MicroscopyEducation
The Undergraduate Student Cohort Te undergraduate student cohort in 2020 comprised 44
students from a range of degrees, including Bachelors of Sci- ence, Engineering, Mathematics, Education, Computer Sci- ence, Biomedical Science, and Technology. Students ranged from first year to fourth (final) year undergraduate levels, and all were enrolled at the University of Newcastle.
A Unique Specimen for Teaching – Structural Color in Maratus (Peacock) Spiders Maratus occasus is a recently discovered Peacock spider
species in Queensland, Australia, belonging to the Mara- tus tasmanicus group (Figures 3a and 3b) [5]. Tis particular specimen was chosen for the workshop to complement the samples examined in previous years when students had stud- ied organic semiconductor nanoparticle films (at Spring and Winter Schools) and self-assembled colloidal nanoparticle photonic crystals (in the PHYS3390 course). Te choice of nat- urally occurring structural-colored specimens in 2020 enabled us to build upon previous years’ work imaging nanostructured materials that interact with light. Structural color occurs in nature in many organisms,
including butterflies [6], peacocks [7], spiders [8,9], and rainfor- est beetles [10]. In addition, structural color occurs in naturally occurring minerals such as opal gemstones and can also be produced synthetically, for example, in self-assembled colloi- dal nanoparticle arrays [11] and 3D printed microscale objects [12]. Structural color results from the interaction of light waves with a structural feature that exhibits the same order of size as the wavelength of light, noting the wavelength of visible light ranges from 380 to 750nm. Spiders employ a variety of struc- tural coloration mechanisms, including multilayer reflectors, three-dimensional photonic crystals, and diffraction gratings. Structural color in Australian Peacock spiders is varied,
from blue in the scales of Maratus occasus and Maratus splen- dens [5,13] to super-black in Maratus speciosus and Maratus karrie [14] to full-spectrum rainbow iridescence in Maratus
robinsoni and Maratus chrysomelas [9]. Te super-black regions in the species Maratus speciosus and Maratus karrie reflect as little as 0.44% and 0.35% of visible light, respectively, owing to their nanoscale structures. Both species evolved microlens arrays, comprising tall and tightly packed cuticular bumps. Te super-black is a combination of pigment and structural effects. Te microlens arrays achieve structurally assisted enhanced absorption of light by melanin pigment. Te spe- cies Maratus robinsoni displays angle-dependent rainbow iridescence attributed to their scales that are comprised of 2D nanogratings on microscale 3D convex surfaces. Maratus occasus is a newly discovered species, hence, a
detailed investigation of the structural and pigmented color ori- gins of its scales does not exist in the literature. Terefore, we draw on the literature from similar species in the spider genus Maratus, such as Maratus splendens, with scales of similar color that have been studied in detail [13]. Maratus occasus has two types of scales, which the students investigated during the work- shop. Type I: plate-like scales, which are blue in color, and Type II: brush-like scales, which are orange in color (Figure 3a, Figure 4). Te brush-like scales, being similar in structure to the red brush- like scales of Maratus splendens, are likely to have pigments that give rise to their color. Te plate-like scales, being similar in structure to the blue plate-like scales of Maratus splendens, are likely to have structural color origins. Te dual thin film struc- ture of the chitin plate-like scales, with an internal filament array (Figure 4b), likely leads to the blue structural color in Maratus occasus, as it does in Maratus splendens. Te interior and exterior structure is evident in the micrograph of a broken plate-like scale in Figures 4a and 4b. For the MyScope Explore demonstration, an intact plate-like scale is zoomed in upon (Figure 5a). While the interior structure of the plate-like scale is not visible, the exterior parallel ridges with a periodicity of 120nm are visible; students were directed to observe these during the workshop. Students were provided with a background tutorial intro-
ducing the origin of both structural color and pigmented color, which is attributed to chemical molecules rather than nanoscale structures. Ommochromes such as xanthommatin [13] have been demon- strated to be the main pigments in spiders, in general [15,16]. In addition, structurally assisted absorption mechanisms have also been proposed for the brush-like scales of some Maratus species [14]. Multiple scat- tering between the spikes and iterative absorption can occur as light propagates through the cuticle into an absorbing layer of melanin pigment in the abdomen [14].
MyScope Explore Background Tis remote learning solution made use
Figure 3: Maratus spiders containing structurally colored blue scales located on the abdomen (dorsal opisthomal plate). (a) Photograph of species Maratus occasus (male) collected from Lake Broadwater, Queensland. (b) Photograph of species Maratus tasmanicus (male) collected from Point Cook, Victoria. The typical species size for Maratus occasus and Maratus tasmanicus is 4mm. In (a) the blue plate-like scales (PLS) (Type I) and orange brush like-scales (BLS) (Type II) are annotated. Photographs courtesy of Joseph Schubert.
2021 November •
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of the existing online learning infrastruc- ture provided by Microscopy Australia. Microscopy Australia (supported by NCRIS, the Australian Government’s National Col- laborative Research Infrastructure Strategy) is a national grid of equipment, instrumen- tation, and expertise in microscopy and microanalysis. Tis national grid provides
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