Imagine a spider-like animal, with jaws up to a third the length of its body that runs so quickly and erratically that the human eye easily mistakes it for a seed blowing in the wind. Such creatures exist all over the world in dry environments including the savannas, steppes, and deserts, of Africa, the Middle East, central Asia, the southwestern United States, Mexico, and South America. Termed solifuges, but also known as camel spiders, sun spiders, wind scorpions, or baardskeerders (beard-cutters), these arachnids form a separate order from spiders and scorpions. Unlike most of their arachnid counterparts, solifuges carry no venom and are basically harmless, belying urban legends claiming otherwise. Members of the order Solifugae have several unique features, such as the racquetshaped sensory organs on their hind legs that detect vibrations or chemical cues. Their most prominent characteristics are their oversized jaws, called chelicerae. However, until my colleagues and I embarked on a ten-year project to survey these fascinating arachnids, no comprehensive overview of solifuge jaw morphology existed.

Solifuges are challenging to study. Most avoid sunlight (indeed, the Latin term “Solifugae” means “those that flee from the sun”). Many species are solely nocturnal. Others come out only during the daytime, usually during the hottest time of the day, sometimes higher than 35°C in deserts. When they do emerge, they move quickly, at speeds up to ten miles per hour. Larger species may measure 15 centimeters long and are easier to spot, but the small species— some only a few millimeters long—are often overlooked. They are also seasonal, abundant only in warm weather, and their lifespans are brief. Finally, their chosen homes are not conducive to research; these hot, dry places tend to be dangerous for humans and pose many logistical challenges.

Despite these hurdles, a smattering of adventurous naturalists over the years have collected and written about roughly 1,100 solifuge species. Unfortunately, these efforts have not been coordinated, and each discoverer has honed in on different characteristics of their specimens, using their own terms to describe their observations. At times the terminology has even been contradictory. For example, some adult male solifuges have two flagellar structures on their jaws, an upper and a lower. Some scientists have used the word “flagellum” to refer to the upper structure, some to the lower structure, and others to both structures together. Multiply such inconsistencies by the number of different features on these complex arachnids, and the confusion quickly escalates. Without a common language, it can become impossible to classify an individual specimen as belonging to one species or another. Lacking this basic starting point, more in-depth studies of solifuge biology cannot proceed.

To address this problem, my colleagues and I set out to define and catalog the different parts and protrusions of solifuge jaws. The jaws have a variety of functions: capturing prey, feeding, fighting, defending, burrowing, mating, and producing sound. Within each of these categories, the jaws may play several roles. For instance, in mating, the jaws might subdue or hold a struggling female, prepare the female for mating, or transfer sperm to the female. In order to serve this wide array of functions, the jaws contain an unusually high concentration of distinctive features, and are therefore the most important section of solifuges’ bodies to be used for their classification.

There is also a clear division between the main function of the jaws in females and males. Females mainly use the jaws for feeding, crushing their prey to a liquid mush they can suck up. Males also use the jaws for feeding, but they tend to eat less than females and do not live as long. With few exceptions, solifuges are generalist predators. Jaws of different species are thus similarly adapted for feeding, and tend to be similar between females of different species. Mating behavior, on the other hand, is more species-specific and most modifications of the jaws are therefore apparent in the males. Hence features of adult male solifuge jaws are crucial for species identification.  

When trying to decide whether structures on different individuals and species are the same or different, scrutinizing actual specimens—rather than reproductions—is necessary to assess the structures’ size and exact position. For our study, we thus needed specimens representative of the major taxonomic groups within the order. We chose Southern Africa as our starting point: globally, it has the highest diversity of solifuges per land area, and the largest scientific collection of solifuges in the world is curated at the National Museum of Namibia. We also went on several expeditions to observe and collect solifuges in such places as Israel, in the harsh climate near the Dead Sea, and in Kazakhstan, where co-author Lorenzo Prendini was stung by a highly venomous buthid scorpion. We scoured the collections of sixteen major museums and scientific institutions across the United States and in Israel, Turkey, France, Belgium, South Africa, Namibia, Germany, and Sweden. We were able to examine 510 specimens representing 188 species with examples from each of the twelve solifuge families, and major taxonomic groupings therein. Additionally, we reviewed roughly 700 publications in English, German, Spanish, French, Italian, Russian, and even Latin, discussing solifuge morphology, systematics, and behavior, going back to the late 1700s.

