Results of the regression analysis using GLM between abdominal pumping and three explanatory variables: T_abdomen, T_thorax and T_ambient.

<p>SE: Standard Error.</p><p>W: Wald Statistic.</p

Tracheal and air sac volume (V_air) of <i>Scarabaeus sacer</i> and <i>S. cicatricosus</i>.

<p>Total body volume (V_T) was measured using the function proposed by Radtke and Williamson (2005) based on total body mass.</p

Heat transfer between the thorax and the abdomen and abdominal pumping behaviour.

<p>Differences between (A) T_thorax and T_abdomen during flight for <i>Scarabaeus sacer</i> (in blue) and <i>S. cicatricosus</i> (in red), and (B) frequency of abdominal pumping. (<i>Scarabaeus cicatricosus</i>, N = 27; and <i>S. sacer</i>, N = 24; U-Mann-Whitney, α<0.05); Boxes labelled with the different letters differ significantly from each other. Black dots indicate the median values.</p

Flight postures of <i>Scarabaeus</i> species.

<p>(A) <i>S. sacer</i> and (B) <i>S. cicatricosus</i>. Arrows indicate regions in which body temperatures were analysed (T_thorax and T_abdomen) and the abdominal movement to generate heat flow (abdominal pumping). The flight posture adopted by <i>S. cicatricosus</i>, with the posterior legs extended from the body, increase turbulence and convective cooling.</p

Flight thermoregulatory behaviour of <i>Scarabaeus sacer</i>.

<p>(A) Individual variation of T_thorax (in red) and T_abdomen (in blue). Environmental temperature (T_ambient) (in green) was maintained during each experiment. (B) Comparison of slopes of T_thorax, T_abdomen and T_ambient variation during flight (<i>N</i> = 27; Kruskal-Wallis test with the Conover-Inman post hoc test for pairwise comparisons, α<0.05); black dots indicate the median values; temperatures with the same letter did not differ significantly from each other. (C) Thermal image of <i>S. sacer</i> during flight showing the strong contrast between T_thorax and T_abdomen.</p

Flight thermoregulatory behaviour of <i>Scarabaeus cicatricosus</i>.

<p>(A) Individual variation of T_thorax (in red) and T_abdomen (in blue). Environmental temperature (T_ambient) (in green) was maintained during each experiment. (B) Comparison of the slopes of T_thorax, T_abdomen and T_ambient variation during flight (<i>N</i> = 27; Kruskal-Wallis test with the Conover-Inman post hoc test for pairwise comparisons, α<0.05); black dots indicate the median values; temperatures with the same letter did not differ significantly from each other. (C) Thermal image of <i>S. cicatricosus</i> during flight, showing lower contrast between T_thorax and T_abdomen, relative to <i>S. sacer</i>.</p

Distribution and development of abdominal air sacs.

<p>(A) <i>Scarabaeus sacer</i> and (B) <i>S. cicatricosus</i>. Abdominal air sacs were coloured in blue. Red lines indicate the separation between the metathorax and the abdomen. Abbreviations: pr: prothorax; ms: mesothorax; mt: metathorax.</p