New research has found an insect species with markings that are so black that they're 'superblack' or 'ultrablack', absorbing nearly all visible and ultraviolet light.
An international team of scientists recently published detailed studies of a Brazilian species of velvet ant that they discovered to have superblack, or ultrablack, patches.
An ultrablack substance is defined as a material that reflects less than 0.5% of the visible and ultraviolet light that hits its surface. This rare structural color is part of the assortment of naturally occurring colors seen in some species of birds-of-paradise, snakes, butterflies, spiders and fishes (Figure 1). Ultrablack components are used in telescopes, cameras, and solar panels to improve the efficiency of light capture as well as providing camouflage for military vessels. Nonetheless, the discovery of the ultrablack cuticle in the Brazilian velvet ant, Traumatomutilla bifurca, makes this species unique amongst their relatives, the bees, wasps and ants.
Despite their name, velvet ants aren't ants at all. They comprise a diverse group of more than 7,000 solitary wasp species. They live in a variety of habitats, including forests, woodlands, deserts, and semi-arid environments in Canada, the United States, Mexico, Europe, and Australia.
Females are wingless and resemble large, very fuzzy ants, hence their common name. Velvet ants in general are quite colorful, often with bright scarlet or orange patches, as well as silver, gold or white, usually with contrasting black patches. Their vibrant coloring serves as a warning to potential predators (velvet ants are sometimes known as 'cow killers', because of the myth that they can kill a cow with their extremely painful stings) but their coloration may serve other functions, too, such as camouflage or protection from solar radiation.
"In the case of Traumatomutilla bifurca, we are still investigating its potential functions," said the study's lead author, entomologist Vinicius Lopez, a postdoc at Federal University of Triângulo Mineiro (UFTM) where he studies the taxonomy and ecogeography of velvet ants (Mutillidae) and other solitary wasps.
"Preliminary unpublished findings suggest that the combination of ultrablack coloration and white hair bands may enhance the females' camouflage abilities in specific environments," Dr Lopez continued in email. "This effectiveness seems to vary depending on the visual systems of potential predators, such as birds or frogs."
Dr Lopez and collaborators used a variety of electron microscopy techniques to carefully document how this structural color is created. For example, they found that the cuticle reflectance of the female velvet ants closely resembles the spectra produced by other animal species with ultrablack coloration (Figure 3). Reflectance spectra records the wavelengths (colors) of light that reflect off the surface of a substance. Different materials produce different spectra because they reflect sunlight in different ways.
Dr Lopez and collaborators pointed out that the velvet ants' ultrablack is not just created by pigmentation. As they write in their paper, these structural colors "are formed in nature by a sophisticated arrangement of microstructures" overlying dark pigments. These microstructures are composed of overlapping stacks or layers of tissue, resembling the pages of a book, underlying dense, hair-like setae. Combined with dense, fuzzy setae, these features in the insect's exoskeleton "minimize reflectance and enhance light absorption," noted to Dr Lopez and collaborators in their paper (ref), thereby trapping visible and ultraviolet light so that almost none of it can escape (Figure 6).
Although there are a few ultrablack butterflies (Figure 1 & 2), female Brazilian velvet ants are the first ultrablack member identified in the large and diverse insect order, Hymenoptera (sawflies, bees, wasps, and ants).
Interestingly, although male velvet ants do have some black patches, these are not ultrablack.
"We also analyzed males, but they did not exhibit ultrablack coloration," Dr Lopez stated in email. "They differ from the females and resemble other male velvet ants."
Why do females have ultrablack patches whilst males don't?
"This question deserves deeper investigation," Dr Lopez responded to me in email. "Unlike males, which can fly, female velvet ants face unique selective pressures as ground-dwelling insects, such as evading ground-level predators. The visual backgrounds encountered by females and males differ significantly, likely shaping their respective anti-predatory strategies. Additionally, females search for hosts (typically the pupae of bees and other wasps) and are more likely to encounter defending adults at nests. This raises intriguing questions about the role of ultrablack in their ecology, potentially as an adaptation to their ground-dwelling lifestyle."
