In 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.
Flying insects that can elevate their body temperature can forage with some independence of ambient air temperature and in this way extend their activity range (Heinrich, 1993). The vespine wasps are capable of endothermic heat production by means of their thoracic muscles (Heinrich, 1984; Stabentheiner and Schmaranzer, 1987). This improves their flight performance and allows a better exploitation of food resources (Kovac and Stabentheiner, 1999; Kovac et al., 2009). In Central Europe, social wasps have a long breading season from the spring till the autumn, where they have to cope with a vast variation of thermal conditions challenging their thermoregulatory capability.
In members of the family Vespidae a great variability of the body mass occurs. The hornet (Vespa crabro) is the largest species of vespine wasps in Central Europe. They may weigh up to 580 mg. Species of the genus Vespula are much smaller, weighing about 40–130 mg. Flying insects with small body size or mass can operate with relatively low thoracic temperatures, large insects must generally have higher thoracic temperatures (Heinrich, 1974; Heinrich, 1993). An obvious advantage of the greater body mass and the higher temperature is the ability to hunt larger prey and to carry heavier loads of food. The thermoregulatory capacity of insects is often a function of their body mass (Bartholomew, 1981; Heinrich and Heinrich, 1983; Heinrich, 1993). A positive relationship between thermoregulatory capability and body size was observed in moths (Bartholomew and Heinrich, 1973), beetles (Bartholomew and Heinrich, 1978), 18 species of Alaskan bees (Bishop and Armbruster, 1999) and Alaskan dragonflies (Sformo and Doak, 2006), in wasps (Heinrich, 1984; Coelho and Ross, 1996) and in solitary bees (Stone and Willmer, 1989; Stone, 1993a; Stone, 1993b). In general, small bees initiate flight at lower thoracic temperatures than larger ones (Stone, 1993a; Stone, 1993b). However, in an interspecific comparison of endothermy in honeybees (Apis), Dyer and Seeley observed deviations from the expected size-related patterns (Dyer and Seeley, 1987).
The knowledge concerning the thermal biology of the European hornet is very sparse. Only a few measurements of body temperature of hornets foraging honey at an artificial feeding place exist (Stabentheiner and Schmaranzer, 1987). The aim of this study was to investigate the thermoregulatory capacity and strategies of this big flying insect in detail. We assumed that hornets have advantages in thermoregulation due to their large body mass. To verify this hypothesis we compared the body temperature of hornets (Vespa crabro) with that of smaller native vespine wasps (Vespula vulgaris, Vespula germanica) living in the same habitat within the same climatic environment. The temperature of leaving and arriving insects was chosen for the assessment of the thermoregulatory performance. The comparison with similar investigations of wasps of different size (Heinrich, 1984; Coelho and Ross, 1996) promised general conclusions.
In order to assess the thermoregulatory performance it is of great advantage to investigate the insects' thermoregulation in their natural environment where they are exposed to the variation of several environmental factors like ambient air temperature, solar radiation and convection, influencing their body temperature. If the greater body mass delivers advantages in thermoregulation this has to be especially expected at low ambient temperatures. Therefore, our experiments covered the entire range of environmental conditions the wasps are likely to be exposed to in their season of activity. Infrared thermography allowed behavioural observations in addition to the non-invasive, undisturbed measurement of body temperature.
