C. Jaco Klok

Insects at low temperatures: an ecological perspective

Abstract Modern climate change has precipitated widespread interest in the responses of organisms to the thermal environment. In insects, it is not only changes in mean environmental temperature and growing season length that are important, but also their responses to environmental extremes. Much is now known about the ways in which insects cope with the ice–water threshold, and with the low temperatures that precede it. Recent work has demonstrated a diversity of physiological responses to cooling and freezing in insects, with extremes of temperature, rates of temperature change, the numbers of freeze–thaw transitions, climatic unpredictability and the state of the surrounding microhabitat being important factors determining the cold tolerance strategy adopted by an insect. Insect low temperature biology now integrates techniques ranging from laboratory-based functional genomics to climatology, making it not only intrinsically fascinating, but also of considerable relevance to investigations of the biological implications of climate change.

Discontinuous gas exchange in insects: a clarification of hypotheses and approaches.

Many adult and diapausing pupal insects exchange respiratory gases discontinuously in a three‐phase discontinuous gas exchange cycle (DGC). We summarize the known biophysical characteristics of the DGC and describe current research on the role of convection and diffusion in the DGC, emphasizing control of respiratory water loss. We summarize the main theories for the evolutionary genesis (or, alternatively, nonadaptive genesis) of the DGC: reduction in respiratory water loss (the hygric hypothesis), optimizing gas exchange in hypoxic and hypercapnic environments (the chthonic hypothesis), the hybrid of these two (the chthonic‐hygric hypothesis), reducing the toxic properties of oxygen (the oxidative damage hypothesis), the outcome of interactions between O2 and CO2 control set points (the emergent property hypothesis), and protection against parasitic invaders (the strolling arthropods hypothesis). We describe specific techniques that are being employed to measure respiratory water loss in the presence or absence of the DGC in an attempt to test the hygric hypothesis, such as the hyperoxic switch and H2O/CO2 regression, and summarize specific areas of the field that are likely to be profitable directions for future research.

Upper thermal tolerance and oxygen limitation in terrestrial arthropods

SUMMARY The hypothesis of oxygen limitation of thermal tolerance proposes that critical temperatures are set by a transition to anaerobic metabolism, and that upper and lower tolerances are therefore coupled. Moreover, this hypothesis has been dubbed a unifying general principle and extended from marine to terrestrial ectotherms. By contrast, in insects the upper and lower limits are decoupled, suggesting that the oxygen limitation hypothesis might not be as general as proposed. However, no direct tests of this hypothesis or its predictions have been undertaken in terrestrial species. We use a terrestrial isopod (Armadillidium vulgare) and a tenebrionid beetle (Gonocephalum simplex) to test the prediction that thermal tolerance should vary with oxygen partial pressure. Whilst in the isopod critical thermal maximum declined with declining oxygen concentration, this was not the case in the beetle. Efficient oxygen delivery via a tracheal system makes oxygen limitation of thermal tolerance, at a whole organism level, unlikely in insects. By contrast, oxygen limitation of thermal tolerances is expected to apply to species, like the isopod, in which the circulatory system contributes significantly to oxygen delivery. Because insects dominate terrestrial systems, oxygen limitation of thermal tolerance cannot be considered pervasive in this habitat, although it is a characteristic of marine species.

Insect gas exchange patterns: A phylogenetic perspective

SUMMARY Most investigations of insect gas exchange patterns and the hypotheses proposed to account for their evolution have been based either on small-scale, manipulative experiments, or comparisons of a few closely related species. Despite their potential utility, no explicit, phylogeny-based, broad-scale comparative studies of the evolution of gas exchange in insects have been undertaken. This may be due partly to the preponderance of information for the endopterygotes, and its scarcity for the apterygotes and exopterygotes. Here we undertake such a broad-scale study. Information on gas exchange patterns for the large majority of insects examined to date (eight orders, 99 species) is compiled, and new information on 19 exemplar species from a further ten orders, not previously represented in the literature (Archaeognatha, Zygentoma, Ephemeroptera, Odonata, Mantodea, Mantophasmatodea, Phasmatodea, Dermaptera, Neuroptera, Trichoptera), is provided. These data are then used in a formal, phylogeny-based parsimony analysis of the evolution of gas exchange patterns at the order level. Cyclic gas exchange is likely to be the ancestral gas exchange pattern at rest (recognizing that active individuals typically show continuous gas exchange), and discontinuous gas exchange probably originated independently a minimum of five times in the Insecta.