Ultrablack appears to have a variety of evolutionary and natural history roles in different species of animals. For example, studies have found that ultrablack coloration in birds-of-paradise and in peacock spiders accentuate brighter colors located next to them, thereby helping to attract potential mates (ref). In contrast, a number of deep sea fishes appear to rely upon ultrablack to remain hidden from both their prey as well as their predators (ref). Other animals, such as snakes, may rely upon ultrablack to help regulate their body temperatures (ref).
Speaking of body temperatures, these velvet ants live in Brazil's caatinga, a semi-arid ecoregion located in the interior of northeastern Brazil. As such, the caatinga is very sunny and warm, which made me wonder how these insects avoid being cooked alive due to intense solar radiation?
"That's an excellent question, and we've pondered the same," Dr Lopez replied in email. "We conducted thermal tests (see Figure 8), and thermal imaging results indicated that velvet ants were consistently 2°C cooler than the ambient temperature. Interestingly, we observed no thermal differences between the white-haired areas and the ultrablack cuticle."
Ultrablack patches may have other physiological and behavioral implications in velvet ants.
"In many cases (including T. bifurca), velvet ants are active in the early morning and late afternoon, when temperatures are milder," Dr Lopez explained in email. "However, your question is very relevant, and we plan to investigate this further. What are the thermal implications of ultrablack and non-ultrablack cuticles? What are the behavioral and physiological strategies to manage these implications? This is what I find fascinating about velvet ants -- there's always more to explore."
It is possible -- indeed, likely -- that there are other species of velvet ants with ultrablack coloration.
"Our preliminary findings revealed that some female velvet ants exhibit ultrablack coloration, while others do not," Dr Lopez said in email. "Interestingly, we couldn't establish a clear pattern related to phylogeny, habitat, or behavior. We haven't yet explored whether similar traits appear in other Hymenoptera, such as ants, bees, and wasps, but it's definitely an area we're eager to investigate."
What was most surprising or interesting about your findings?
"For me, it was already fascinating to discover an ultrablack species! Specifically, I'd say the thermal results surprised me the most," Dr Lopez said in email. "Surprisingly, the white hairs did not exhibit distinct thermal patterns compared to the dark areas. While I expected differences, this could be explained by the species' behavior."
When I asked co-author Stanislav Gorb, a professor of zoology at the University of Kiel, the same question in email, he replied: this is "another structural solution of generating ultrablack," then added: "Of course, it is always a kind of microstructure combined with the dark pigments, but in velvet ants it is different from that of butterflies or snakes."
Despite their small size (well, small compared to a human shoe), unsquashable exoskeleton, and their endearing ability to chirp loudly when alarmed, Brazilian velvet ants are endangered, mainly by climate change. This is hardly surprising because globally, the percentage area that is affected by drought has doubled since 1970 through early 2000. Additionally, there is strong evidence that those climate extremes are becoming more frequent (think: California wildfires), and are probably linked to human-induced climate change (i. e.; ref, ref and ref).
"The Caatinga region, where we collected Traumatomutilla bifurca, is undergoing severe impacts linked to climate change," Dr Lopez explained in email. "Alarmingly, projections suggest that parts of this unique biome could transition into desert-like conditions in the coming years."
Such a dramatic climate change could not only spell trouble for the continued existence of velvet ants, but it could also endanger other animals living in the caatinga as well as the human communities living there. This developing climatic disaster highlights the urgency of studying and preserving all of the remarkable species in this unique ecoregion.
Vinicius Marques Lopez, Wencke Krings, Juliana Reis Machado, Stanislav Gorb and Rhainer Guillermo-Ferreira (2024). Ultrablack color in velvet ant cuticle, Beilstein Journal of Nanotechnology 15, 1554-1565 | doi:10.3762/bjnano.15.122