Critical PO2 is size-independent in insects: implications for the metabolic theory of ecology.

Insects, and all animals, exhibit hypometric scaling of metabolic rate, with larger species having lower mass-specific metabolic rates. The metabolic theory of ecology (MTE) is based on models ascribing hypometric scaling of metabolic rate to constrained O2 supply systems in larger animals. We compiled critical PO2 of metabolic and growth rates for more than 40 insect species with a size range spanning four orders of magnitude. Critical PO2 values vary from far below 21kPa for resting animals to near 21kPa for growing or flying animals and are size-independent, demonstrating that supply capacity matches oxygen demand. These data suggest that hypometric scaling of resting metabolic rate in insects is not driven by constraints on oxygen availability.

Single and multigenerational responses of body mass to atmospheric oxygen concentrations in Drosophila melanogaster : evidence for roles of plasticity and evolution.

Greater oxygen availability has been hypothesized to be important in allowing the evolution of larger invertebrates during the Earth's history, and across aquatic environments. We tested for evolutionary and developmental responses of adult body size of Drosophila melanogaster to hypoxia and hyperoxia. Individually reared flies were smaller in hypoxia, but hyperoxia had no effect. In each of three oxygen treatments (hypoxia, normoxia or hyperoxia) we reared three replicate lines of flies for seven generations, followed by four generations in normoxia. In hypoxia, responses were due primarily to developmental plasticity, as average body size fell in one generation and returned to control values after one to two generations of normoxia. In hyperoxia, flies evolved larger body sizes. Maximal fly mass was reached during the first generation of return from hyperoxia to normoxia. Our results suggest that higher oxygen levels could cause invertebrate species to evolve larger average sizes, rather than simply permitting evolution of giant species.

Responses of terrestrial insects to hypoxia or hyperoxia.

Oxygen is critically important for catabolic ATP generation but is also a dangerous source of reactive oxygen species. Insects respond to short-term exposure to hypoxia or hyperoxia with compensatory changes in spiracular opening and ventilation that reduce variation in internal Po2. Below critical Po2 values (Pc), nitric oxide and hypoxia inducible factor (HIF)-mediated pathways induce long-term responses such as compensatory tracheal growth, suppressed development, and acclimation of ventilation. Pc values are strongly affected by activity and ontogeny, due to changes in the ratio of tracheal conductance to metabolic rate. Although growth rates and development are suppressed by significant hypoxia in all species studied to date, adult body size is only affected in some species. Severe hyperoxia causes major oxidative stress and reduces survival, while moderate hyperoxia increases development times and body sizes in some species by unknown mechanisms.

Temperature- and body mass-related variation in cyclic gas exchange characteristics and metabolic rate of seven weevil species: Broader implications.

The influence of temperature on metabolic rate and characteristics of the gas exchange patterns of flightless, sub-Antarctic Ectemnorhinus-group species from Heard and Marion islands was investigated. All of the species showed cyclic gas exchange with no Flutter period, indicating that these species are not characterized by discontinuous gas exchange cycles. Metabolic rate estimates were substantially lower in this study than in a previous one of a subset of the species, demonstrating that open-system respirometry methods provide more representative estimates of standard metabolic rate than do many closed-system methods. We recommend that the latter, and especially constant-pressure methods, either be abandoned for estimates of standard metabolic rate in insects, or have their outputs subject to careful scrutiny, given the wide availability of the former. V(.)CO(2) increase with an increase in temperature (range: 0-15 degrees C) was modulated by an increase in cycle frequency, but typically not by an increase in burst volume. Previous investigations of temperature-related changes in cyclic gas exchange (both cyclic and discontinuous) in several other insect species were therefore substantiated. Interspecific mass-scaling of metabolic rate (ca. 0.466-0.573, excluding and including phylogenetic non-independence, respectively) produced an exponent lower than 0.75 (but not distinguishable from it or from 0.67). The increase of metabolic rate with mass was modulated by an increase in burst volume and not by a change in cycle frequency, in keeping with investigations of species showing discontinuous gas exchange. These findings are discussed in the context of the emerging macrophysiological metabolic theory of ecology.

Inertia in physiological traits: Embryonopsis halticella caterpillars (Yponomeutidae) across the Antarctic Polar Frontal Zone.

