The mutilids' darkest secret: how ultra-black animals inspire technology

Very few people have taken up the challenge of studying mutilids. These insects, commonly known as velvet ants, are as interesting as they are difficult to research. As one classic work on them puts it: “None [Hymenoptera species] can provide more hours of fascination and frustration than the velvet ants.”

Although it is true that they have a velvety appearance due to their bodies being covered by a dense layer of hair, they are not true ants, as they are more closely related to wasps. Currently, the Mutillidae family comprises 4,693 species, but for the vast majority we have no information at all .

The reasons for this are found in their biology. Males and females show extreme sexual dimorphism : females are wingless and large, while males have wings and are usually smaller. This makes taxonomy difficult because the same species has often been described as different genera.

Females seek out nests of other insects, such as solitary wasps, to lay their eggs. Upon hatching, the larvae feed on the host pupa, acting as parasites . This behavior also complicates their study, as it requires observing and understanding interactions between species in their natural environments. In addition, their solitary lifestyle makes them especially difficult to locate.

However, there are people for whom difficulties, rather than being a discouragement, are an attraction. This is the case of Vinicius López, an entomologist at the Federal University of the Mining Triangle in Brazil. As he tells EL PAÍS: “When you study mutilids, each discovery seems to open up more questions than answers . That feeling of mystery and the constant challenge of understanding them is, for me, absolutely charming.”

In particular, there is a species called Traumatomutilla bifurca , which caught his attention due to its peculiar black and white colouring. “On an expedition in Chapada dos Guimarães National Park, I saw this species walking on light sand and I had the impression that the white bands were floating, as if the black parts did not exist,” explains Vinicius López, “it was a moment I will never forget.”

Years later, a spectrometer arrived in his laboratory that could measure the light reflected by different surfaces, and he and his team decided to measure the black present in the cuticle of T. bifurca . As part of its calibration, this instrument includes specific references for white (representing maximum reflection) and for black (minimum reflection). They discovered that the black of T. bifurca reflected less light than the black calibrator of the spectrometer. “The discovery confirmed to us that we were dealing with a unique phenomenon, worthy of study ,” says the entomologist.

What Vinicius and his team had found was a colour known as ultrablack, which is characterised by reflecting less than 0.5% of the incident light. In nature this colour is extremely rare, found only in a few animals such as the peacock spider ( Maratus tasmanicus ), the bird of paradise ( Lophorina niedda ), the Gaboon viper ( Gaboon viper ) or the Greek slipper butterfly ( Catonephele numilia ).

In each animal, this colour has evolved independently and confers distinct evolutionary advantages. In peacock spiders and birds of paradise, ultra-black highlights other bright colours, increasing visual contrast and attracting mates. In the Gaboon viper, it allows it to better regulate temperature and camouflage itself, while in the Greek slipper butterfly it helps confuse predators.

In the case of Traumatomutilla bifurca , the function of the ultra-black is not entirely clear. The sexual behaviour of mutilids is still poorly understood, but it does not seem that the females' colouration serves to attract males. Instead, it could help them evade predators.

Mutilids are known for theiranti-predator strategies . Females look like small armoured tanks: they have an incredibly resistant exoskeleton and a very powerful venomous stinger. They also share colour patterns with other species in their environment that are also venomous, which is known as Mullerian mimicry. Curiously, interactions between these ants and insectivorous predators have hardly been observed. As Vinicius López explains, “it remains a mystery who their natural predators are, which makes studying them even more intriguing.”

However, not everything is a mystery when it comes to mutilids. Using advanced techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Vinicius López and his team have found out how these insects manage to produce such a dark colour. They have recently published their results in the Belstein Journal of Nanotechnology .

The surface of the T. bifurca cuticle is covered by a dense layer of spines that act as light traps, directing light into the cuticle. Beneath this surface layer, there is a series of overlapping lamellae that form a three-dimensional network. This arrangement further increases light absorption, as the light bounces multiple times within the lamellae until it is finally absorbed by pigments, probably melanin. This is a perfect combination of nanoengineering and biochemistry.

Studies like this open up opportunities for practical applications in technology. Science has already managed to replicate these mechanisms in laboratories using carbon nanotubes and structures similar to those of these animals. According to Stanislav Gorb, professor at the Zoological Institute of the University of Kiel and one of the leading experts in biomimetic materials, one of their most successful applications is in optical devices such as microscopes and telescopes, where ultra-black surfaces reduce internal reflections, improving the quality of the images obtained.

For example, Vantablack , one of the best-known ultra-black materials, made its space debut in 2015, coating the star tracker on the Kent Ridge 1 microsatellite. Although it appears to be a dark environment, space is very bright. Vantablack absorbs light entering the tracker’s sensors, improving its ability to monitor position relative to stars.

Stanislav Gorb believes that one of the most promising developments for ultra-black is its application in solar panels. Compared to current coatings, which can reflect a portion of the incident light, ultra-black will ensure that virtually all available light energy is captured by photovoltaic cells. In the long term, this technology could play a key role in improving the performance of renewable energy, driving the development of more sustainable and competitive solutions.

When Vinicius López first came across T. bifurca on the white sand, he probably didn't think of telescopes or solar panels. His interest in mutilids was none other than to understand animals that refuse to reveal their secrets. But in nature everything is connected and the darkest secret of an insect can be a source of inspiration for a curious primate that has gone into space.