Materials and Methods
Animals, field site and measuring conditions
Measurements were conducted on four hornet colonies (Vespa crabro, Vespidae, Hymenoptera) and six wasp colonies, five Vespula vulgaris and one Vespula germanica (Vespidae, Hymenoptera). The hornet nests were located in tree hollows or abandoned bird nest boxes. The wasps nested in wall hollows or lofts (V. vulgaris) and in a ground nest (V. germanica). Data were collected on 16 days from July till October in the years 2009 and 2010, and on 2 days in October and November 1989. The entire range of ambient temperature (Ta: ∼0–40°C) where the insects exhibited foraging flights was investigated in V. crabro and V. vulgaris. The insects' body surface temperature was investigated during take-off, landing, and other activities at the nest entrance. To ensure similar environmental conditions for the comparison of hornet (V. crabro) and wasp (V. vulgaris) thermoregulation only measurements conducted in shade were taken into consideration in this case. The influence of solar radiation on body temperature of leaving and arriving insects was investigated in another set of experiments comparing hornets with wasps (V. germanica) when the nest entrance was in sunshine. Foraging wasps and hornets on fruits (pear) or lilac were investigated in an orchard. The hornets foraging on lilac bitted open the bark of the lilac branches and sucked the sap. Hornets and wasps foraging on pears sucked the sap of the fruits or gnawed off the fruit pulp. The insects were observed during their whole foraging stay. Infrared recordings were started immediately after landing and ran until the insects' take-off. Foraging stays lasted from a few seconds (∼ 5 s) to maximal 15 minutes. After departure of the observed insect the next arriving forager was chosen for measurement. Hornets preying on honeybees were also observed in the orchard. Patrolling hornets were followed until they were in the focus of the infrared camera. The fight between hornets was observed and measured on the ground in the vicinity of a nest.
The insects were filmed with an infrared camera (AGA 782 SW, ThermaCam SC2000 NTS or i60; FLIR, Stockholm, Sweden). The infrared cameras were calibrated periodically by slotting in a precision-calibrated AGA1010 reference source (FLIR) or a self-constructed peltier-driven reference source of known temperature and emissivity (Stabentheiner and Schmaranzer, 1987; Schmaranzer and Stabentheiner, 1988; Stabentheiner et al., 2012). Thermographic data were stored on a VHS videorecorder at 25 frames s−1 with the AGA 782 SW, digitally with 14 bit resolution on a portable computer (DOLCH Flexpac-400-XG, Munich, Germany) at a rate of 3–5 frames s−1 with the SC2000, or digitally (14 bit) on the internal memory card of the i60. The ambient air temperature (Ta) was measured with thermocouples near the insects (∼ 3 cm). Thermocouple measurements were corrected for errors caused by solar radiation according to Stabentheiner et al. (Stabentheiner et al., 2012). The solar radiation was measured with a miniature global radiation sensor (FLA613-GS mini spezial, AHLBORN, Holzkirchen, Germany). The temperature and radiation data were stored every second with ALMEMO data loggers (AHLBORN, Holzkirchen, Germany).
For determination of the body weight, from each colony insects leaving the nest were captured and weighed with a balance to the nearest 0.1 mg (AB104, METTLER-TOLEDO, Greifensee, Switzerland).
Data evaluation and statistics
The surface temperature of head (Thd), thorax (Tth) and abdomen (Tab) was calculated from the infrared thermograms (Fig. 1) by means of proprietary software in the AGA 782 SW, or by the AGEMA Research software (FLIR, Stockholm, Sweden) controlled by a proprietary Excel VBA-macro (Microsoft Corporation, Santa Rosa, California) in the other two cameras. Values were taken from pictures immediately before take-off, after landing or during flight (if the insects were in focus) at the nest entrance. From foraging insects values were evaluated immediately after landing and subsequently in intervals of about five seconds. The surface temperature of the three body parts was calculated with an infrared emissivity of 0.97, determined for the honeybee cuticle (Stabentheiner and Schmaranzer, 1987; Schmaranzer and Stabentheiner, 1988; Stabentheiner et al., 2012). Because the SC2000 and i60 infrared cameras work in the long-wave infrared range (7.5–13 µm) the reflected solar radiation from the wasps' cuticle produced only a small measurement error (0.218°C for 1000 Wm−2), which was compensated for (Stabentheiner et al., 2012). In this way we reached an accuracy of 0.7°C for the insect body surface temperature at a sensitivity of 0.1°C.
Infrared thermograms of a hornet (left) and a wasp (right) at the nest entrance.
The temperature gradient between the thorax and the ambient air (thorax temperature excess = Tthorax − Ta) was used as a measure to assess the insects' endothermic capability. To evaluate the influence of the radiative heat gain on the body temperature three classes of solar radiation were established, shade: <200 Wm−2, overcast sky: 200–500 Wm−2, and sunshine: >500 Wm−2. For comparing our results of returning insects with results of Heinrich (Heinrich, 1984) and Coelho and Ross (Coelho and Ross, 1996), means or single values of their data were digitized and appropriate functions were fitted.
