Thorax Temperature Research Articles

Altering developmental oxygen exposure influences thermoregulation and flight performance of Manduca sexta.

Endothermic, flying insects are capable of some of the highest recorded metabolic rates. This high aerobic demand is made possible by the insect's tracheal system, which supplies the flight muscles with oxygen. Many studies focus on metabolic responses to acute changes in oxygen to test the limits of the insect flight metabolic system, with some flying insects exhibiting oxygen limitation in flight metabolism. These acute studies do not account for possible changes induced by developmental phenotypic plasticity in response to chronic changes in oxygen levels. The endothermic moth Manduca sexta is a model organism that is easy to raise and exhibits a high thorax temperature during flight (∼40°C). In this study, we examined the effects of developmental oxygen exposure during the larval, pupal and adult stages on the adult moth's aerobic performance. We measured flight critical oxygen partial pressure (Pcrit-), thorax temperature and thermoregulating metabolic rate to understand the extent of developmental plasticity as well as effects of developmental oxygen levels on endothermic capacity. We found that developing in hypoxia (10% oxygen) decreased thermoregulating thorax temperature when compared with moths raised in normoxia or hyperoxia (30% oxygen), when moths were warming up in atmospheres with 21-30% oxygen. In addition, moths raised in hypoxia had lower critical oxygen levels when flying. These results suggest that chronic developmental exposure to hypoxia affects the adult metabolic phenotype and potentially has implications for thermoregulatory and flight behavior.

Water loss, not overheating, limits the activity period of an endothermic Sonoran Desert bee

Abstract Desert animals must manage the physiological stress caused by heat and desiccation; evaporative heat loss mitigates overheating but exacerbates water stress. Small endothermic flying insects may be particularly vulnerable to overheating and water stress as a result of high surface area to volume ratios, but we lack quantitative understanding of the relative magnitude of these abiotic stressors in flying desert invertebrates, despite their ecological importance. During the hottest and driest weeks of year, many thousands of males of the Sonoran Desert digger bee (Centris caesalpiniae) flew near‐continuously at elevated thorax temperatures for hours at mating aggregation sites, while fighting other males and digging for females. To determine whether incapacitating high temperatures or water loss limited the activity period of male C. caesalpiniae, we assessed wet and dry body mass and water content through the activity period, flight durations using mark–recapture methods, crop volume and sugar content, microclimate selection, water balance during flight, critical water content and maximum critical temperature. Body masses and sizes of males declined through the morning. Body water content scaled isometrically with total body wet mass. Crop volume and sugar content did not vary throughout the day or with bee size. Maximum critical temperature during flight was 51°C for large‐morph C. caesalpiniae males, similar to those measured for other bees, and well above body temperatures reached in the field, suggesting that avoidance of overheating does not limit activity in this desert bee. The critical water content of Centris bees averaged 50%, at the low end of those measured for other bees. Measures of net water loss rate indicated that males approached lethal water loss limits within 4 h, a duration in the range of measured daily flight activity, suggesting that desiccation tolerance limits activity. Remarkably, male C. caesalpiniae were not observed to forage at floral or water sources during the activity period, yet they returned over multiple days, suggesting that these males have a mechanism to rehydrate when not at the mating aggregation. Read the free Plain Language Summary for this article on the Journal blog.

Substrate use and temperature effects in flight muscle mitochondria from an endothermic insect, the hawkmoth Manduca sexta

