Arachnids: primeval survivors of an ever-changing world

Under ultraviolet light scorpions fluoresce. It is not well understood why they do this.

With their origins tied deep into the ancient oceans of the Cambrian, arachnids have come a long way. Modern arachnids have diversified into many different families. Most people are familiar with at least some: spiders, scorpions, ticks and mites. Some people have never even seen others like the psuedoscorpions and some are just grouped with spiders with people not seeing the uniqueness of families like solifuges and harvesters.

Gasteracantha, also known as a kite spider. These spiders can reach phenomenal densities in wooded areas, the amount of insects that are caught per day in an area of a square kilometer must be astounding.

The arachnids are invertebrates that belong to the phylum Arthropoda and the sub-phylum Cheliceriformes which includes horseshoe crabs, spiders, ticks, mites, scorpions, solifuges and other groups that are not as well known. The sub-phylum Cheliceriformes once had a member known as Pterygotus buffaloensis which attained a length of close to three meters. This now extinct animal used to inhabit the oceans, in fact this is where the phylum has its origins, the ancient oceans of the Cambrian. [i] That points to an emergence about 485 million years ago. Today the majority of the sub-phylum Cheliceriformes are terrestrial in habits. The aquatic forms include sea-spider and horseshoe crabs. On land the Cheliceriformes have adapted to many situations and lifestyles and there are many families.

Two Uroplectes triangulifer, these small scorpions are very common in the veld around Johannesburg.

Platyoides also known as scorpion spiders are bizarrely shaped, an adaption for living under bark and stones.

Just to give you an idea of  diversity of the phylum chelicerata there are estimated to be  “7,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species”. [ii] This kind of diversity is only rivaled by the Hexapoda (insects and their close relations).  There is just so much out there.

Platythomisus, a bright coloured spider that has made its retreat amongst some dead blades of grass.

Anyphops, Flat Wall Spider or flatties as the are sometimes affectionately called are always a nice surprise to find under rocks or in the house. They are able to run forwards or sideways very quickly.

With this post I’d like to introduce those that are unfamiliar with arachnids to these amazing creatures and to those who are already familiar with them I hope you can enjoy the pictures. I will look exclusively at arachnids here, starting with the most familiar arachnids – spiders.

 

Ceratogyrus darlingi, the Rear Horned Baboon spider. These spiders get relatively big and when found moving around at night like this one, it is often a male in search of a female. When he finds a female, he will perform a complicated courtship ritual that involves drumming and touch, she will then allow him to mate with her. He may or may not be consumed by the female, but he is doomed to die anyway. Once he has mated he will not feed again and will not live very long. Male baboon spiders do not live as long as females which are able to live up to 15 years.

 

Stegodyphus, a small spider that has adapted to live in a community made up of others of the same species. Most spiders are solitary, but these Community Nest Spiders are one of the handful of species that are not.

The "nest" of the  Community Nest Spiders is built up overtime and they tend to accumulate the exoskeletons of their consumed prey.

There is some ambiguity as to when exactly spiders emerged, some estimates say about 400 million years ago but small soft bodied arthropods are not easily fossilized. The oldest fossil of a spider is from 374 million years ago and is called Attercopus fimbriunguis, and it is said that when the fossil was being cleaned and prepared a silken thread was still attached to the spinnerets. [iii]

Idiops, a large Trap-door Spider. These spiders are considered to be closer to primitive spiders in shape and habits than those that build webs. It is important to remember that they are not actually primitive in anyway.

Today spiders are commonly split into two groups. The Mygalomorphae (usually live in silk-lined burrows or other retreats) and Araneomorphae (web spinners). The Mygalomorphae include New World Tarantulas, Bird-eaters and Baboon Spiders.  Araneomorphae are considered more modern and are far less limited in where they can live. Members of the Areneomorphae are commonly found in houses and have colonized the entire world except for the continent of Antarctica. 

Scytodes, the Spitting Spider is unusual among spiders and only has six eyes rather than eight. They use a sticky glue type of venom to catch their prey. In this image the sticky venom can be seen in the spiders jaws.

One of the reasons spiders have been so successful is through the ability to produce silk and spin webs. This allowed them to exploit a biological niche and capture insects that had begun to take flight. A theory on the development of web spinning is outlined in Leroy and Leroy’s Spiderwatch:

“Webs might have developed from silken lines that radiated from the mouths of burrows or retreats, which these early spiders, like their modern counterparts, probably laid down as they moved around. These silken trails would have been similar to the slime trails of slugs and snails, and could have enabled them to find their way around and, because they probably contained pheromones, to recognize one another by scent. At the same time, they might have snared crawling insects and, as the millennia passed, the spiders with the most elaborate and efficient silk trails would have caught more insects, eaten better, bred more successfully and evolved increasingly more efficient webs” (11-12).

It is interesting to think of the spider’s web as an extension of its sensory organs and not just as a device for capturing its prey. When the spider sits on its web in anticipation of potential insect prey it is spreads its senses over the entire area the web covers and is made immediately aware when something is caught in the web, from the impact it can tell the size and even the location in the web were the impact occurred. 