Activity and behaviour of the hornets at the nest entrance were classified from the infrared video sequences. Means reported for leaving, arriving and foraging insects are average values derived from the fitted regression lines, means of the different hornet activities are calculated means ± SD. The relationship between body temperature, temperature excess and ambient temperature (Ta) was described by linear, exponential or polynomial regression functions and tested with ANOVA if possible (comparing linear regressions). Data analysis and statistics were performed by using the Statgraphics package (Statistical Graphics Corporation, Warrenton, Virginia) and ORIGIN software (OriginLab Corporation, Northampton, Massachusetts).
The difference in the body mass of the three investigated Vespidae was significant (t-test, P<0.05). Vespula vulgaris weighed on average 84.1±19.0 mg (n = 147), Vespula germanica weighed 74.1±9.6 mg (n = 23) and the hornets had a mean weight of 477.5±59.9 mg (n = 50). The hornets weighed the 5.7 fold of V. vulgaris and the 6.4 fold of V. germanica.
Departure and arrival measured in shade
Surface temperature of thorax, head and abdomen of hornets and wasps in dependence on ambient temperature (Ta).
The hornets' thorax surface temperature (Tth; derived from the regression line) was 38.0°C at a low Ta of 2°C. With increasing Ta it first declined to 33.3°C at Ta = 15°C, and increased again to 36.7°C at Ta = 27°C. By contrast, the wasps had a more constant Tth during departure. When leaving the nest at a low Ta of 2°C we measured a Tth of 39.4°C, which decreased to 36.4°C at Ta = 15°C, and increased again to 40.4°C at a high Ta of 37°C. The temperature of the head exhibited a stronger dependence on Ta. In the hornets it was 23.6°C at Ta = 2°C and increased to 33.5°C at a high Ta of 27°C, and in the wasps it increased from 26.2°C at Ta = 2°C to 37.7°C at Ta = 37°C. The abdomen was the coolest body part. Its temperature depended strongly on Ta. In the hornets Tab increased from 15.5°C to 31.9°C, and in the wasps from 22.8°C to 35.8°C in the investigated range of Ta (Fig. 2A; Table 2).
This equation fitted the data better than Eqn 1. The hornets' average Tth after arrival was ∼0 to 2.5°C higher than that of V. vulgaris (Fig. 2). Their Tth decreased from ∼34°C to ∼31°C as Ta increased from 1°C to 10°C, and increased continuously at higher Ta. It reached 37.5°C when it was warm (Ta = 27°C). The Tth of V. vulgaris upon arrival decreased from ∼32 to 30°C in the lowest range of Ta (1–10°C) and, like in the hornets, increased continuously at higher Ta. The Tth reached 40.7°C when Ta was 37°C. V. germanica regulated the Tth at a similar level as the hornets (Tth = 34.5–40.1°C; Ta = 20–30°C) (Fig. 2C). The hornets' Thd was ∼22.8°C at low Ta (1–10°C) and increased continuously to 33.0°C at high Ta (27°C). The Thd of V. vulgaris was 22.2°C at low Ta (1–10°C) and increased to 38.6°C at very high Ta (37°C). The Tab of both hornets and wasps depended strongly on ambient air temperature. It was ∼2 to 5°C higher than Ta. Only at very low Ta the abdominal temperature elevation reached a maximum of 10°C above the ambient air in both species. At high Ta (27°C) the hornets' abdomen was only slightly elevated above Ta and the wasps had an abdominal temperature ∼2°C lower than the ambient air (Fig. 2B; Table 2). The Thd of the hornets resembled that of the wasps. The Tab of the hornets, however, was similar to or warmer than that of the wasps (difference: ∼0–3°C) (Fig. 2C).