Manduca sexta are endothermic insects, requiring adult thorax temperatures to be elevated above 35 °C for flight muscles to produce the wing beat frequencies necessary for flight. During flight, these animals rely on aerobic production of ATP by flight muscle mitochondria with several potential metabolic pathways providing the fuel. Along with typical carbohydrate substrates, mitochondria of other endothermic insects including bumblebees and wasps can use the amino acid proline or glycerol 3-phosphate (G3P) as metabolic fuel for prewarm up and flight. Here we examine flight muscle mitochondria physiology and the role of temperature and substrates in oxidative phosphorylation from 3-day old adult Manduca sexta. Mitochondria oxygen flux from flight muscle fibers were temperature sensitive with Q10 values ranging from 1.99 to 2.90, with a large increase in LEAK respiration with increased temperature. Mitochondria oxygen flux was stimulated by carbohydrate-based substrates, with flux through Complex I substrates providing the greatest oxygen flux. Neither proline nor G3P produced an increase in oxygen flux of the flight muscle mitochondria. Unlike other endothermic insects, Manduca are unable to supplement carbohydrate oxidation with either proline or G3P entering through Coenzyme Q and rely on substrates entering at complex I and II.

A desert bee thermoregulates with an abdominal convector during flight.

Flying endothermic insects thermoregulate, likely to improve flight performance. Males of the Sonoran Desert bee, Centris caesalpiniae, seek females at aggregations beginning at sunrise and cease flight near midday when the air temperature peaks. To identify the thermoregulatory mechanisms for C. caesalpiniae males, we measured tagma temperature, wingbeat frequency, water loss rate, metabolic rate and tagma mass of flying bees across shaded air temperatures of 19-38°C. Surface area, wet mass and dry mass declined with air temperature, suggesting that individual bees do not persist for the entire morning. The largest bees may be associated with cool, early mornings because they are best able to warm themselves and/or because they run the risk of overheating in the hot afternoons. Thorax temperature was high (38-45°C) and moderately well regulated, while head and abdomen temperatures were cooler and less controlled. The abdominal temperature excess ratio increased as air temperature rose, indicating active heat transfer from the pubescent thorax to the relatively bare abdomen with warming. Mass-specific metabolic rate increased with time, and air and thorax temperatures, but wingbeat frequency did not vary. Mass-specific water loss rate increased with air temperature, but this was a minor mechanism of thermoregulation. Using a heat budget model, we showed that whole-body convective conductance more than doubled through the morning, providing strong evidence that the primary mechanism of regulating thorax temperature during flight for these bees is increased use of the abdomen as a convector at higher air temperatures.

Insect thermal limits in warm and perturbed habitats: Dragonflies and damselflies as study cases.

Disturbance (e.g. loss of plant cover) increases ambient temperature which can be lethal for ectotherm insects especially in hot places. We compared the thorax temperatures of 26 odonate species as a function of body size, habitat quality ("conserved" and cooler vs "perturbed" and warmer) and suborder (Anisoptera vs Zygoptera), as well as critical thermal maximum (CTmax) and as a function of habitat quality in Argia pulla (Zygoptera) and Orthemis ferruginea (Anisoptera). We expected thorax temperatures to differ between suborders based on their differences in body size and habitat quality status, and that populations in perturbed sites would have higher critical thermal maxima compared to those in conserved sites. This study was done in a tropical region with high ambient temperatures. Anisopterans had a higher body temperature than zygopterans, with no difference between habitats. Thoracic and air temperature were positively related, yet body temperatures were higher than the ambient temperature. A. pulla had higher CTmax in the perturbed sites, while O. ferruginea showed the opposite trend. Microenvironmental changes increase the ambient temperature, perhaps filtering insect species. The apparent resilience of odonates to disturbance should be examined more closely (using more species), especially in small species like the zygopterans which appear to be more strongly affected by ambient temperature.

Varied Air Temperature and Helium‐Nitrogen Ratios Reveal the Thermal Performance Curve of a Free‐Flying Endotherm

