Spiders' abdomen secrets exposed

Editorial

Rebecca Pool

Wednesday, May 15, 2019 - 12:45
Image: A male Maratus volans peacock spider [Jürgen Otto]
 
Using SEM, US-based researchers have uncovered a distinctive microstructure within the deep black surfaces of male jumping peacock spiders, renowned for their enthusiastic courtship displays.
 
Male peacock spiders have splendidly coloured abdomens which they erect and wave side-to-side during mating displays to females.
 
But next to the vivid abdomen colours lie velvety black patches akin to the super black, extremely low reflectance, plumage found near the bright colours on birds of paradise.
 
To shed light on these patterns, Dakota McCoy from the Department of Organismic and Evolutionary Biology, Harvard University, and colleagues set out to determine the spectral reflectance and surface microstructures of the deep black areas in two peacock spider species, Maratus speciosus and Maratus karrie.
 
Using a FESEM Ultra55, alongside hyperspectral analysis and finite-difference time-domain modelling of the interaction between the structures and incident electromagnetic field, the researchers analysed spider specimens obtained from the Harvard Museum of Comparative Zoology.
 
Investigations revealed that the spiders' super black regions reflect less than 0.5% of light, thanks to distinctive microscale structures.
 
Both M. speciosus and M. karrie species were found to have unusually high, tightly packed cuticular bumps - microlens arrays - with the M. karrie having an additional dense covering of black brush-like scales.
 
“We observed a new, distinct type of microstructure in super black spiders different than those previously described in birds of paradise,” highlights McCoy. “Maratus has brush-like scales similar to the bird of paradise feathers, but also has novel anti-reflective microlens arrays.”
 
(a,b) SEMs of the brown region of M. speciosus (c,d) SEMs of super black region in M. speciosus (e,f) SEMs of super black region in M. karrie g) Diagram of the proposed structurally assisted absorption mechanisms by peacock spider microstructures. Scale bars: (a) 30 µm, (b) 10 µm, (c) 30 µm, (d) 10 µm, (e) 50 µm and (f) 10 µm. Read more in McCoy et al , 'Structurally assisted super black in colourful peacock spiders', 286, Proc. R. Soc. B.
 
According to the researchers, the spiders' microlens arrays behave similarly to engineered microlenses found in anti-reflective applications, such as solar cells.
 
What's more, the microlenses also bear a striking resemblance to the anti-reflective surface structures in organisms such as flower petals, tropical shade plant leaves, ommatidea in moth eyes and a range of birds, including birds of paradise.
 
As McCoy points out, this 'super black' makes nearby colours appear brighter, even luminous, to vertebrates, and this so-called sensory bias has also led to the convergent evolution of super black for mating displays in non-vertebrate, jumping spiders.
 
Research is published in the Proceedings of the Royal Society B.
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