The Tth of hornets in flight was ∼1 to 2°C lower than immediately after landing at a Ta of 16 to 27°C (Fig. 2B). Linear regression lines of flying and landing hornets differed significantly in this range of ambient temperature (ANOVA, P<0.0001, DF = 3, F-Ratio = 85.27, n = 447).
Thorax temperature excess and solar radiation
The hornets and wasps were always endothermic at departure and arrival, i.e. their thorax was clearly elevated above the ambient air. The thorax temperature excess depended strongly on Ta (Tth−Ta ∼ 3–38°C) (Fig. 2, Fig. 3; Table 3). To quantify the influence of solar radiation on the body temperature, measurements were conducted also in sunshine. For hornets and V. germanica the values of the thorax temperature excess were partitioned according to our classification of solar radiation (Fig. 3; Table 3) (shade: <200 Wm−2, sunshine: >500 Wm−2).
Thorax temperature excess (Tth−Ta) of hornets (Vespa crabro) and wasps (Vespula germanica) in dependence on ambient temperature (Ta) during departure (left) and arrival (right) at the nest entrance.
The hornets leaving the nest in sunshine had a ∼0.2–0.4°C higher temperature excess than in the shade. An ANOVA comparing regressions of Tth−Ta in dependence on Ta revealed significant differences (P<0.0001, DF = 3, F-Ratio = 469.38, n = 788). The difference in Tth−Ta at arrival was ∼1°C. An ANOVA comparing regressions also revealed significant differences (P<0.0001, DF = 3, F-Ratio = 364.88, n = 765).
The wasps leaving the nests in the sun had a similar temperature excess than in the shade (Tth−Ta ∼ 0–0.3°C). However, the regression lines differed significantly (ANOVA, P<0.0001, DF = 3, F-Ratio = 224.82, n = 246). In arriving wasps the difference in temperature excess between sunshine and shade was smaller than in the hornets (Tth−Ta ∼ 0–0.7°C; ANOVA: P<0.0001, DF = 3, F-Ratio = 209.58, n = 247).
Thorax temperature and body mass
The thorax temperature of arriving insects of each colony was plotted in dependence on ambient temperature. From the calculated regressions functions the mean Tth was determined for three Tas (10, 20, 30°C) and afterwards related with the colonies' mean body mass. An ANOVA revealed a dependence on Ta (P<0.05, DF = 1, F-Ratio = 74.26, n = 13), but no dependence on colony or body mass. For a further comparison with data from the literature our values were pooled for each species (because values for different colonies were not available in these papers) and analysed in the same way. Fig. 4 shows the relation of thorax temperature and body mass, comparing our results with results of Heinrich, and Coelho and Ross (Heinrich, 1984; Coelho and Ross, 1996). The Tth was fitted with exponential functions in dependence on body mass for three Tas (10, 20, 30°C).
Thorax temperature in dependence on body mass of hornets (Vespa crabro) and wasps (Vespula vulgaris and Vespula germanica) and other vespine wasps of Heinrich (**), and Coelho and Ross (*) (Heinrich, 1984; Coelho and Ross, 1996).
A dependence of the Tth on body mass could be detected, but it was small and more distinct at lower temperatures (ANOVA, P<0.01, DF = 6, F-Ratio = 6.18, n = 20). However, the heaviest species (V. crabro, this paper), weighing 477.5 mg on average, had a somewhat lower Tth than Dolichovespula maculata with a medium body mass of 185.5 mg (Heinrich, 1984). The Tth values of the other wasps were mostly below that of D. maculata and V. crabro. For statistical details see Table 4.
Foraging and other activities
Linear regression lines were fitted for the temperature values of thorax, head and abdomen of foraging hornets on lilac. The results revealed differences in the dependence of body part temperatures on ambient temperature as expected (Fig. 5A; Table 5). The Tth was regulated nearly independent of Ta (Ta = 15°C: Tth = 33.0°C; Ta = 30°C: Tth = 34.7°C). The head was cooler and exhibited a stronger dependence on Ta (Ta = 15°C: Thd = 27.6°C; Ta = 30°C: Thd = 33.6°C). The abdomen was the coolest body part. It was ∼3 to 5°C warmer than Ta. The temperature increased nearly parallel to Ta (Ta = 15°C: Tab = 21.2°C; Ta = 30°C: Tab = 32.9°C).