While thermal performance curves are central to understanding the thermal biology of ectotherms and are key parameters for predicting effects of climate change, a thermal performance curve has never been described for free flight of any endotherm. This is likely the case for two reasons: 1) it is challenging to vary the body temperature of most flying endotherms, and 2) thermal performance curves require assessment of maximal performance at each body temperature, and maximal performance is challenging to elicit during fight. We overcame both of these challenges by flying honey bees at 23°C and 35°C air temperatures and at air densities ranging from 1.288-0.441 kg·m-3 (21% O2/79% N2 to 21% O2/79% He). Decreasing air density increases lift requirements, providing a way to test maximal aerobic performance during hovering flight. Helium has a six-fold higher thermal conductance than nitrogen, so interactively changing air temperature and He-N2 content provides a system for varying body temperature. Decreasing air density produced an increase in flight metabolic rate of up to 40% (Fig. 1), and the range of air temperatures and He-N2 content produced thorax temperatures that ranged from 29-44°C (Fig. 2). Plotting the maximal performance at each thorax temperature revealed a clear thermal performance curve, with an optimal temperature of 39°C, and maximal performance decreasing by about 5% with increasing or decreasing 1°C thorax temperature (Fig. 2). These findings help resolve past disagreements about how honey bees thermoregulate during hovering flight; some prior studies have found that flight metabolic rates were constant across air temperatures, while others have reported that decreased metabolic heat production at higher air temperatures contributes to thermoregulation. We found that the range of thorax temperatures differed in these prior studies, and our results suggest that honey bees use different mechanisms to thermoregulate during hovering depending on thorax temperature. Because maximal flight performance will determine capacities to carry loads and fly in challenging environments, the thermal performance curve during flight likely affects the fitness of honey bee colonies, and thus may be useful for developing predictions for effects of climate change on these important pollinators. Also, documentation of the strong effects of thorax temperature on flight performance provides a quantitative measure of the benefits of thermoregulation for this partially homeothermic periodic endotherm.

Thermal response of two sexually dimorphic Calopteryx (Odonata) over an ambient temperature range.

Organisms may internally or behaviorally regulate their body temperatures or conform to the ambient air temperatures. Previous evidence is mixed on whether wing pigmentation influences thermoregulation in various odonates.We investigated the thermal response of sympatric North American Calopteryx aequabilis and Calopteryx maculata with a thermal imaging study across a 25°C ambient temperature range.We found that regressions of thorax temperature on ambient temperature standardized by species had similar slopes for male and female C. maculata, but females were consistently 1.5°C warmer than males. In contrast, the sexes of C. aequabilis differed in slope, with C. aequabilis females having a slope less than 1.0 and males having a slope greater than 1.0.We found that regressions of thorax temperature on ambient temperature standardized by sex had similar slopes for males and females of both species, but C. maculata females were consistently 2.1°C warmer than C. aequabilis females.Given that C. aequabilis is strongly sexually dimorphic in pigment, but C. maculata is not, our findings suggest that wing pigmentation may influence thermal response rate in sympatric populations of both species.

Green-brown polymorphism in alpine grasshoppers affects body temperature.

Ectothermic animals depend on external heat sources for pursuing their daily activities. However, reaching sufficiently high temperature can be limiting at high altitudes, where nights are cold and seasons short. We focus on the role of a green‐brown color polymorphism in grasshoppers from alpine habitats. The green‐brown polymorphism is phylogenetically and spatially widespread among Orthopterans and the eco‐evolutionary processes that contribute to its maintenance have not yet been identified.We here test whether green and brown individuals heat up to different temperatures under field conditions. If they do, this would suggest that thermoregulatory capacity might contribute to the maintenance of the green‐brown polymorphism.We recorded thorax temperatures of individuals sampled and measured under field conditions. Overall, thorax temperatures ranged 1.7–42.1°C. Heat up during morning hours was particularly rapid, and temperatures stabilized between 31 and 36°C during the warm parts of the day. Female body temperatures were significantly higher than body temperatures of males by an average of 2.4°C. We also found that brown morphs were warmer by 1.5°C on average, a pattern that was particularly supported in the polymorphic club‐legged grasshopper Gomphocerus sibiricus and the meadow grasshopper Pseudochorthippus parallelus.The difference in body temperature between morphs might lead to fitness differences that can contribute to the maintenance of the color polymorphism in combination with other components, such as crypsis, that functionally trade‐off with the ability to heat up. The data may be of more general relevance to the maintenance of a high prevalence polymorphism in Orthopteran insects.

