The literature on the response of insect species to the changing environments experienced along altitudinal gradients is diverse and widely dispersed. There is a growing awareness that such responses may serve as analogues for climate warming effects occurring at a particular fixed altitude or latitude over time. This review seeks, therefore, to synthesise information on the responses of insects and allied groups to increasing altitude and provide a platform for future research. It focuses on those functional aspects of insect biology that show positive or negative reaction to altitudinal changes but avoids emphasising adaptation to high altitude per se. Reactions can be direct, with insect characteristics or performance responding to changing environmental parameters, or they can be indirect and mediated through the insect's interaction with other organisms. These organisms include the host plant in the case of herbivorous insects, and also competitor species, specific parasitoids, predators and pathogens. The manner in which these various factors individually and collectively influence the morphology, behaviour, ecophysiology, growth and development, survival, reproduction, and spatial distribution of insect species is considered in detail. Resultant patterns in the abundance of individual species populations and of community species richness are examined. Attempts are made throughout to provide mechanistic explanations of trends and to place each topic, where appropriate, into the broader theoretical context by appropriate reference to key literature. The paper concludes by considering how montane insect species will respond to climate warming.

Terrestrial insects along elevation gradients: species and community responses to altitude

In recent years, structural colors have attracted great attention in a wide variety of research fields. This is because they are originated from complex interaction between light and sophisticated nanostructures generated in the natural world. In addition, their inherent regular structures are one of the most conspicuous examples of non-equilibrium order formation. Structural colors are deeply connected with recent rapidly growing fields of photonics and have been extensively studied to clarify their peculiar optical phenomena. Their mechanisms are, in principle, of a purely physical origin, which differs considerably from the ordinary coloration mechanisms such as in pigments, dyes and metals, where the colors are produced by virtue of the energy consumption of light. It is generally recognized that structural colors are mainly based on several elementary optical processes including thin-layer interference, diffraction grating, light scattering, photonic crystals and so on. However, in nature, these processes are somehow mixed together to produce complex optical phenomena. In many cases, they are combined with the irregularity of the structure to produce the diffusive nature of the reflected light, while in some cases they are accompanied by large-scale structures to generate the macroscopic effect on the coloration. Further, it is well known that structural colors cooperate with pigmentary colors to enhance or to reduce the brilliancy and to produce special effects. Thus, structure-based optical phenomena in nature appear to be quite multi-functional, the variety of which is far beyond our understanding. In this article, we overview these phenomena appearing particularly in the diversity of the animal world, to shed light on this rapidly developing research field.

https://iopscience.iop.org/article/10.1088/0034-4885/71/7/076401/pdf

Physics of structural colors

Natural structural color materials, especially those that can undergo reversible changes, are attracting increasing interest in a wide variety of research fields. Inspired by the natural creatures, many elaborately nanostructured photonic materials with variable structural colors were developed. These materials have found important applications in switches, display devices, sensors, and so on. In this critical review, we will provide up-to-date research concerning the natural and bio-inspired photonic materials with variable structural colors. After introducing the variable structural colors in natural creatures, we will focus on the studies of artificial variable structural color photonic materials, including their bio-inspired designs, fabrications and applications. The prospects for the future development of these fantastic variable structural color materials will also be presented. We believe this review will promote the communications among biology, bionics, chemistry, optical physics, and material science (196 references).

Bio-inspired variable structural color materials

This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead-in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors.

Photonic Crystals for Chemical Sensing and Biosensing

Foreword. Preface. 1. Friction and adhesion: fundamentals. 2. Insect cuticle: structure and properties of the material. 3. Cuticular protuberances of insets. 4. Principles of cuticular attachment in Arthropoda. 5. Frictional systems. 6. Dragonfly and damselfly head-arresting system. 7. Intersegmental frictional devices. 8. Wing-locking devices. 9. Attachment pads. 10. Secretions in frictional systems. References. Index.

Attachment devices of insect cuticle

Members of the order Coleoptera are sometimes referred to as ‘living jewels’, in allusion to the strikingly diverse array of iridescence mechanisms and optical effects that have arisen in beetles. A number of novel and sophisticated reflectance mechanisms have been discovered in recent years, including three-dimensional photonic crystals and quasi-ordered coherent scattering arrays. However, the literature on beetle structural coloration is often redundant and lacks synthesis, with little interchange between the entomological and optical research communities. Here, an overview is provided for all iridescence mechanisms observed in Coleoptera. Types of iridescence are illustrated and classified into three mechanistic groups: multilayer reflectors, three-dimensional photonic crystals and diffraction gratings. Taxonomic and phylogenetic distributions are provided, along with discussion of the putative functions and evolutionary pathways by which iridescence has repeatedly arisen in beetles.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2008.0354.focus

Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)