Geographic variation is characteristic of many physiological traits at the population and species levels. However, several recent studies have suggested that population-level variation is either limited or that it is mostly a consequence of phenotypic plasticity. Here we show that there is considerable physiological inertia in cold hardiness, upper thermal tolerance limits and desiccation resistance in caterpillars of the sub-Antarctic moth Embryonopsis halticella Eaton, such that populations from two climatically different islands are physiologically very similar. Both populations are moderately chill tolerant, with no difference in the supercooling points of caterpillars (-17 to -20 degrees C). Within their host plants caterpillars of both populations freeze at substantially higher, and statistically equivalent temperatures (-9.5 to -11.5 degrees C). The populations also have similar upper lethal limits (38 degrees C), and survival times of dry conditions (6-170 h depending on mass). The previously inexplicably low freezing point of caterpillars at the climatically less severe Marion Island seems likely a consequence of physiological inertia given that the freezing point of caterpillars within their hosts is only a few degrees below absolute minima at the older, and colder, Heard Island. Lack of adaptive geographic variation in physiological traits has consequences for models of range limits, and highlights the importance of exploring phenotypic plasticity as a response to climatic variation.

Metabolic rate variation in Glossina pallidipes (Diptera: Glossinidae): gender, ageing and repeatability.

Despite the importance of metabolic rate in determining flight time of tsetse and in mediating the influence of abiotic variables on life history parameters (and hence abundance and distribution), metabolic rate measurements and their repeatability have not been widely assessed in these flies. We investigate age-related changes in standard metabolic rate (SMR) and its repeatability, using flow-through respirometry, for a variety of feeding, gender and pregnancy classes during early adult development in laboratory-reared individuals of the tsetse fly, Glossina pallidipes. Standard metabolic rate (144-635 microW) was generally within 22% of previous estimates, though lower than the values found using closed system respirometry. There was no significant difference between the genders, but metabolic rate increased consistently with age, probably owing to flight muscle development. Repeatability of metabolic rate was generally high (r=0.6-.09), but not in younger teneral adults and pregnant females (r approximately equal to 0.05-0.4). In these individuals, low repeatability values are a consequence of muscle or in utero larval development. Tsetse and other flies generally have a much higher metabolic rate, for a given size, than do other insect species investigated to date.

Interactions between desiccation resistance, host-plant contact and the thermal biology of a leaf-dwelling sub-antarctic caterpillar, Embryonopsis halticella (Lepidoptera: Yponomeutidae).

During May 1997 thermal tolerance, supercooling point (SCP), low and high temperature survival, and desiccation resistance were examined in field-fresh Embryonopsis halticella Eaton larvae from Marion Island. SCPs were also examined in acclimated larvae, larvae starved for seven days, larvae within their leaf mines, and in larvae exposed to ice crystals. Field-fresh larvae had a critical minimum temperature (CT(Min)) and critical maximum temperature (CT(Max)) of 0 degrees C and 39.7 degrees C, respectively. Mean SCP of field-fresh caterpillars was -20.5 degrees C and this did not change with starvation. Field-fresh larvae did not survive freezing and their lower lethal temperatures (70% mortality below -21 degrees C) and survival of exposure to constant low temperatures (100% mortality after 12hrs at -19 degrees C) indicated that they are moderately chill tolerant. SCP frequency distributions were unimodal for field-fresh larvae, but became bimodal at higher acclimation temperatures. Contact with ice-crystals caused an increase in SCP (-6.5 degrees C), but contact with the host plant had less of an effect at higher subzero temperatures. It appears that the remarkable desiccation resistance of the larvae is selected for by the absence of a boundary layer surrounding their host plant, caused by constant high winds. This suggests that the low SCPs of E. halticella larvae may have evolved as a consequence of pronounced desiccation resistance.

Fluvoxamine and clomipramine in depressed patients. A double-blind clinical study.

In a double-blind study we compared fluvoxamine, a new selective serotonin re-uptake inhibitor, with clomipramine. In 36 female inpatients with vital depression, the antidepressant properties of fluvoxamine and clomipramine were studied. Eight patients did not complete the study. During a 4-week treatment period the Hamilton-, Zung-, Clinical Global Impression- and Leyden ratings showed, apart from the last scale in the fluvoxamine group, significant improvement in both groups. In the latter scale, a statistically significant difference was found in favour of clomipramine. Additional anxiolytic-sedative medication was required equally in both groups. CSF data in 10 patients are discussed. Non-specific electrocardiographic (ECG) repolarization disturbances were observed in both groups. Anticholinergic side effects were more prominent with clomipramine than with fluvoxamine; gastrointestinal symptoms and agitation were more prominent with fluvoxamine than with clomipramine. Fluvoxamine did not show particular advantages or disadvantages over clomipramine.