The energetics and thermoregulation of water collecting honeybees

Honeybees need water for different purposes, to maintain the osmotic homeostasis in adults as well as to dilute stored honey and prepare liquid food for the brood. Water is also used for cooling of the hive. Foraging in endothermic insects is energy-intensive and the question arises how much energy bees invest in a resource without any metabolically usable energy. We investigated the energy demand of water collecting bees under natural conditions. The thermoregulation and energetic effort was measured simultaneously in a broad range of experimental ambient temperatures (Ta = 12–40 °C). The thorax temperature as well as the energetic turnover showed a great variability. The mean Tthorax was ranging from ~ 35.7 °C at 12 °C to nearly 42.5 °C at 40 °C. The energy turnover calculated from CO2-release was highest at a Ta of 20 °C with about 60 mW and lowest at 40 °C with about 22 mW per bee. The total costs during collection decreased from 10.4 J at 12 °C to 0.5 J at 40 °C. The energetic effort of the water collectors was comparable with that of 0.5 M sucrose foraging bees. Our investigation strongly supports the hypothesis that the bees’ motivational status determines the energetic performance during foraging.

Variation of thorax flight temperature among twenty Australian butterflies (Lepidoptera: Papilionidae, Nymphalidae, Pieridae, Hesperiidae, Lycaenidae)

Thermal requirements for flight in butterflies is determined by a combination of external factors, behaviour and physical constraints. Thorax temperature of 152 butterflies was monitored with an infra-red thermometer in controlled laboratory conditions. The temperature at take-off varied from 13.4°C, for a female Heteronympha merope to 46.3°C, for a female Junonia villida. Heteronympha merope, an understorey species, had the lowest recorded take-off temperatures, with females flying at a much lower thorax temperatures than males. Among the tested butterfly species, warming-up rate was positively correlated with take-off temperature and negatively with body mass. Wing loading is a major variable in determining the thorax flight temperature. Butterflies with the highest wing-loadings experienced the highest thorax temperatures at take-off. A notable exception to this rule is Trapezites symmomus, the only Hesperiidae of our data set, which had thorax flight temperatures of 31.5°C and 34.5°C, well within the range of the observed butterflies, despite a wing load ca. five times higher. The high thorax temperature recorded in J. villida is probably linked to its high flight speed. The results highlight the importance of physical constraints such as body size on the thermal requirements for flight across a range of butterfly species.

Effects of a neonicotinoid pesticide on thermoregulation of African honey bees (Apis mellifera scutellata)

Thiamethoxam is a widely used neonicotinoid pesticide that, as agonist of the nicotinic acetylcholine receptors, has been shown to elicit a variety of sublethal effects in honey bees. However, information concerning neonicotinoid effects on honey bee thermoregulation is lacking. Thermoregulation is an essential ability for the honey bee that guarantees the success of foraging and many in-hive tasks, especially brood rearing. We tested the effects of acute exposure to thiamethoxam (0.2, 1, 2ng/bee) on the thorax temperatures of foragers exposed to low (22°C) and high (33°C) temperature environments. Thiamethoxam significantly altered honey bee thorax temperature at all doses tested; the effects elicited varied depending on the environmental temperature and pesticide dose to which individuals were exposed. When bees were exposed to the high temperature environment, the high dose of thiamethoxam increased their thorax temperature 1–2h after exposure. When bees were exposed to the low temperature, the higher doses of the neonicotinoid reduced bee thorax temperatures 60–90min after treatment. In both experiments, the neonicotinoid decreased the temperature of bees the day following the exposure. After a cold shock (5min at 4°C), the two higher doses elicited a decrease of the thorax temperature, while the lower dose caused an increase, compared to the control. These alterations in thermoregulation caused by thiamethoxam may affect bee foraging activity and a variety of in-hive tasks, likely leading to negative consequences at the colony level. Our results shed light on sublethal effect of pesticides which our bees have to deal with.