Temperature of thorax, head and abdomen of hornets (Vespa crabro) and wasps (Vespula sp.) foraging from natural sources in dependence on ambient temperature (Ta).
By contrast, foraging hornets on pears exhibited a strong dependence of all body parts on Ta (Fig. 5B, regression lines; Table 5). The thorax was the warmest body part (Ta = 15°C: Tth = 27.5°C; Ta = 30°C: Tth = 38.0°C), followed by the head (Ta = 15°C: Thd = 24.8°C; Ta = 30°C: Thd = 35.5°C), and the abdomen (Ta = 15°C: Tab = 24.9°C; Ta = 30°C: Tab = 33.5°C). However, wasps foraging on pears showed no dependence of Tth on Ta (Ta = 13°C: Tth = 31.5°C; Ta = 25°C: Tth = 31.6°C). A weak dependence on Ta was measured in Thd (Ta = 13°C: Thd = 26.6°C; Ta = 25°C: Thd = 28.5°C), and a strong dependence on Ta in Tab (Ta = 13°C: Tab = 19.9°C; Ta = 25°C: Tab = 26.4°C).
The hornets' mean body temperatures for the observed activities at the nest entrance, in the vicinity of the nest and during foraging and hunting are summarized in Table 1. The highest Tths were measured in fighting (37.2°C) and attacking (36.8°C) hornets. The Tth during hunting, flight and arrival after flight was somewhat lower (35.8, 35.7 and 35.3°C, respectively) and they were lowest during nest-building activities at the entrance (29.6°C). The mean Tth during foraging was in the intermediate range (compare with Fig. 5). Guards examining other hornets at the nest entrance could be observed in three individuals. In one case the examined hornet exhibited a heating bout similar to a typical thermal behaviour observed in honeybees involved in such guard – examinee interactions (Stabentheiner et al., 2002). After some seconds of inspection the examined hornet started to heat up the thorax very strongly, without making any attempts to escape. The thorax temperature at the beginning of inspection was 26°C. After about 80 seconds the guard stopped the inspection and the Tth of the examined hornet reached the maximum of 31°C. When the guard left the inspected hornet the Tth decreased strongly to 28°C. In honeybees such heating bouts are presumed to improve the recognition of examined individuals (Stabentheiner et al., 2002; Stabentheiner et al., 2007).
Our results demonstrate that hornets (Vespa crabro) and wasps (Vespula sp.) are capable of pronounced and similar endothermy and thermoregulation despite the large difference in body mass. Both maintained their thorax surface temperature (Tth) not only elevated but also relatively constant in a vast range of Ta (Figs 2, 3, 5). A high thermal performance (large temperature excess, i.e. gradient between thorax and ambient air) was exhibited especially at low ambient temperatures. The decline of the thoracic temperature excess with increasing Ta in the landing insects revealed that both hornets and wasps were able to regulate their body temperature even during flight. Measuring the insects immediately after landing provided values comparable with the flight temperature (Fig. 2B). The somewhat lower Tth of hornets during flight than after landing was possibly due to a higher convective cooling of the thorax surface in flight. The hornets' mean thorax temperature after arrival was maximal ∼2.5°C higher than that of V. vulgaris and similar to that of V. germanica (Fig. 2C). In contrast to the arrival, V. vulgaris had a similar or a somewhat higher Tth than the hornets during departure from the nest (Fig. 2C). Perhaps they need a higher Tth for take-off to compensate for a higher heat loss in the initial phase of flight due to their more unfavourable relation of body surface area to mass.
