Body size rather than reflectivity explains thermal constraints on colour variation in an aposematic jewel bug

Theory posits that warning signals should converge phenotypically to reinforce predator memory, yet many aposematic species show substantial variation in warning signals within and between populations. This may reflect alternative selection pressures, such as thermoregulation, though empirical tests are limited and often overlook the full solar spectrum. We tested whether thermal trade-offs could explain warning colour variation in the aposematic cotton harlequin bug (Tectocoris diophthalmus), which is sexually dichromatic, varies within sexes, and shows clinal shifts in colour—iridescent blue-green in cooler regions, red–orange in warmer ones. We measured reflectivity across the full solar spectrum and assessed the role of ultraviolet–visible (300–700 nm) and near-infrared (700–1700 nm) light on heating, using a solar simulator and temperature controlled-chamber to isolate the effect of radiative heating. Reflectivity differences between iridescent and non-iridescent patches were greatest in the NIR, but these did not translate into significant heating differences. However, reflectivity was tightly linked to body size, with smaller males reflecting less and heating faster. Given the strong correlation between colour and body size, thermal constraints may contribute to clinal colour variation.

Methods
We measured the total hemispherical reflectance for a subset of individuals (n = 11), using an integrating sphere. We measured reflectance of the spectral range of 400–2100 nm, and measurements were calibrated against a 99% white reflectance standard and a 2% black reflectance standard. We measured the bugs alive and immediately after euthanasia to validate that reflectance remained similar post-mortem. Although our reflectance measurements do not include UV, our multispectral imaging showed that the bugs do not reflect UV and thus there is no difference in UV reflectance.

We conducted heating experiments using a solar simulator and a closed glass thermal chamber which kept the ambient air temperature in the chamber constant at 20º C. This was to isolate the effect of radiation from the solar simulator and to minimise the effects of convection and conduction. Inside the chamber, the sample bug was placed dorsal side up on a transparent acrylic platform and with a thermocouple inserted in the posterior to record body temperature. Temperature readings of both the air and the bug were taken every 10 s using a digital thermometer. We ran each individual sample once for each heating treatment: full (300–1700 nanometres), NIR (700–1700 nm), and UV–visible (300–700 nm). We used optical filters over the silicon window to isolate the effect of radiation in different wavelength ranges. After placing the bug in the chamber, we allowed a 10 min period for the bug and air to achieve an equilibrium temperature of 20º C. We then opened the solar simulator shutter to begin the 10 min heating period. Although each heating trial ran for 10 min, all bugs reached a stable maximum temperature by 5 min; therefore, all heating estimates were calculated based on the first 5 min of the heating period.