Thermal biology of Pacific cicada killers, Sphecius convallis Patton, in the Upper Sonoran Desert

A comprehensive investigation of the Pacific cicada killer, Sphecius convallis Patton, was undertaken to examine the behavioral and physiological mechanisms by which they are able to complete their life cycle in the thermal extremes of the Upper Sonoran Desert. S. convallis were endothermic, exhibiting elevated and relatively constant thorax temperatures during many activities. Males basked in trees at dawn to warm up, then used a variety of behaviors and perching strategies to maintain thorax temperature during territorial behavior. The thorax temperature of females was highest during provisioning and orientation flights, somewhat lower while investigating burrows, and lowest while digging burrows. The optimal thorax temperature for flight was about 40°C, which was approximated most closely by males resting in the shade during the afternoon. In mating clusters, the mated male was the hottest, the female was coolest and the other males were intermediate. Wasps lost about 5% of body mass during heating treatments, and may use evaporative water loss for cooling. Pacific cicada killers use a complex suite of behavioral and physiological adaptations to regulate body temperature during their nesting season.

The fungicide Pristine® inhibits mitochondrial function in vitro but not flight metabolic rates in honey bees

Honey bees and other pollinators are exposed to fungicides that act by inhibiting fungal mitochondria. Here we test whether a common fungicide (Pristine®) inhibits the function of mitochondria of honeybees, and whether consumption of ecologically-realistic concentrations can cause negative effects on the mitochondria of flight muscles, or the capability for flight, as judged by CO2 emission rates and thorax temperatures during flight. Direct exposure of mitochondria to Pristine® levels above 5ppm strongly inhibited mitochondrial oxidation rates in vitro. However, bees that consumed pollen containing Pristine® at ecologically-realistic concentrations (≈1ppm) had normal flight CO2 emission rates and thorax temperatures. Mitochondria isolated from the flight muscles of the Pristine®-consuming bees had higher state 3 oxygen consumption rates than control bees, suggesting that possibly Pristine®-consumption caused compensatory changes in mitochondria. It is likely that the lack of a strong functional effect of Pristine®-consumption on flight performance and the in vitro function of flight muscle mitochondria results from maintenance of Pristine® levels in the flight muscles at much lower levels than occur in the food, probably due to metabolism and detoxification. As Pristine® has been shown to negatively affect feeding rates and protein digestion of honey bees, it is plausible that Pristine® consumption negatively affects gut wall function (where mitochondria may be exposed to higher concentrations of Pristine®).

What do foraging wasps optimize in a variable environment, energy investment or body temperature?

Vespine wasps (Vespula sp.) are endowed with a pronounced ability of endothermic heat production. To show how they balance energetics and thermoregulation under variable environmental conditions, we measured the body temperature and respiration of sucrose foragers (1.5 M, unlimited flow) under variable ambient temperature (Ta = 20–35 °C) and solar radiation (20–570 W m−2). Results revealed a graduated balancing of metabolic efforts with thermoregulatory needs. The thoracic temperature in the shade depended on ambient temperature, increasing from ~37 to 39 °C. However, wasps used solar heat gain to regulate their thorax temperature at a rather high level at low Ta (mean Tthorax ~ 39 °C). Only at high Ta they used solar heat to reduce their metabolic rate remarkably. A high body temperature accelerated the suction speed and shortened foraging time. As the costs of foraging strongly depended on duration, the efficiency could be significantly increased with a high body temperature. Heat gain from solar radiation enabled the wasps to enhance foraging efficiency at high ambient temperature (Ta = 30 °C) by up to 63 %. The well-balanced change of economic strategies in response to environmental conditions minimized costs of foraging and optimized energetic efficiency.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-015-1033-4) contains supplementary material, which is available to authorized users.

