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Solifuge from Arizona
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Order:	Solifugae Sundevall, 1833

Solifugae is an order of arachnids known variously as solifuges, sun spiders, camel spiders, and wind scorpions. The order includes more than 1,000 described species in about 147 genera. Despite the common names, they are neither true scorpions (order Scorpiones) nor true spiders (order Araneae). Because of this, it's less ambiguous to call them "solifuges".

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Etymology

The order's name is derived from the Latin "sol" meaning "sun" and "fugere" meaning "to flee". Put together, it means something along the lines of "those who flee from the sun". These animals have a number of common names including sun spiders, wind scorpions, wind spiders, red romans, and camel spiders. In Afrikaans, they are known as "haarskeerders" ("hair cutters"), and "baardskeerders" ("beard cutters"). This is in reference to myths that they cut hair to be used as nest bedding.

Anatomy and physiology

Eyes

Solifuge eyes with bristly setae

Solifuges have a pair of large central eyes known as median ocelli These eyes are oriented at the very front of its cephalothorax and are placed close together. These eyes have a pigment-cup structure and are covered by a domed outer lens made from the animal's exoskeleton. Below the dome is the animal's retina, a multi-tiered structure with a layer of cells called the vitreous body at its top. Underneath is the thin preretinal membrane, acting as a barrier between the vitreous body above and the rhabdomeres beneath. Rhabdomeres are light-sensitive and function as the eye's photoreceptors. Interspersed between the rhabdomeres are pigment cells. The eye's optic nerve begins at its center and is connected to the axons of numerous rhabdomeres.

In addition to the median eyes, solifuges possess a pair of vestigial lateral ocelli. These eyes are found in pits on the animal's cephalic lobes near the chelicerae. The ocelli's lenses are usually atrophied. However, in some species both nerves and pigment cells are present. In species where lateral eyes are functional, they probably aid in detecting motions or changes in light intensity.

Habitat and distribution

Most solifuges live in tropics and subtropical deserts in the Americas, Southern Europe, Africa, the Middle East, and South Asia. Surprisingly, these animals are absent in Australia and Madagascar. Within the desert, solifuges live in a variety of micro-habitats. These include sand dunes, sand flats, floodplains, rocky hillsides, desert shrublands, gravel plains, and mountain valleys. In addition to the desert, certain solifuges live in more arid grasslands and forests.

Depending on the species in question, solifuges may be more sedentary or on the move. Sedentary species are often fossorial, living in relatively permanent burrows underground. Transitory species spend most of their time up the surface, occasionally seeking refuge in cracks or under rocks and vegetation.

Behavior and life history

Diet and hunting

Gluvia dorsalis eating a cabbage bug (Eurydema oleracea)

Solifuges are carnivores and typically generalists, feeding on a wide variety of prey in their given environment. For most species, insects comprise the bulk of their diet. However, these animals have been known to consume anything they can subdue. This includes other arachnids like spiders, scorpions, and smaller solifuges, other arthropods like millipedes, and small lizards, birds, and mammals. Additionally, solifuges are voracious eaters. It's common for adult females to eat so much that they're temporarily unable to walk.

When looking for prey, most solifuges rapidly move about while tapping their pedipalps on the ground. The only exception is the majority of termite-loving species, as they prefer to be more sedentary. In addition to using their pedipalps, solifuges have a variety of methods to locate prey. These include seeing movements with their eyes, feeling with their long hairlike setae, smelling with their malleoli, and sensing vibrations. How much the animal relies on each sense depends on the species. While all hunt on the ground, some species are great climbers, able to search for prey on trees, shrubs, and on artificial structures.

Solifuges hunt their prey using three main hunting strategies: stalking, chasing, and ambushing. Depending on the meal's size, prey is seized with the animal's pedipalps or massive chelicerae. When the pedipalps are used, prey is initially caught with the limb's suction cups, then rapidly pulled towards the chelicerae to be chewed. These motions happen so fast that they can't be distinguished. Before eating, solifuges prepare their food by removing any parts they find unfavorable. In arthropods, these are typically areas that have a high amount of chitin (heads, antennae, wings, etc).

Solifuges eat in different ways based on the shape of their food. Prey that is long and narrow is held perpendicular to the chelicerae and chewed from one end to another. More round prey is chewed by rotating the body all at once. This chewing motion turns the food into a liquidized paste which is then swallowed by the animal's pharynx. Solifuges that haven't fed for long periods are known to eat faster than ones that fed recently. Larger solifuges are also known to eat faster than smaller ones.

Classification and phylogeny

Solifuges are an order of arachnids comprised of over 1200 species in 146 genera assigned to 16 different families. Solifuges can be divided into two groups of families which are recognized as distinct suborders. These are the Australosolifugae which live predominantly in the Southern Hemisphere and the Boreosolifugae which live mostly in the Northern Hemisphere. This phylogeny is considered congruent with a Gondwanan origin for Australosolifugae and a Laurasian origin for Boreosolifugae. When looking at their relationships, the families Ammotrechidae and Daesiidae were found to be paraphyletic, leading to multiple clades without a name. Because of this, a later genomic study established three additional families: Dinorhaxidae, Lipophagidae, and Namibesiidae.

Suborder Boreosolifugae

• Eremobatidae Kraepelin, 1901
• Galeodidae Sundevall, 1833
• Gylippidae Roewer, 1933
• Karschiidae Kraepelin, 1899
• Rhagodidae Pocock, 1897

Suborder Australosolifugae

• Ammotrechidae Roewer, 1934
• Ceromidae Roewer, 1933
• Daesiidae Kraepelin, 1899
• Dinorhaxidae (Roewer, 1933)
• Hexisopodidae Pocock, 1897
• Melanoblossiidae Roewer, 1933
• Mummuciidae Roewer, 1934
• Lipophagidae (Wharton, 1981)
• Namibesiidae (Wharton, 1981)
• Solpugidae Leach, 1815

incertae sedis

• †Protosolpugidae Petrunkevitch, 1953

Phylogeny

Below is a family tree of the various solifuge families based on phylogenomics.

Solifugae	Boreosolifugae    Gylippidae Eremobatidae Karschiidae   Galeodidae Rhagodidae  Australosolifugae    Ceromidae   Hexisopodidae Solpugidae Others

References