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

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).