However, during foraging the results were not so consistent. In wasps foraging on pears, the Tth (mean ∼31.5°C) was regulated constantly high and nearly independent from Ta (Fig. 5B). By contrast, the hornets' Tth (mean ∼28 to 38°C) showed a strong increase with Ta. On the other hand, the Tth of hornets foraging on lilac (mean ∼33 to 35°C) was regulated at a relatively high level and rather independent from Ta (Fig. 5A). These results confirm that the body temperature in wasp species depends not only on physiological requirements but also on other parameters like motivation, type of activity and behavioural context (Kovac and Stabentheiner, 1999; Eckles et al., 2008; Kovac et al., 2009). The dependence of thermoregulation on the behavioural context is pronounced by the great differences in the hornets' body temperature observed during different activities at the nest entrance (mean Tth 29.6 to 37.2°C) (Table 1).
The investigated hornets had more than fivefold the body mass (Mb) of the wasps. However, this great difference in body mass was only partly reflected in the measured thorax temperatures. In Fig. 2C we compare our results of arriving hornets and wasps with similar measurements of other authors (Heinrich, 1984; Coelho and Ross, 1996). The investigated species cover nearly the entire range of body mass occurring in vespine wasps (mean Mb ∼ 53–477 mg). Comparing our Tths of arriving hornets and wasps with measurements of Heinrich revealed an astonishing coincidence (Heinrich, 1984). The Tths of our hornets (Mb = 477.5 mg) and of D. maculata (Mb = 185.5 mg) from Heinrich were quite similar and the values of V. vulgaris from Heinrich were nearly the same as the Tths of our V. vulgaris (Heinrich, 1984). However, the V. vulgaris (Mb = 57.2 mg) from Heinrich had a lower body mass than our V. vulgaris (Mb = 84.1 mg). By contrast, our V. germanica (Mb = 74.1 mg) exhibited a similar thorax temperature as the hornets. However, the thorax temperatures of V. germanica (Mb = 78.7 mg) and V. maculifrons (Mb = 53.4 mg), measured by Coelho and Ross, were lower than in the other wasps (Coelho and Ross, 1996), although the body mass of V. germanica was similar to our wasps' mass (Fig. 2C). These results demonstrate that investigations on the same or related species in varying geographical and climatic areas (Europe, USA) may reveal different results. These differences in the flight body temperature may be caused by local adaptation of strains of this species as well as by differences in foraging motivation.
To show the relationship between thorax temperature and body mass of all the aforementioned species, the temperature was plotted against the mass for three different Tas. The result revealed a significantly dependence of the Tth on the body mass (Fig. 4) (ANOVA: P<0.01). However, Tth at a given mass varied considerably. A similar relationship between metathoracic temperature and body mass of flying dung beetles was obtained by Bartholomew and Heinrich (Bartholomew and Heinrich, 1978). Beetles did not show appreciable endothermy in continuous flight until they reached a body mass of ∼100 mg. Methathoracic temperature of beetles with a mass between ∼100 and 250 mg was strongly correlated with body mass. The critical mass for obvious endothermy in beetles seems to be about 50 mg. A further comparison of the Tth with body mass of 12 species of moths from Costa Rica (Bartholomew and Heinrich, 1973) (range Mb ∼70 to 1200 mg) also showed a strong correlation of Tth with mass.
There are some studies investigating the relationship between body mass and thermoregulatory performance in bees (Apoidea). They show that large bees can generally regulate Tth better than small ones. Stone and Willmer reported a positive correlation between body mass (range Mb ∼10 to 1300 mg) and thoracic temperature in flight in a comparison of 55 species of bees at Ta = 22°C (Stone and Willmer, 1989). A similar result was obtained by Stone, who investigated endothermy in the solitary bee Anthophora plumipes (Stone, 1993a). Thoracic temperatures measured during free flight in the field correlated positively with the bees' body mass (range Mb ∼120 to 220 mg). In another study of Stone on thermoregulation of tropical solitary bees, he could show that in Coelioxys frontalis and Amegilla sapiens thoracic temperatures correlated positively with both ambient temperature and body mass (Stone, 1993b). A similar result was obtained by Bishop and Armbruster with regression analysis of species and family means of 18 Alaskan bees (Bishop and Armbruster, 1999). Thermoregulatory capability, and minimum thoracic temperature necessary for initiating flight, increased with body size. Bees having a dry mass smaller than 15 mg (∼46 mg fresh mass) showed no appreciable ability to regulate their thorax temperature. This is probably due to the extreme increase of cooling constants at a mass below 50 mg fresh weight (Bishop and Armbruster, 1999). Heinrich and Heinrich reported a similar relationship in bumblebees (Heinrich and Heinrich, 1983). In large queens foraging Tth did not decrease much in the mass range of 750 to 300 mg. Only the smaller workers, ranging in mass from about 150 to 90 mg, showed about 1 to 3°C lower thorax temperatures. This coincided with a steep increase of cooling constants at a fresh mass below 200 mg.