Using these resources, we developed a dictionary of nearly eighty terms with accompanying images to describe the different parts of solifuge jaws. Structures with similar appearance and position on different specimens were given the same name. Our hope is that all current and future researchers will use these terms as a common language when writing and speaking about solifuges.

Some of the terms we set forth were already in use. For example, most recent scientists agreed upon the general term “cheliceral fingers” to refer to the opposing sections on the far end of the jaws on which the teeth are situated. But we also chose a single appropriate term for structures that went by various names. For instance, the mucron— the usually toothless far sections of the cheliceral fingers— has previously been called the terminal fang, “toothless terminal portion” of the jaw, and untoothed finger.

Moreover, we identified several new structures that have never before been described. We named one such structure the mucron organ, which is a small, round structure, paler than the surrounding cuticle that appears on the mucron of the jaw. While we observed the mucron organ on many species, its purpose is not known. We also found larger round-to-oval depressions situated in the lower part of the jaws. Although visible in various published images of solifuge jaws, they have been readily dismissed as slight abnormalities in the surface of the cuticle, or perhaps as cuticular damage. Only after looking at many specimens did we appreciate that these “irregularities” were fairly consistent. We hence referred to them as the medioventral organ, but, as with the mucron organ, their function remains unknown.

Beyond streamlining the terminology for parts of the jaw and identifying new structures, we proposed novel hypotheses about functions of known structures. For example, mole solifuges belong to a small, burrowing family of solifuges found only in southern Africa. Their unique morphology is clearly associated with their behavior of “swimming through the sand.” All male mole solifuges have a structure on each jaw that we termed the “prolateral dental process.” Previously interpreted simply as a tooth, this structure hampers or prevents another important structure, the flagellum, from rotating past a certain point when manipulated, thus functioning as some kind of stopper to flagellar movement. Such a stopping mechanism can be very useful in preventing damage to the flagellum during burrowing.

We also drew novel conclusions about the evolution and behavior of solifuges. For instance, each of the two jaws present in a solifuge contains a smooth area, often with well-defined ridges. These areas on the inner sides of the jaws are known as the stridulatory apparatus; when rubbed together, they produce a sound, which seems to be amplified in some species by modified hairs associated with these areas. The function of this sound—and whether the stridulatory apparatus plays a role other than noisemaking— has been unclear. Based on our review of the literature, we believe the stridulatory apparatus serves two roles. The first is in chewing. Prey can be crushed between the ridges of the stridulatory apparatus. Female solifuges eat more than males, and the stridulatory apparatus tends to be more prominent in females than males, supporting its use in chewing. The second role is to produce sound for defensive purposes. For example, solifuges may imitate the sound of other, more dangerous species with whom they share a habitat, such as vipers. These dual functions raise the question of which evolved first: did a chewing apparatus later become useful for defense, or did a defensive apparatus become useful for chewing? At this point, we don’t have enough information to say, but we have laid the foundation for testing these hypotheses in future research. 

One evolutionary question we were able to answer concerns the origins of the flagellum, an appendage on the inside of the jaw of adult male solifuges. Its appearance varies widely across species. A partly rotatable, spoon-shaped flagellum is characteristic of the family Galeodidae, found from North Africa to central Asia. The similarly distributed but more robust, short-legged Rhagodidae have two flagella which overlap in such a manner that they form a halfmoon- shaped tube. 

