Exoskeleton may influence the internal body temperatures of Neotropical dung beetles (Col. Scarabaeinae)

Abstract

The insect exoskeleton is a multifunctional coat with a continuum of mechanical and structural properties constituting the barrier between electromagnetic waves and the internal body parts. This paper examines the ability of beetle exoskeleton to regulate internal body temperature considering its thermal permeability or isolation to simulated solar irradiance and infrared radiation. Seven Neotropical species of dung beetles (Coleoptera, Scarabaeinae) differing in colour, surface sculptures, size, sexual dimorphism, period of activity, guild category and altitudinal distribution were studied. Specimens were repeatedly subjected to heating trials under simulated solar irradiance and infrared radiation using a halogen neodymium bulb light with a balanced daylight spectrum and a ceramic infrared heat emitter. The volume of exoskeleton and its weight per volume unit were significantly more important for the heating rate at the beginning of the heating process than for the asymptotic maximum temperature reached at the end of the trials: larger beetles with relatively thicker exoskeletons heated more slowly. The source of radiation greatly influences the asymptotic temperature reached, but has a negligible effect in determining the rate of heat gain by beetles: they reached higher temperatures under artificial sunlight than under infrared radiation. Interspecific differences were negligible in the heating rate but had a large magnitude effect on the asymptotic temperature, only detectable under simulated sun irradiance. The fact that sun irradiance is differentially absorbed dorsally and transformed into heat among species opens the possibility that differences in dorsal exoskeleton would facilitate the heat gain under restrictive environmental temperatures below the preferred ones. The findings provided by this study support the important role played by the exoskeleton in the heating process of beetles, a cuticle able to act passively in the thermal control of body temperature without implying energetic costs and metabolic changes.

Keywords: Body size; Cuticle properties; Heating process; Interspecific differences; Passive physiology.

Figure 1. Dorsal habitus of Canthon rutilans rutilans (A), Deltochilum brasiliense (B), Coprophanaeus saphirinus (C), Phanaeus splendidulus (D), Homocopris sp. nov (E), Dichotomius sericeus (F) Dichotomius opalescens (G) showing their sexual dimorphism as well as main within species coloration variations.

The maps represent the general distribution of each species.

Figure 2. Mean ± one standard error of heating rates at the beginning of the heating trials in seven dung beetles (Scarabaeinae) under two sources of radiation: infrared radiation (infrared) and luminous radiation (light) resembling the electromagnetic spectrum of sun radiation.

The values presented are adjusted means after controlling for the effect of the three covariates included in the ANCOVA model of Table 3 (control temperature, animal volume and weight per volume unit).

Figure 3. Mean ± one standard error of the maximum temperature internally reached by beetle exoskeletons in heating experiments (asymptotic temperature attained after ten minutes) in seven dung beetles (Scarabaeinae) under two sources of radiation: infrared radiation (infrared) and luminous radiation (light) resembling the electromagnetic spectrum of sun radiation.

The values presented are adjusted means after controlling for the effect of the three covariates included in the ANCOVA model of Table 4 (control temperature, animal volume and weight per volume unit).