DOI: 10.5061/dryad.3xsj3txjw
title: A desert bee thermoregulates with an abdominal convector during flight

description: Flying endothermic insects thermoregulate, likely to improve flight
 performance. Males of the Sonoran Desert bee, Centris caesalpiniae
 Cockerell, seek females at aggregations beginning at sunrise and cease
 flight near midday when air temperature peaks. To identify the
 thermoregulatory mechanisms for C. caesalpiniae males, we measured tagma
 temperatures, wingbeat frequency, water loss rates, metabolic rates, and
 tagma masses of flying bees across shaded air temperatures of 19 to 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 temperatures were high (38 to
 45°C) and were moderately well-regulated, while head and abdomen
 temperatures were cooler and less controlled. Abdomen temperatures
 converged on thorax temperatures as air temperature rose, indicating
 active heat transfer from the pubescent thorax to the relatively bare
 abdomen with warming. The mass-specific metabolic rate increased with
 time, air, and thorax temperatures, but wingbeat frequency did not vary.
 Mass-specific water loss rate increased with air temperature. Using a heat
 budget model, we showed that whole-body convective conductance increased
 through the morning, and 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. 
description: <strong>Carbon dioxide and water vapor emission
 measurements</strong>: To determine the metabolic and water loss
 rates of <em>C. caesalpiniae</em> large morph males during
 flight, we used flow-through respirometry. We stationed the setup
 underneath an outdoor, shaded porch less than 100 m from the locations
 where bees were captured to measure bees in conditions as close as
 possible to that of their natural, ambient conditions. Shaded air
 temperatures ranged from 19 to 38°C across and within four days of
 measurements. We calculated average
 CO<sub>2</sub> and H<sub>2</sub>O levels for 2-3
 minute periods when bees were observed to be steadily hovering. We
 recorded flight behaviors for each bee, but all bees flew well and
 consistently, and we found no relationship between our flight behavior
 scores and flight metabolic rates, so these behavioral data are not
 reported. To measure shaded air temperature, we used a
 BAT-12 thermometer and thermocouple. To assess that the metabolic chamber
 was perfectly air-tight, we measured CO<sub>2</sub> and
 H<sub>2</sub>O levels over three to four minutes without an
 animal in the chamber; under these conditions, there were no significant
 changes in CO<sub>2</sub> concentration. The 95% washout of
 CO<sub>2</sub> from the metabolic chamber occurred in
 approximately 90 seconds. <strong>Bee tagma
 temperatures</strong>: Immediately following the respirometry
 measures, we transferred the bee into a plastic bag, which we flattened
 onto a Styrofoam board to reduce conduction and restrict the bee’s
 movement. We then measured the head, thorax, and abdomen temperatures
 (T<sub>h</sub>, T<sub>th</sub>,
 T<sub>ab</sub>, respectively) in random order within five
 seconds after cessation of flight by inserting a hypodermic thermocouple
 (Physitemp, MT-29/5HT Needle Microprobe, time constant = 0.025·s) into the
 center of each tagma. We recorded the tagma temperature data with a Pico
 Technology USB TC-08 Thermocouple Data Logger (Tyler, TX, USA). We
 recorded the shaded air temperature values following the temperature
 measurements for each individual. We calculated the temperature excess
 ratio (R<sub>tagma</sub>) using Equation 1 (Baird,
 1986). <strong>Wingbeat frequency and flight
 score</strong>: We recorded the sound of wing movements during
 hovering flight in the flight chamber prior to each respirometry
 measurement for 20 to 30 seconds using the iPhone 7+ microphone. After the
 wingbeat frequency measurement, we closed the chamber to flush
 CO<sub>2</sub> and H<sub>2</sub>O before the
 respirometry measurement. Using a sound editing program, Audacity version
 2.4.2 for Windows, we visualized the wingbeats. We calculated average
 wingbeat frequency by dividing the number of wing beats by the time
 duration for three separate measures of 10 wingbeats.
 <strong>Total body surface area
 calculations</strong>: We used a digital caliper (accurate to 0.01
 mm) to approximate body surface area using geometrical calculations. We
 assumed that the head of the bee is a cylinder, measuring head width as
 the diameter and head thickness as the height. We assumed that the thorax
 is a sphere measuring thorax width as the diameter. We assumed that the
 abdomen is a cylinder and a cone, with the 1st through 3rd terga of the
 abdomen being the cylinder and the 4th and 5th tergi being a cone (Roberts
 and Harrison, 1999). <strong>Heat budget model
 calculations</strong>: We assumed that bees were flying at thermal
 equilibrium between 19°C and 38°C in steady-state conditions. This
 assumption is supported by observations for honey bees that body
 temperatures are stable during 1-5 min of flight (Roberts and Harrison
 1999), the prolonged steady hovering exhibited by most of our bees, and
 the steady CO<sub>2</sub> emission traces we observed.
 Q<sub>metabolic</sub> and Q<sub>evaporation</sub>
 were calculated from VCO<sub>2</sub> and
 VH<sub>2</sub>O. Bees have mostly been reported to utilize
 carbohydrates as fuel for flight (Bertsch, 1984; Gäde and Auerswald, 1999;
 Suarez et al., 2005). Therefore, we assumed a respiratory quotient of 1
 and 21.4 J*mL<sup>-1</sup> CO<sub>2</sub> to
 calculate metabolic heat production in Watts. We then multiplied by 0.96
 (the fraction of power input liberated as heat during flight) (Ellington,
 1984; Harrison et al., 1996; Roberts and Harrison, 1999). To calculate
 evaporative heat loss in Watts, we multiplied VH<sub>2</sub>O
 by the latent heat of evaporation of water, 2.45
 J*mg<sup>-1</sup> H<sub>2</sub>O.
 As we performed respirometry measurements in shade, we assumed
 shortwave radiation to be negligible. We summed the longwave (infrared)
 net radiation (R<sub>loss</sub> –
 R<sub>gain</sub>) for the head, thorax, and abdomen of each
 bee using the Stefan-Boltzmann equation. We assumed
 that the bee’s emissivity, ε<sub>s</sub>, is 0.97, and that
 bee surface temperature equals bee internal temperature (Stupski and
 Schilder, 2021). We assumed that the bee’s body surface absorptivity, a,
 is 0.97, that the emissivity, ε<sub>c</sub>, of the glass
 metabolic chamber is 0.90, and that air temperature equals the wall
 temperature, T<sub>i</sub>, of the glass chamber (Campbell,
 1977; Ray E. Bolz and Tuve, 1973; Stupski and Schilder, 2021). To estimate
 whole-bee radiative exchange, we summed Q<sub>radiation</sub>
 for the head, thorax, and abdomen and multiplied by tagma surface
 area.