Thermoregulation of individual paper wasps (Polistes dominula) plays an important role in nest defence and dominance battles

Paper wasps, like Polistes dominula, are considered as primitively eusocial. Hence, they are often used as model species for studies about the evolution of eusociality and dominance hierarchies. However, our knowledge about basic physiological processes in these wasps remains limited. In particular, the thermoregulation of individual wasps in their natural habitat has not yet been investigated in detail. We conducted a comprehensive field study to test their ability to respond to external hazards with elevated thorax temperatures. We presented artificial threats by applying smoke or carbon dioxide simulating fire and predator attacks, respectively, and monitored the thorax temperature of wasps on the nest using infrared thermography. We found that P. dominula workers recognized smoke and CO2 and reacted almost instantaneously and simultaneously with an increase of their thorax temperature. The maximal thorax temperature was reached about 65 s after the application of both stressors, but subsequently, the wasps showed a different behaviour pattern. No rise of the thorax temperature was detectable after an air blast was applied or in wasps resting on the nest. These observations provide evidence that P. dominula is able to heat up its thorax and that thermoregulation is employed in escape and defence reactions. Additionally, we investigated the thorax temperatures of queens during dominance battles. We found that the thorax temperature of the dominant queens rose up to 5 °C compared to that of subordinate queens that attacked the former, suggesting that the dominant queen defends herself as well as her nest.

Energetic optimisation of foraging honeybees: flexible change of strategies in response to environmental challenges.

Heterothermic insects like honeybees, foraging in a variable environment, face the challenge of keeping their body temperature high to enable immediate flight and to promote fast exploitation of resources. Because of their small size they have to cope with an enormous heat loss and, therefore, high costs of thermoregulation. This calls for energetic optimisation which may be achieved by different strategies. An ‘economizing’ strategy would be to reduce energetic investment whenever possible, for example by using external heat from the sun for thermoregulation. An ‘investment-guided’ strategy, by contrast, would be to invest additional heat production or external heat gain to optimize physiological parameters like body temperature which promise increased energetic returns. Here we show how honeybees balance these strategies in response to changes of their local microclimate. In a novel approach of simultaneous measurement of respiration and body temperature foragers displayed a flexible strategy of thermoregulatory and energetic management. While foraging in shade on an artificial flower they did not save energy with increasing ambient temperature as expected but acted according to an ‘investment-guided’ strategy, keeping the energy turnover at a high level (∼56–69 mW). This increased thorax temperature and speeded up foraging as ambient temperature increased. Solar heat was invested to increase thorax temperature at low ambient temperature (‘investment-guided’ strategy) but to save energy at high temperature (‘economizing’ strategy), leading to energy savings per stay of ∼18–76% in sunshine. This flexible economic strategy minimized costs of foraging, and optimized energetic efficiency in response to broad variation of environmental conditions.

Does size matter? – Thermoregulation of ‘heavyweight’ and ‘lightweight’ wasps (Vespa crabro and Vespula sp.)

SummaryIn insect groups with the ability of endothermy, the thermoregulatory capacity has a direct relation to body mass. To verify this relationship in vespine wasps, we compared the thermoregulation of hornets (Vespa crabro), the largest species of wasps in Central Europe, with two smaller wasps (Vespula vulgaris and Vespula germanica) in the entire range of ambient temperature (Ta: ∼0–40°C) where the insects exhibited foraging flights.Despite the great difference in body weight of Vespula (V. vulgaris: 84.1±19.0 mg, V. germanica: 74.1±9.6 mg) and Vespa (477.5±59.9 mg), they exhibited similarities in the dependence of thorax temperature on Ta on their arrival (mean Tth = 30–40°C) and departure (mean Tth = 33–40°C) at the nest entrance. However, the hornets' thorax temperature was up to 2.5°C higher upon arrival and up to 3°C lower at departure. The thorax temperature excess (Tth−Ta) above ambient air of about 5–18°C indicates a high endothermic capacity in both hornets and wasps. Heat gain from solar radiation elevated the temperature excess by up to 1°C. Results show that hornets and wasps are able to regulate their body temperature quite well, even during flight. A comparison of flight temperature with literature reports on other vespine wasps revealed a dependence of the Tth on the body mass in species weighing less than about 200 mg.