A typical whip-like flagellum is long and slender; depending on the family, this type of flagellum is either fixed immovably to the jaw (as in the largely African family Solpugidae) or is able to make a half-circle rotation around an axis at the base of the flagellum. Even within the Solpugidae, what seem to be secondary modifications include, for example, the claw-like flagellum typical of one of its genera. Irrespective of these modifications, all whip-like flagella are unified by the presence of two canals that run along the length of the shaft of the flagellum: one blind-ended canal that seems to connect to the hemolymph in the jaw and which might function to extend or rotate the flagellum, and a second canal containing a fluid of unknown function and which opens to the outside at the apex of the flagellum.  

Some flagella are membranous, such as the non-rotatable bowl-shaped flagellum of the New World family Ammotrechidae, or the rotatable flagellum varying from husk- to bowl-shaped in most Daesiidae, a speciose family with a wide distribution in both Old and New World continents. In other families, such as the North American family Eremobatidae and the southern African family Melanoblossiidae, the flagellum is less distinctive and strongly resembles the setae, or stiff bristles, from which it is presumed to have evolved. This diversity makes it seem plausible that different species’ flagella are not in fact the same structure, and could have evolved from multiple distinct origins.

Rather than looking at all species as a group and seeing only the diversity of flagella, we considered species one by one and noticed strong similarities between flagella structure and position on pairs of species. This approach enabled us to line up all the species such that the flagellum changed very little from one species to its neighbor. For example, the two seemingly disparate categories of a whip-like and a bowlshaped flagellum are bridged by the unique flagellum of the daesiid Ammotrechelis. This species’ flagellum comprises an elongated shaft, resembling the shafts of the whip-like flagella, complete with two canals, and also a broad, membranous base, resembling the bowl-shaped flagella. We found similar links to connect the flagella of all solifuge species. By clearly defining the components that form a flagellum and meticulously examining it on many species, we hypothesized that the flagella are homologous: they share a single origin on the common ancestor of all solifuges.

The new insights we have gained about solifuges are only a sampling of the sorts of questions it is now possible to explore. Much remains unknown about these arachnids’ ecology. Armed with our atlas of terms to describe the various jaw structures and classify species, researchers can more systematically study how each body part functions to fill different needs. For instance, we now know which structures constitute flagella—but what do they do? It is clear that they contribute to reproduction, but how, exactly? Some reports suggest that they may play a role in copulation, such as sperm retention and transfer, or species- specific mate recognition. The structural differences of flagella across families, or even species, likely relate to functional differences. In the family Ammotrechidae, the male has been observed to place sperm in its bowl-shaped flagellum, which is then used to transfer the sperm to the female. In the family Solpugidae, the jaws themselves, not the flagellum, have been observed to transfer sperm to the female. However, in this family the male’s long, whip-like flagellum is inserted deeply into the female’s reproductive tract post-sperm transfer. It may deposit the contents from one of the flagellar canals, perhaps to facilitate fertilization in some way. The Eremobatidae family takes yet another approach: they transfer sperm directly, and the jaws—which still participate in other aspects of mating— are not involved in sperm transfer. It is probably no coincidence that the flagellum in this family, when present, is less elaborate than in other families, instead forming part of a cluster of modified setae. These bristles may detect whether the female is ready for sperm transfer before insemination, or whether sperm was actually transferred to the female post-insemination. More measurements of flagella and the reproductive tracts of different species, as well as observations of mating and other behavior, are needed to determine whether and how the flagella facilitate mating, or perhaps serve other functions.

In addition to defining terms, our initial goal, we were also able to analyze the features of the chelicera, and break down features of sexual dimorphism. Most features associated with feeding and defense are situated on the inside surfaces of the jaws, which show the least differences between species, but also between sexes. Parts of the jaws that seem more closely associated with reproduction are the outer sides of the jaws, which in males nearly always are clothed with a diversity of hairs and setae of various levels of robustness and shape, and the teeth and upper finger of the jaw, which differ in shape in males of different species and on which the flagellum is also situated.

Although at the end of the study, we were left with even more unanswered questions, we hope that our atlas and our findings will form a base for other studies, and that they will stimulate research into the mysteries of these animals, which can now be communicated with a unified terminology.