Metabolism of the sub-Antarctic caterpillar Pringleophaga marioni during cooling, freezing and thawing.

SUMMARY Although general models of the processes involved in insect survival of freezing exist, there have been few studies directly investigating physiological processes during cooling, freezing and thawing, without which these models remain hypothetical. Here, we use open-flow respirometry to investigate the metabolism of the freeze-tolerant sub-Antarctic caterpillar Pringleophaga marioni Viette (Lepidoptera: Tineidae) during cooling, freezing and thawing and to compare animals exposed to non-lethal (–5.8°C) and lethal (–6.0°C, after which caterpillars are moribund for several days, and –18°C, after which caterpillars are completely unresponsive) freezing stress. We found a large decrease in metabolic rate (that is not associated with freezing) at– 0.6±0.1°C and calculated a Q10 of 2.14×103 at this breakpoint. This breakpoint is coincident with the critical thermal minimum (CTmin) and is hypothesised to be a metabolic manifestation of the latter, possibly a failure of the Na+/K+-ATPase pump. This provides a plausible link between processes at the cellular level and observations of the action of the CTmin at tissue and whole-organism levels. Caterpillars froze at –4.6±0.1°C and had detectable metabolism when frozen. Post-thaw, metabolic rates were lower than pre-freezing measurements. Post-thaw metabolic rates did not differ between temperatures that did and did not kill the caterpillars, which suggests that mortality may be a result of a breakdown in processes at the organismal, rather than cellular, level of organisation.

Determinants of terrestrial arthropod community composition at Cape Hallett, Antarctica

The distribution and abundance of free-living arthropods from soil and under stones were surveyed at the Cape Hallett ice-free area (ASPA No. 106), North Victoria Land, Antarctica. A total of 327 samples from 67 plots yielded 11 species of arthropods comprised of three Collembola: Cryptopygus cisantarcticus, Friesea grisea and Isotoma klovstadi and eight mites: Coccorhagidia gressitti, Eupodes wisei, Maudheimia petronia, Nanorchestes sp., Stereotydeus belli, S. punctatus, Tydeus setsukoae and T. wadei. Arthropods were absent from areas occupied by the large Adélie penguin colony. There was some distinction among arthropod communities of different habitats, with water and a lichen species (indicative of scree slope habitats) ranking as significant community predictors alongside spatial variables in a Canonical Correspondence Analysis. Recent changes to the management plan for ASPA No. 106 may need to be revisited as the recommended campsite is close to the area of greatest arthropod diversity.

Water‐Balance Characteristics Respond to Changes in Body Size in Subantarctic Weevils

Several environmental factors leading to size‐dependent mortality influence insect body size. Few investigations have been concerned with the ways in which the mechanisms underlying variation in water‐balance characteristics evolve in response to changes in body size that occur independently of water‐balance requirements. Using an explicitly phylogenetic analysis, we show how body size has changed over time in the Ectemnorhinus group of weevils and how water‐balance characteristics have evolved in response to this change and changes in habitat use. The basal species in the group are all large bodied and from moist environments. In response to a change in resource availability, there was a marked decline in size within the group. Despite the reduction in water content and dehydration tolerance that this meant, evolution of low whole‐animal water‐loss rates and high tolerance of dehydration resulted in conservation of desiccation resistance. The return to moist habitats in the group resulted in a reduction in dehydration tolerance and an increase in water‐loss rate. Thus, dehydration tolerance and water‐loss rate respond rapidly both when there is selection for water conservation and when this requirement is relaxed. Future laboratory selection experiments might usefully explore both directions of water‐balance evolution.

OXYGEN SUPPLY INDEX: UNIFYING PHYSIOLOGICAL AND ECOLOGICAL APPROACHES

![Figure][1] Compared with air, water contains about 33 times less oxygen and it diffuses ∼300,000 times slower. Also well established is that oxygen availability in aquatic ecosystems varies with temperature and atmospheric pressure. As a consequence, it is an important ecological factor

HEAVY LOADS SLOW TRUCK DRIVERS AND ANTS

![Figure][1] When carrying resources from a collecting point to the nest, one would assume that animals would attempt to carry as much as possible to maximize their foraging efforts. However, among social insects that is not always the best strategy. Foragers carrying large loads might

TRACHEAE & TRACHEOLES: A FORTUITOUS DISCOVERY REFINES A KEY DEFINITION

![Figure][1] It is always interesting when a seemingly minor study actually contributes significantly to our understanding of a key concept in animal biology. A key concept in the field of insect respiratory biology is understanding the structure, function and formation of the insect tracheal