Fluctuations in oxygen influences facultative endothermy in bumblebees

Bumblebees are facultative endotherms, having the ability to elevate thorax temperature above ambient temperature by elevating metabolism. Here we investigated the influence of hypoxia on metabolic demands and thermoregulatory capabilities of the bumblebee Bombus terrestris. We measured thorax temperature, oxygen consumption, carbon dioxide production, and ventilation rates of bees exposed to oxygen levels of 21, 15, 10, and 5 kPa at 26°C. Under normoxia, bumblebees maintained an elevated thorax temperature of 35.5 °C. There was no significant change in thorax temperature at 15 kPa oxygen (33.4°C). Mean thorax temperature decreased significantly at 10 kPa (31.6 °C) and 5 kPa (27.3 °C). Bees were able to maintain an elevated metabolic rate at 15 kPa and 10 kPa. In normoxia, thermoregulating bees exhibited periods of rapid abdominal pumping (327 s−1) interspaced by periods of no abdominal pumping. At 10 kPa oxygen, abdominal pumping rate deceased (255 s−1) but became continuous. Upon exposure to 5 kPa, metabolic rate and abdominal pumping rate (152 s−1) dropped rapidly although the animals continued abdominal pumping at the reduced rate throughout the exposure period. Bumblebees are able to meet the energetic demands of endothermy at 15 kPa oxygen, but begin to become compromised at levels of 10 kPa oxygen.

Near-Optimal Foraging in the Pacific Cicada Killer Sphecius convallis Patton (Hymenoptera: Crabronidae)

This study evaluated foraging effectiveness of Pacific cicada killers (Sphecius convallis) by comparing observed prey loads to that predicted by an optimality model. Female S. convallis preyed exclusively on the cicada Tibicen parallelus, resulting in a mean loaded flight muscle ratio (FMR) of 0.187 (N = 46). This value lies just above the marginal level, and only seven wasps (15%) were below 0.179. The low standard error (0.002) suggests that S. convallis is the most ideal flying predator so far examined in this respect. Preying on a single species may have allowed stabilizing selection to adjust the morphology of females to a nearly ideal size. That the loaded FMR is slightly above the marginal level may provide a small safety factor for wasps that do not have optimal thorax temperatures or that have to contend with attempted prey theft. Operational FMR was directly related to wasp body mass. Smaller wasps were overloaded in spite of provisioning with smaller cicadas, while larger wasps were underloaded despite provisioning with larger cicadas. Small wasps may have abandoned larger cicadas because of difficulty with carriage.

Thermoregulation of foraging honeybees on flowering plants: seasonal variability and influence of radiative heat gain

1. During nectar and pollen foraging in a temperate climate, honeybees are exposed to a broad range of ambient temperatures, challenging their thermoregulatory ability. The body temperature that the bees exhibit results from endothermic heat production, exogenous heat gain from solar radiation, and heat loss. In addition to profitability of foraging, season was suggested to have a considerable influence on thermoregulation. To assess the relative importance of these factors, the thermoregulatory behaviour of foragers on 33 flowering plants in dependence on season and environmental factors was investigated.2. The bees (Apis mellifera carnica Pollman) were always endothermic. On average, the thorax surface temperature (Tth) was regulated at a high and rather constant level over a broad range of ambient temperatures (Tth = 33.7–35.7°C, Ta = 10–27°C). However, at a certain Ta, Tth showed a strong variation, depending on the plants from which the bees were foraging. At warmer conditions (Ta = 27–32°C) the Tth increased nearly linearly with Ta to a maximal average level of 42.6 °C. The thorax temperature excess decreased strongly with increasing Ta (Tth−Ta = 21.6 − 3.6°C).3. The bees used the heat gain from solar radiation to elevate the temperature excess of thorax, head, and abdomen. Seasonal dependance was reflected in a 2.7 °C higher mean Tth in the spring than in the summer. An anova revealed that season had the greatest effect on Tth, followed by Ta and radiation.4. It was presumed the foragers' motivational status to be the main factor responsible for the variation of Tth between seasons and different plants.