1. Organisms may internally or behaviourally regulate their body temperatures or conform to the ambient air temperatures. Previous evidence is mixed on whether wing pigmentation influences thermoregulation in various odonates.

2. 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.

3. We found that regressions of thorax temperature on ambient temperature standardised 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.

4. We found that regressions of thorax temperature on ambient temperature standardised 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.

5. 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.

This study was conducted at three sites along the AuSable River near Grayling, MI, USA (Finley’s Riverside Cabins [N = 589]: 44.657871, -84.728130; Burton’s Landing [N = 66]: 44.663861, -84.647406; Keystone Landing [N = 538]: 44.665036, -84.627466) from June 25 to July 2, 2018 between 0800 and 1800 h. At these sites, the river was slow-moving and variable but mostly shallow in depth (0.5-1.5 m). Streams were 10-30 m wide, so damselflies were commonly exposed to full sun. These damselflies basked on vegetation along the banks and in the stream on rocks and debris dams, some of which had live vegetation. They roosted overnight in trees along the stream, and adult females oviposited in submerged vegetation. C. aequabilis was more common in faster flowing water (Burton’s Landing and Keystone Landing), and C. aequabilis females were most scarce. Otherwise, all damselflies were extremely plentiful.

Surface temperature of perching damselflies was quantified from 20-30 cm distance with an infrared thermal camera (Seek Thermal Compact Pro) attached to the lightning port of an iPhone 6s (Apple). Thorax surface temperatures were measured using “Hi/Lo” mode (16:9 aspect ratio), as the thorax was usually the hottest point in the frame. Thorax temperatures of 89 C. aequabilis females, 294 C. aequabilis males, 324 C. maculata females, and 485 C. maculata males were measured. Only hardened adult individuals were measured (i.e., no tenerals). For a subset of individuals (N = 409), wing temperature was also recorded with manual “Spot” mode pointed at melanised regions. Damselflies were not marked, so individuals may have been measured more than once. However, the large population greatly decreased this chance of pseudoreplication.

Ambient air temperature was simultaneously measured within 1 m of each individual with a digital thermometer (Nielson-Kellerman Kestrel 3000 Pocket Weather Meter).

Wing temperature data was not recorded for many individuals. For a subset of individuals (N=409), we recorded wing temperature and thorax temperature with the thermal camera. However, we chose to only analyse thorax temperatures in our study; we had a much larger sample size and there is a strong Pearson correlation with thorax temperature (r=0.754). Additionally, using the “Hi/Lo” (automatic) setting on the infrared camera to quantify thorax temperature was much easier/quicker than the “Spot” (manual) setting needed to find the wing temperature. The thorax temperature was found very quickly as the hottest point in the frame, while the wing temperature needed to be found manually with a different setting. So, we chose to maximise our sample size by using the automatic setting to measure thorax temperatures and only measuring the wing temperature of damselflies that perched for a long time.