We compiled data on the occurrence and frequency of distinct female variants among Holarctic Odonata and interpreted the data in light of harassment-based hypotheses. The major source of male confusion for male mimicry hypotheses is predicted to be signal similarity between andromorphs and male distractors; for the learned mate recognition hypothesis (LMR), it is predicted to be variation in female signals. Mapping morphism state onto molecular phylogenies of Ischnura and Enallagma failed to resolve the general ancestral female condition. However, it appeared that the andromorphic state may be ancestral in one case, and that blue structural colors were ancestral to orange and green pigmentations. Of the polymorphic species surveyed, 13% had more than two morphs, 4% had multiple heteromorphs but no andromorph, and 7% of ‘monomorphic’ congeners were functionally polymorphic because developmental variants mate. Such female signal variation lies beyond the scope of simple male mimicry, but neverthele...

The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?

Insect mate recognition is often viewed as stereotypic, innate, and species-specific. However, male damselflies can learn to identify female-specific color morphs as potential mates. A suite of male mimicry hypotheses assume that heteromorphic females, which differ from males in color pattern, are more easily recognized as “female” and thus lack the inherent, anti-harassment advantage that the more male-like signal provides for andromorphs. Using two measures of male preference, we investigated whether naive males have a preexisting sensory bias for a given morph color in Enallagma civile, a species that appeared to exhibit extreme plasticity in morph expression across generations within a breeding season. E. civile males raised in the absence of females exhibited no preference for either morph, whereas males raised with one female type exhibited a learned sensory bias for that morph. Male Enallagma also lacked a bias toward conspecific females over a congeneric sister species. In a naturally naive population of Enallagma ebrium, males reacted sexually to both morphs of Enallagma hageni as often as they did to conspecific females, whose thoracic spectra were nearly identical with those of E. hageni. Moreover, despite the similar thoracic spectra of males and andromorphs, both of which reflected UV, males rarely reacted sexually to other males. Our results falsified implicit assumptions of male mimicry hypotheses, supported learned mate recognition, and suggested a scenario for speciation via sexual conflict.

Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies

Summary
1.  Structural coloration is common among animals that produce sexual displays involving motion or ultraviolet reflection. Different sources of colour may provide multicomponent signals that indicate the location, sex, and fitness of a potential mate or rival. We investigated the proximate basis and ultimate function of the wing coloration of the territorial damselfly Megaloprepus caerulatus, which produces a dynamic, high contrast display during flight. The wings of both sexes have blue and white bands, but the location of the white patches are sex specific.

2.  Wax filaments produce diffuse, white areas through broadband scattering of wavelengths between 300 and 700 nm. Blue bands reflect wavelengths between 300 and 500 nm (λmax = 398 nm) and shift in hue with viewing angle, the result of thin layer interference produced by layers of cuticle and pigment within the wing membrane. Both wing bands strongly reflect UV wavelengths.

3.  Both the iridescent UV-blue and white wing patches provide high contrast against the vegetation in forest light gaps where mating occurs. Moreover, the iridescent signal oscillates during flight. Angle-dependent UV-blue iridescence is periodically extinguished during each wing beat cycle, in contrast to the white areas, which remain bright.

4.  Males distinguish potential mates from rivals by the presence of a female’s white wing tip. Blackening the white wing bands of males and adding white wing tips to resemble a female elicits a sexual rather than aggressive response from males. Conversely, blackening the white wing tips of females reduces sexual responses.

5.  The proportional area of the white wing bands of males is indicative of wing symmetry, correlated with body size, and in turn, territory residency suggesting that it may serve as a signal of male condition during intra- and intersexual interactions.

6.  We propose that the flashing iridescent UV-blue wing bands provide a beacon to potential mates across forest light gaps, whereas the white patches serve in mate recognition and may indicate male quality or territorial status. Our study identifies a unique combination of interference and broadband reflectors that provide a dynamic multicomponent signal.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2009.01584.x

Structural colours create a flashing cue for sexual recognition and male quality in a Neotropical giant damselfly

Tiger beetles of the genus Cicindela exhibit iridescent structural coloration due to the presence of a non-ideal multilayer interference reflector located in the outermost 2 μm of the integument. The reflector is composed of alternating layers of electron-lucent and electron-dense material. This series of layers was distinguished from chitinous procuticle by its position, ultrastructure and solubility in dilute KOH. The reflector appears homologous with the inner epicuticle of current models. Measurements of surface reflectance, refractive index and the dimensions of the alternating layers suggests that the dense layer has a refractive index (RI) near 2.0 and may be a melanoprotein.

Tom D. Schultz

Papers

Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)

The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?

Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies

Structural colours create a flashing cue for sexual recognition and male quality in a Neotropical giant damselfly

The Ultrastructure of the Epicuticular Interference Reflectors of Tiger Beetles (Cicindela)