essentially two ways in which the relationship between physiological tolerances and species range boundaries have been investigated. The correlational approach involves physiological inference, physiological prediction, isocline analyses and climatic matching, and has often been criticized for a lack of rigour, while the experimental approach seeks to examine experimentally the relationships between physiological variables and range edges. Second, we use the recent debate on processes underlying latitudinal patterns in body size to caution against the conflation of patterns and processes operating at intraspecific and interspecific levels, the dangers inherent in invoking single explanatory variables, and an undue focus on adaptationist (e.g. optimization) rather than non-adaptationist explanations or some combination of the two. We show that both positive and negative relationships between body size and latitude have been found at the intraspecific level and suggest that interactions between temperature-induced heterochrony, and the relationship between habitat durational stability, growing season length, and generation time can be used to explain these differences. Similar variation in documented patterns is demonstrated at the interspecific level, and the mechanisms usually proffered to explain such clines (especially the starvation/desiccation-resistance hypothesis) are discussed. Interactions between various environmental factors, such as host-plant quality, and their effects on size clines are also discussed. Third, we argue that respiratory metabolism, as a measure of ATP cost, and its spatio-temporal variation are critical to many explanations of macroecological patterns. Adaptive changes in metabolism reputedly involve both depression (stress resistance) and elevation of metabolic rate, although recent studies are increasingly calling these ideas into question. In particular, flow-through respirometry is revolutionizing results by allowing careful separation of resting (or standard) and active metabolic rates. These techniques have rarely been applied to studies of metabolic cold adaptation in insects, one of the most polemical adaptations ascribed to high-latitude and high-altitude species. We conclude by arguing that physiological investigations of species tolerances are important in the context of macroecology, especially species distributional patterns and the possible impact of climate change thereon. However, we caution that relationships between abiotic variables, species tolerances, and distributional ranges may be non-linear and subject to considerable modification by the presence of other species, and that many of the pressing questions posed by macroecology have not been addressed by insect physiologists. Nonetheless, we suggest that because an understanding of the dynamics of species distributions is of considerable importance, especially in the context of current conservation problems, insect physiological ecology has much future scope.

A programme of research into phase change in the desert locust, Schistocerca gregaria, is described. The ability to change phase between solitarious and gregarious forms in response to population density is a key feature of locusts and is central to their occasional yet catastrophic impact on humans. Phase polymorphism is an extreme form of phenotypic plasticity. The most labile phase characteristic is behaviour. It is argued that a fully integrated study of behavioural phase change provides a powerful tool for understanding both the mechanisms of phase change and locust population dynamics, both of which offer possibilities for improved management and control of desert locust plagues. An assay for measuring behavioural phase-state in individual locusts was derived, based on logistic regression analysis. Experiments are described that used the assay to quantify the time-course of behavioural change, both within the life of individual locusts and across generations. The locust-related stimuli that provoke behavioural gregarization were investigated. Complex interactions were found between tactile, visual and olfactory stimuli, with the former exerting the strongest effect. Behavioural analysis also directed a study of the mechanisms whereby adult females exert an epigenetic influence over the phase-state of their developing offspring. Female locusts use their experience of the extent and recency of being crowded to predict the probability that their offspring will emerge into a high-density population, and alter the development of their embryos accordingly through a gregarizing agent added to the foam that surrounds the eggs at laying. There is also a less pronounced paternal influence on hatchling phase-state. An understanding of the time-course of behavioural phase change led to a study of the effect of the fine-scale distribution of resources in the environment on interactions between individual locusts, and hence on phase change. This, in turn, stimulated an exploration of the implications of individual behavioural phase change for population dynamics. Cellular automata models were derived that explore the relationships between population density, density of food resources and the distribution of resources in the environment. The results of the simulation showed how the extent of gregarization within a population increases with rising population size relative to food abundance and increasing concentration of food resources. Of particular interest was the emergence of critical zones across particular combinations of resource abundance, resource distribution and population size, where a solitarious population would rapidly gregarize. The model provided the basis for further laboratory and field experiments, which are described.

Amongst the diversity of methods used by organisms to reduce damage caused by ultraviolet (UV) radiation, the synthesis of UV-screening compounds is almost ubiquitous. UV-screening compounds provide a passive method for the reduction of UV-induced damage and they are widely distributed across the microbial, plant and animal kingdoms. They share some common chemical features. It is likely that on early earth strong selection pressures existed for the evolution of UV-screening compounds. Many of these compounds probably had other physiological roles, later being selected for the efficacy of UV screening. The diversity in physiological functions is one of the complications in studying UV-screening compounds and determining the true ecological importance of their UV-screening role. As well as providing protection against ambient UV radiation, species with effective screening may also be at an advantage during natural ozone depletion events. In this review the characteristics of a wide diversity of UV-screening compounds are discussed and evolutionary questions are explored. As research into the range of UV-screening compounds represented in the biosphere continues, so it is likely that the properties of many more compounds will be elucidated. These compounds, as well as providing us with insights into natural responses to UV radiation, may also have implications for the development of artificial UV-screening methods to reduce human exposure to UV radiation.

Theorists and experimental researchers have long debated whether animals are able to imitate. A variety of definitions of imitation have been proposed to describe this complex form of social learning. Experimental research on imitation has often been hampered by either a too loose ‘anthropomorphic’ approach or by too narrow ‘behaviourist’ definitions. At present neither associative nor cognitive theories are able to offer an exhaustive explanation of imitation in animals. An ethological approach to imitation offers a different perspective. By integrating questions on function, mechanism, development and evolution one can identify possible directions for future research. At present, however, we are still far from developing a comprehensive theory of imitation.

A functional approach to imitation shows that, despite some evidence for imitative learning in food processing in apes, such learning has not been shown to be involved in the social transmission of either tool-use skills or communicative signals. Recently developed procedures offer possible ways of clarifying the role of imitation in tool use and visual communication. The role of imitation in explorative play in apes is also investigated and the available data suggest that copying during play might represent a behavioural homologue of human imitation.

It is proposed that the ability to copy the behaviour of a companion is under a strong genetic influence in many social species. Many important factors have not been examined experimentally, e.g. the effect of the demonstrator, the influence of attention and memory and the ability to generalize. The potential importance of reinforcement raises the possibility that copying abilities serving divergent functions might be partly under the control of different mechanisms.