No abstract

Summary 1.The nutrient supply network model of the metabolic theory of ecology predicts that metabolic rate scales as mass 0·75 at all hierarchical levels.2. An alternative, cell size, model suggests that the scaling of metabolic rate is a by-product of the way in which body size changes, by cell size or number, or some combination thereof. It predicts a scaling exponent of mass 0·75 at the widest interspecific level, but values of mass 0·67 − 1·0 for lower taxonomic groups or within species. 3. Here these predictions are tested in insects using 391 species for the interspecific analysis, and the size-polymorphic workers of eight ant species at the intraspecific level. In the latter, the contribution of ommatidium size and number to variation in body length, which is closely related to eye size, is used to assess the relative contributions of changes in cell size and number to variation in body size. 4. Before controlling for phylogeny, metabolic rate scaled interspecifically as mass 0·82 . Following phylogenetic correction, metabolic rate scaled as mass 0·75 . 5. By contrast, the intraspecific scaling exponents varied from 0·67 to 1·0. Moreover, in the species where metabolic rate scaled as mass 1·0 , cell size did not contribute significantly to models of body size variation, only cell number was significant. Where the scaling exponent was < 1·0, cell size played an increasingly important role in accounting for size variation. 6. Data for one of the largest groups of organisms on earth are therefore inconsistent with the nutrient supply network model, but provide support for the cell size alternative.

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 O(2) and CO(2) 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 H(2)O/CO(2) regression, and summarize specific areas of the field that are likely to be profitable directions for future research.

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.

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.

Using the tsetse, Glossina pallidipes, we show that physiologic plasticity (resulting from temperature acclimation) accounts for among-population variation in thermal tolerance and water loss rates. Critical thermal minimum (CT(Min)) was highly variable among populations, seasons, and acclimation treatments, and the full range of variation was 9.3 degrees C (maximum value = 3.1 x minimum). Water loss rate showed similar variation (max = 3.7 x min). In contrast, critical thermal maxima (CT(Max)) varied least among populations, seasons, and acclimation treatments, and the full range of variation was only approximately 1 degree C. Most of the variation among the four field populations could be accounted for by phenotypic plasticity, which in the case of CT(Min), develops within 5 days of temperature exposure and is lost rapidly on return to the original conditions. Limited variation in CT(Max) supports bioclimatic models that suggest tsetse are likely to show range contraction with warming from climate change.

Chown, S. L. and Klok, C. J. 2003. Altitudinal body size clines: latitudinal effects associated with changing seasonality. -Ecography 26: 445-455.Within ectotherms, increases in body size with latitude are thought to be the consequence of the effect of a decline in development temperature, which results in a larger final body size. In contrast, latitudinal declines in body size are usually ascribed to limited resources. It has been suggested that if generation time is similar to or constitutes a significant proportion of the growing season length, then season length is likely to have a considerable influence on body size because of constraints on resource availability, so resulting in a decline in body size with temperature (latitude). However, if generation time declines relative to season length, resources effectively become available for longer. Temperature influences on growth and differentiation are likely to be most significant, resulting in an increase in body size with latitude. Here, we test the hypothesis by comparing intraspecific altitudinal body size clines in a monophyletic group of weevils from two regions that differ substantially in seasonality. On the relatively aseasonal Marion Island, body size increases with altitude, whereas on the more seasonal Heard Island the opposite is found. In addition, overlapping generations on Marion Island indicate year-round resource availability, whereas more discrete generations on Heard Island indicate winter cessation of growth and development. Our data provide support for the hypothesis that the seasonality of resource availability has a major influence on body size clines. Furthermore, we argue that analysis of interspecific body size clines should be preceded by nested analyses of variance to determine the influence of clinal replacement of higher taxa on these patterns.

Much of the work on the responses of terrestrial arthropods to high and low temperatures has been done on model organisms such as Drosophila . However, considerable variation in thermotolerance is partitioned at the family level and above, raising questions about the broader applicability of this work to other taxa. Here we investigate resistance to high and low temperatures, following different temperature treatments, in ten species and 31 populations of weevils found on sub-Antarctic Heard Island and Marion Island, which have substantially different climates. In these weevils there is considerable interspecific and among-population variation in critical thermal minimum (CTmin) and critical thermal maximum (CTmax), but most of this variation in critical limits can be ascribed to phenotypic plasticity. We find no relationship between CTmin and CTmax at the species level, and this is true also of populations and of responses to the temperature treatments. In general, plastic (acclimation) changes in CTmin are larger than those in CTmax. Our data therefore provide support for the idea that resistance to heat and to cold are decoupled in terrestrial arthropods. Furthermore, our results suggest that investigations of physiological limits to species borders should incorporate the effects of phenotypic plasticity on physiological capabilities. continuous expression or persistence of the 72kDa heatshock protein. Journal of Insect Physiology 38: 603-609. Zatsepina OG, Velikodvorskaia VV, Molodtsov VB, Garbuz D, Lerman DN, Bettencourt BR, Feder ME, Evgenev MB. 2001. A Drosophila melanogaster strain from sub-equatorial Africa has exceptional thermotolerance but decreased Hsp70 expression. Journal of Experimental Biology 204: 1869-1881.