Incidence of the thermal transition in the range of 45-5°C in chromatophores present in the wings of Schistocerca americana

Introduction

Melanism, the occurrence of darker pigmentation in specimens within species is well-known in insects^1,2. It is also known that chromatophores to those cells present in the animal integumentary system containing pigments, and these cells respond to hormonal factors and/or neuronal (response to stimuli) factors². There are several studies focusing on why melanism has evolved, and a hypothesis about this is known as thermal melanism hypothesis^2,3. This states that at low temperatures, darker insects are at an advantage compared to light ones because they warm up more quickly. There are several studies that support the thermal melanism hypothesis related to butterflies^3–5 and hoverflies⁶.

Insects have a chemical or pigmentary coloration represented mainly by carotene and melanin, which are found in the cuticle, hypodermis or sub-hypodermis⁷. In the exocuticle, it is more common to find the variants of carotene and melanin as yellow, brown and black colorings, and in the hypodermis as yellow, red, green and orange colorings⁸. Adult Schistocerca americana have wings with large brown spots (melanin) on a light-colored background⁹. The study of the Arthropoda phylum and its Insecta class is highly wide-spread, however, the majority of the studies regarding grasshoppers' behavior under different temperature conditions, focus on habitat selection only^10,11.

The main objective of this study was to analyze the effect of thermal transition on the intensity of the color of the chromatophores present in the wings of the species S. Americana. This study was inspired by the observation in the literature^3–6 that temperature has a direct effect on change and/or color variations of the chromatophores located in other species and phyla of the Animalia kingdom.

Methods

Housing of specimens

Three terrariums were used in the experiment: one with the dimensions 40x40x30 cm and two with the dimensions 70x70x50 cm (Figure 1A). The first terrarium was used for the collection of specimens in the and the others of greater longevity for the creation of an adaptation habitat and for experimentation, respectively. As a substrate, a layer of soil (approx. 4cm) collected in the same location, one layer of stones (approx. 2 cm), and finally a layer of grass (approx. 2cm) was used along with branches of bushes and leaves (Figure 1A). A plastic mesh was used to cover the terrariums. Hydroponic lettuce, cabbage and the grass were used as food for the specimens.

Figure 1. Terrariums used in the study and evidence of adaption.

(A top) Mini terrarium for collection; (A bottom) habitat for the specimens during the experiment; (B left) Evidence of adaptation, molt; (B right) Evidence of adaptation, reproduction.

Results and Discussions

Adaption to the terrarium environment was considered successful as after a one-week period the specimens began molting (Figure 1B left) and reproducing (Figure 1B right).

At 45°C, degradation of color intensity can be observed in chromatophores present in the wings (Figure 2A). At 40°C chromatophores of the melanin-type begin to darken (Figure 2B); however, between the temperature of 45°C and 40°C there are no huge differences in color change. At 35°C chromatophores of the wings take a light brown color, intensify their color (Figure 2C). At 30°C the chromatophores turn dark brown, indicating an increase in their color intensity (Figure 2D). A temperature of 25°C corresponds to the optimum temperature for the normal development of the species in the region¹⁴ (Figure 2E). This is the reference color of the chromatophores used for all analysis. At 5°C chromatophores of the melanin-type intensify their color to brownish-black (Figure 2F). This temperature was the extreme minimum that S. americana could tolerate.

Figure 2. Results of the incidence of the thermal transition in the range of 45–5°C in chromatophores present in the wings of Schistocerca americana.

Chromatophores (melanin) at a temperature of (A) 45 ^⍜C; (B) 40 ^⍜C; (C) 35^⍜C; (D) 30^⍜C; (E) 25 ^⍜C; (F) 5^⍜C. Magnifications: 3.5 x (left), and 10x (right).

Temperature, precipitation and solar radiation are the meteorological elements that most affect the distribution, rate of growth, reproduction, migration and adaptation of insects¹⁵. The thermal melanism in tests with Phaulacridium vittatum and its exposure to the heat of lights within the range 300 to 700 nm represented an intensification in its color with a variation in percentage of between 2.49% to 5.65%³. The majority of the studies that examine the thermal melanism have focused on species with very distinct color morphs representing a wide range in melanism.

We found negative correlation between the temperature and the degree of darkness (R2 = 0.8038). Our results are in accordance with a previously published study³ in which P. vittatum was examined - a decrease of temperature caused darkening color change in melanin-type chromatophores.

Conclusions

The present investigation can be considered as the first initial study of its kind for Schistocerca americana, in terms of examining the changes in the color intensity of the chromatophores present in the wings caused by thermal transition under laboratory conditions. The color change can be considered as an indicator of a reaction to the increase or decrease of temperature^2,3 as the clear surface (in this case the wings) reflects the radiation emitted by the lightbulbs and thus absorb less heat, while in the case of a decrease in temperature, the wings absorb more heat.

Data availability