These results of bees, beetles and moths are partly in agreement with our results of wasps. However, from our analysis of own and literature data (Heinrich, 1984; Coelho and Ross, 1996) we obtained no simple linear correlation between Tth and body mass (Fig. 4) as shown in the solitary tropical bee Anthophora by Stone (Stone, 1993a). Even a double logarithmic plot does not show a linear relationship as shown in a comparison of 55 species of bees by Stone and Willmer (Stone and Willmer, 1989). Extrapolation of curves of Fig. 4 to body masses below 50 mg suggests a critical mass of about 40 to 50 mg for a pronounced endothermic performance to be also valid for wasps. However, all vespine wasps investigated so far were heavier than ∼50 mg. The V. maculifrons (Coelho and Ross, 1996) with a mean weight of ∼53 mg has the lowest thorax temperature especially at low ambient temperatures (Fig. 4, 10°C and 20°C). With increasing mass the Tth increases to a plateau at Mb ∼180 mg. Above this value occurs no appreciable increase in Tth between the intermediate species (D. maculata with Mb = 185.5 mg) (Heinrich, 1984) and the largest species (V. crabro with Mb = 477.5 mg).
In contrast to these findings there are investigations with results deviating from the expected size-related patterns. Kovac et al. reported in water foraging wasps (Vespinae and Polistinae) a great difference in their thermoregulatory behaviour (Kovac et al., 2009). At moderate Ta (22 to 28°C) Vespula exhibited distinctly higher thoracic temperatures (mean Tth 35.5–37.5°C) than Polistes (mean Tth 28.5–35.5°C). Polistinae showed only a weak endothermic activity, despite their larger size and body mass. In honeybees Heinrich found two races of Apis mellifera, A. m. adansonii and A. m. mellifera, to have the same average thorax temperature excess, even though A. m. mellifera is about 30% larger in mass than A. m. adansonii (Heinrich, 1979). Dyer and Seeley made an interspecific comparison of endothermy in honeybees (Apis) arriving at the nest (Dyer and Seeley, 1987). The smallest species, A. florea, showed the lowest thorax temperature excess above ambient air, but the intermediate-sized A. cerana and A. mellifera both showed a higher excess temperature than the largest species, A. dorsata. They found that the rate of passive convective heat loss from the thorax scales linearly and inversely with body size in the four species and there was no anatomical evidence for differences in efficiency with which heat flow from the thorax to the abdomen may be restricted. Dyer and Seeley reported that wing-loading was disproportionately high in A. cerana and A. mellifera relative to A. dorsata and A. florea (Dyer and Seeley, 1987). A higher Tth in flight may be necessary to improve muscular efficiency (Coelho, 1991), and this way compensate for the higher wing loading. Such parameters could also be responsible for the small differences in flight Tth of the vespine wasps.
Concluding we can say that thermoregulation in vespine wasps depends on body mass, but not in a simple linear relation. The great variability in the Tth, especially in the smaller sized wasps, confirms a statement of Dyer and Seeley: “… general scaling relationships based on body mass alone may fail to predict qualitative physiological differences even within a closely related group of species” (Dyer and Seeley, 1987).
The research was funded by the Austrian Science Fund (FWF): P20802-B16, P7371-BIO. We greatly appreciate the help with electronics and software by G. Stabentheiner and S. K. Hetz, with data evaluation by P. Kirchberger, A. Lienhard, L. Mirwald and for technical support by H. Käfer.
The authors have no competing interests to declare.