By Tharina Bird, Curator General Entomology
DITSONG: National Museum of Natural History
I asked a friend what she knows about solifuges. Her answer: “Apart from them being repulsive, nothing.” Unfortunately, this would be the answer from most – that is, from those that even know what a solifuge is (or camelspider as the Americans like to call them). For those who do know them, they are often the villains of the world. There is even a horror movie called “Camel Spiders” – a rather badly produced, and highly inaccurate production. Before I studied solifuges, I admittedly also thought solifuges was amongst the less pretty creatures that roam the earth. But, the more I study them, the more I am in awe: the way that they are build (morphology, anatomy), the way that they function (physiology), and their behaviour. And the more I am convinced that, if comic writers and movie producers knew more about solifuges, they would not make these animals the villains of the story, but use them as the blueprint for future superheroes!
What are solifuges?
In South Africa, solifuges are better known as red romans, sun- or windspiders, or even sun- or windscorpions (Figs. 1a-f). Yet, although they are arachnids, as are spiders and scorpions, solifuges are neither a spider, nor a scorpion. They are an order on their own – the order Solifugae. They produce no venom, and are completely harmless. They don’t have the sting of scorpions, they also don’t have the fangs of spiders, and they are easily recognized by their huge jaws (called chelicerae). For their size, solifuges probably have the largest jaws in the animal kingdom. In some species in Africa, the jaws are nearly a third the length of the body (Fig. 2). As with all arachnids, solifuges have four pairs of legs, plus an extra pair of appendages, called pedipalps (or “palps”), in front of the legs. Pedipalps take different forms in different arachnids. In scorpions, for example, the pedipalps are the appendages that bear the claws. In solifuges, however, the pedipalps look like an extra pair of robust legs. To the untrained eye it may thus look as if solifuges have ten legs (Fig. 3). The pedipalps are full of hairs and other sensory structures, and are held in front of the solifuge while it moves about.
But what are the superhero characteristics?
Solifuges can run fast. Really fast. They are built to run fast: long legs allow for long strides, a combination of hinge and pivot joints, and the angles of the joints at different parts of the legs mean that there is no time wasted as each leg segment bends forward or backward with every stroke. They also use only three of their four pairs of legs for locomotion — the front pair is always held in the air – resulting in less legs to trip over while running! Achieving great speeds are also facilitated by greater oxygen use efficiency: most arachnids use a so-called book lung system to exchange respiratory gasses with the environment. However, solifuges, abandoned the book lung system long ago and evolved a tracheal respiratory system instead, which comprises a system of tubes in the body that opens to the outside through openings called spiracles (Fig. 4). The tracheal system, also present in insects, is much more efficient in getting oxygen from the environment into the body than a respiratory system that revolves around book lungs.
Masters of illusion (the disappearing act)
Solifuges not only run extremely fast, they can also “disappear”. Not literally off course, but by using visual tricks. Combining speed, erratic running with sudden changes of direction, and unexpected complete stops, they confuse their predators – and any human that “think they saw something”. Suddenly you see it, and then you don’t – pulling the same tricks that master magicians use to create illusions that fool their audiences! Some day-living solifuges add clever camouflage by being covered with long hairs (Fig. 5) and when they run around in such fast, erratic fashion, they seem like a fluffy seed blowing in the wind. No observer thinks twice about the identity of the “seed”, unless he or she registers that “the seed” is blowing against the wind!
The jaws of solifuges can be seen as built-in multitools. They are used to catch and tear prey, yet when grinding food, jaws do not just chew. No! Together with the normal “chewing” motion of each jaw, the two jaws work together in up-down and forward-backward movements, in effect putting the food through a tearing motion called a “cheliceral mill”. In this way, the food is ground to such an extent that every drop of juice is sucked out, leaving only morsels of dry chitenous exoskeleton behind. Ridges on the inner side of the jaws assist with the grinding. In some solifuge species, jaws are also used to make a sound, called stridulation. This is done by scraping these ridges (hence also called stridulatory ridges) against each other by rubbing the jaws against each other. Scientists speculate that stridulation forms part of courtship, but another hypothesis put forward is that it is a way of scaring predators off. In some of the larger species, stridulation can be audible to the human ear!
Surprisingly, their jaws are also used in mating. Detail differ between species, but all species so far observed use their jaws extensively during mating. The male uses his jaws to subdue the female, but in most species the male will even use his jaws as an actual intromittent organ. What this means is that he uses his jaws, or a certain structure on the jaw, to pick up his sperm, and then use the jaw to put the sperm into the female’s genital opening! In one species the male has been observed to pick the female up in his jaws, lift her in the air, and run with her before mating!
Suctorial organs to scale glass and grab objects from the air
And then there is the suctorial organ! This is a unique structure situated at the tip of the pedipalps. The sectorial organ can “grab” an airborne insect (e.g. jumping cricket) and bring it to the ground, using suction force only. Solifuges can also be seen pulling themselves up on smooth glass surfaces, only by their pedipalps, showing the strength of the suction force that the suctorial organ can produce!
Our own African superhero?
Although solifuges occur on all continents except Australia and Antartica, their true diversity is in Africa, but it is really in southern Africa that we have the most spectacular diversity. This diversity, not only in species, but in major taxonomic and ecological groups, forms, colours, shapes, and sizes (Figs. 1, 5), is reflected in the rich collection of words in local languages, each descriptive in its own way, or associated in some way with many believes or myths: jaagspinnekoppe or haarskeerders in Afrikaans, eyambaula-hungi in Oshiwanbo, isibadwa in isiNdebele, selaalii or mmalebelwana in Setswana, bhora mkantsha in isiZulu, to name but a few.
Maybe it is time to create our own African superhero, with an African identity. And an African name.
Bird, T.L., Wharton, R.A. & Prendini, L. 2015. Cheliceral morphology in Solifugae (Arachnida): primary
homology, terminology, and character survey. Bulletin of the American Museum of Natural
History 394: 1–355.
Willemart, R.H., Santer, R.D., Spence, A.J. & Hebets, E. 2011. A sticky situation: Solifugids (Arachnida, Solifugae) use adhesive organs on their pedipalps for prey capture. Journal of Ethology. 29(1):177-80.
Captions to figures:
Solifuge research and Ditsong collections – an afterword
Why study solifuges? We still know surprisingly little about solifuges. The taxonomy of solifuges still needs much sorting, and new species are waiting to be described. Such information is not only crucial for solifuge conservation, but also for conservation in general given i.) the role that solifuges play in ecosystems as top predators, and ii.) the apparent diversification of solifuges in areas that are biodiversity hotspots. The unique aspects of their morphology and physiology could provide insight into evolutionary advances and adaptations in this arachnid lineage. Similarly, structures such as the suctorial organ, the manner in which the cheliceral mill functions, and other potentially yet undiscovered morphological and behavioural features, hold great potential for innovative projects based on biomimicry.
The DITSONG: National Museum of Natural History’s collection provides a valuable resource for the study of solifuges. The Museum has about 1500 solifuge specimens in the collection, many of which are type specimens (the specimen(s) on which the name of a species is based). The collection is especially valuable because of its historic importance. The oldest specimen dates back to 1896. A large percentage of the specimens housed in the collection were collected in the early 1900s, and were examined and are cited in publications authored by historic taxonomists such as Pocock, Hewitt, Lawrence, and Lamoral. These publications, in turn, form the basis of our current knowledge of solifuges in southern Africa, and to an extent Africa. The solifuge collection at Ditsong is therefore an incredibly valuable resource for 1.) taxonomic studies, and 2.) a time-slice that provides us with an insight into changes in species distributions.