During such events every single movement of legs was accompanied by exceptional spikes in the CO2 production curve at these low temperatures, which could be clearly distinguished from the common resting gas exchange pattern (our own unpublished results). Thus we assume the wasp forced activity CTmin to be below 5.8 °C (our lowest experimental temperature with IR video observation). In any case our investigations demonstrate an increased cold hardiness of Vespula sp. foragers in comparison to A. mellifera. MacMillan and Sinclair (2011) proposed that in insects chill coma and CTmin are not caused by failure of cell respiration or the
circulatory system but by disruption of signal transmission leading to failure in the neuromuscular system. Hazell et al. (2008) and Hazell and Bale (2011) opine that selleck screening library voluntary and forced activity show an insects’ CTmin. Respiration data seem not to be of so much significance for them regarding the lower thermal limit. Insect respiration,
however, depends on active spiracle control and abdominal respiratory movements to achieve sufficient exchange of respiration gases via the tracheae. So respiration and muscular and neural activity are closely related. Like in CTmax determination, selleck chemicals the combination of respiratory and behavioral data seems to provide the most accurate results in defining CTmin. Our investigation showed that even closely
related groups like honeybees and wasps Monoiodotyrosine may show significant differences of resting metabolism (Fig. 7, compare No. 7 A. mellifera ( Kovac et al., 2007), No. 8 Vespula sp. (this study) and No. 9 P. dominulus ( Weiner et al., 2009)). In a comparison over several taxa Vespula sp. stands out with a high resting metabolic rate over the entire temperature range ( Fig. 7). At Ta = 20 °C the CO2 production of Vespula sp. is 60% higher than that of A. mellifera, an insect with similar body shape, weight and active thermoregulation: 18.054 μl mg−1 min−1 vs. 11.16 μl mg−1 min−1. This might be based on differences in the thermal activity range as well as diverse overwintering strategies (single Polistes- and Vespula-queens vs. whole Apis colony). Nowickia sp. has a comparable body mass, but an even lower resting metabolism of only 2.304 μl mg−1 min−1 ( Chappell and Morgan, 1987). This is only 13% of Vespula’s turnover. Measurements at only one temperature ( Fig. 8) or in the species’ preferred temperature range do not always show differences between species clearly. Only respiratory data gathered over the animals’ entire active temperature range allows profound comparison. The metabolic theory of ecology links the metabolic rate to mass and ambient temperature. It predicts a general decrease of mass-specific metabolism with body mass for all organisms (see e.g. Clarke, 2006).