Abstract
BackgroundThe exoskeleton of an insect could be an important factor in the success of its evolutionary process. This reaches its maximum expression in beetles, which constitute the most diversified animal taxon. The involvement in the management of environmental radiation could be one of the most important functions of the exoskeleton due to the passive contributions to the thermoregulation of body temperature. We study whether the elytra of two sympatric and closely related beetle species respond differentially to the radiation of distinct wavelengths in agreement with their ecological preferences.MethodsOnthophagus coenobita (Herbst) and O. medius (Kugelaan) occupy different habitats and environmental conditions (shaded vs. unshaded from solar radiation). The potential adaptive variations to thermoregulation under these different ecological conditions were studied using the responses of their exoskeletons to radiation of different wavelengths (ultraviolet, visible and near-infrared). For these two species, the amounts of the three wavelengths that were reflected, transmitted or absorbed by the exoskeleton were measured using of a spectrophotometer. In addition, the darkness and thickness of the elytra were examined to determine whether these two features influence the management of radiation by the exoskeleton.ResultsBoth species differ in the management of visible and near-infrared radiation. In agreement with habitat preferences, the species inhabiting shaded conditions would allow infrared and visible radiation to penetrate the elytra more easily to heat internal body parts, while the elytra of the heliophilous species would have increased absorbance of these same types of radiation. An increase in body size (and therefore in elytron thickness) and the quantity of dark spots may serve as barriers against exogenous heat gain. However, the maintenance of between-species differences independent of the effects of these two morphological features led us to suspect that an unconsidered elytron characteristic may also be affecting these differences.DiscussionThe results of the involvement of the exoskeleton thickness and spots in the thermoregulation of insects opens new research lines to obtain a better understanding of the function of the exoskeleton as a passive thermoregulation mechanism in Coleoptera.
Highlights
The radiation emitted by the sun can be considered the ultimate cause of the functioning of biogeochemical cycles in nature, and the flux of energy created by this radiation is a decisive force that conditions the behavioral, ecological, morphological, metabolic and physiological characteristics of living organisms (Hessen, 2008; Angilletta, 2009)
The evidence obtained in these studies suggested that there are interspecific differences in internal body temperatures when these specimens are exposed dorsally to simulated sunlight (Amore et al, 2017), and that these internal temperatures are lower when these specimens are exposed to infrared radiation (Carrascal, Jiménez-Ruiz & Lobo, 2017; Amore et al, 2017)
This temperature increase probably results from the transmittance and/or absorbance of non-infrared wavelengths by the dorsal cuticle. The elytra of these species could absorb most of the highly energetic radiation from the ultraviolet and visible parts of the spectrum and convert it into body heat (Alves, Hernández & Lobo, 2018; Pavlović et al, 2018). All these results suggest that the beetle exoskeleton may allow for the “passive thermoregulation” of body temperatures
Summary
The radiation emitted by the sun can be considered the ultimate cause of the functioning of biogeochemical cycles in nature, and the flux of energy created by this radiation is a decisive force that conditions the behavioral, ecological, morphological, metabolic and physiological characteristics of living organisms (Hessen, 2008; Angilletta, 2009) This phenomenon is especially true for the animals, such as insects, that depend on radiation and external temperatures to warm their internal parts, enhancing metabolic processes and increasing evolutionary rates (Brown et al, 2004). The potential adaptive variations to thermoregulation under these different ecological conditions were studied using the responses of their exoskeletons to radiation of different wavelengths (ultraviolet, visible and near-infrared) For these two species, the amounts of the three wavelengths that were reflected, transmitted or absorbed by the exoskeleton were measured using of a spectrophotometer. Discussion: The results of the involvement of the exoskeleton thickness and spots in the thermoregulation of insects opens new research lines to obtain a better
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