Nephila senegalensis, the Golden orb-webbed spider feeding on a locust she has caught in her web. On the ventral side of the spiders abdomen is the tiny male. It is waiting to eat some of what the huge female has caught. These spiders can produce more than 300 meters of silk daily and their webs are strong enough to catch small birds, which the spider will proceed to eat like any other meal they catch in their web.

Silk in spiders is produced as a liquid in the cells of the silk glands. The liquid silk changes to a solid form as soon as it is pulled out from the spinnerets. Spiders can produce a lot of silk in short periods of time. The Golden Orb Web Spiders (Nephila species) can produce more than 300 metres of silk per day. [iv] Spiders also recycle silk by eating it and it has been found that 80-90% of an old web is reused in new webs. [v]

Mexcala, the ant mimicking jumping spider does a good impression of an ant. They have become ant-prey specialists with a prediliction for ants of the species Camponotus cinctellus. To think how this spider came to favour and mimic ants of this specific species is just one of things in nature that bends ones mind.

The next group I would like to look at is a lesser-known group that is often mistaken for spiders. These are the harvestmen or harvesters of the order Opiliones. They superficially resemble spiders and have long legs and a very round body that is not distinctly divided into an abdomen and cephalothorax. These animals are commonly found in leaf litters of forested areas and are not very often seen.  Harvesters are generally predatory and eat mites and aphids, some are scavengers and some eat rotting plant material.

Opiliones, the Harvestman. Over 6 500 species have been described globally. This particular one was photographed in Amsterdam. Picture courtesy of Francis Burger.

The next arachnid order I would like to share is Amblypygi, commonly called whip scorpions but more "correctly" should be called Whip Spiders. These are large flat arthropods get their name from the large sensory “whips” which are sensitive to smell, touch and environmental factors such as heat and humidity. [vi] These whip spiders have powerful modified pedipalps with spines which they use catching their prey which is generally insects and other arthropods, but they have been known to catch small geckos and frogs. [vii]

Damon variegatus, the Whip Spider. These strange looking arachnids can be found in the moister and warmer regions of Southern Africa. In some areas they are very common in dark, moist spaces, sometimes with high densities of individuals inhabiting the same space.

These whip spiders can move suddenly in any direction and their only defense when threatened is to scuttle into the nearest crack. There is something about their creeping movements that unnerves people. They are, like most arachnids, completely harmless and inoffensive and can be handled safely. I have also heard that they make interesting pets.

Pseudoscorpiones are very small arachnids. This individual was photographed in my hand. 

Another overlooked order of arachnids are the Pseudoscorpiones. These are very small and easy to miss. There are said to be around 2 000 species worldwide and 135 species have been found in South Africa. Psuedoscorpions look like scorpions but don’t have a tail. They do have venom though: on the pedipalps (pincers) there is a small joint with venom gland and a “sharp tubular tooth” with which they can inject venom into their prey. The pseudoscorpions prey predominantly on mites and one species Chelifer cancroides has become associated with human dwellings and come to hunt mite and fish moths in book shelves (this is where the common name book scorpion comes from). [viii]

Like scorpions, Pseudoscorpiones have pedipalps modified into pincers. They lack the tail but do possess venom which is located in the modified pedipalps.

Scorpions are the next order of arachnids I’d like to look at. Southern Africa has a high diversity of scorpions. Of the 1 500 species found worldwide there are said to be more than 130 in Southern Africa. [ix] Scorpions can be found nearly anywhere in Southern Africa and there are three families that occur in South Africa: Buthidae, Ischnuridae and Scorpionidae. Of these, some members of the Buthidae are medically important. 

Opistophthalmus pugnax. These scorpions are a burrowing species and can be found in the veld and koppies around Johannesburg. The name pugnax comes from the Italian word pugno and has combative associations.

Parabuthus transvaalicus, the Transvaal Thick-tailed Scorpion is a formidable creature. Their venom is considered medically important and they are also known to spray venom at attackers. This is a good reason to shake out ones clothing before putting it on in areas where these scorpions occur in high densities.

The Buthidae that are likely to be encountered in Southern Africa come from three genres: Hottentotta, Uroplectes and Parabuthus. These scorpions all have thick tails and small pincers. All individuals from these groups can deliver painful stings and stings from large Parabuthids have resulted in a few deaths. That said, the strength of the venom in these animals is often exaggerated. Many people are stung by full-grown Parabuthus transvaalicus scorpions and although they experience a lot of pain and discomfort suffer no adverse symtoms.

Hottentotta trilineatus, this scorpions are also considered to have very strong venom. A member of this genus from India
Hottentotta tamulus is considered the most lethal scorpion in the world. As with most scorpions, fatalities are usually children and those whose health is compromised in other ways.

Uroplectes flavoviridus.

The Ischnuridae family is represented here by members of the Opisthacanthus and Hadogenes genus. These scorpions are characterized by small tails and large pincers indicating that they have relatively weak venoms and mainly rely on their pincers to overpower prey. These scorpions are generally incredibly docile and reluctant to sting in self-defense.

Hadogenes troglodytes, these are scorpions associated with rocky areas. This species of scorpion can grow up to 21cm long, making it the longest scorpion in the world. They are very docile and inoffensive.

Opisthacanthus, the massive pincers and small tail indicate that this species does not have a potent venom. They are also docile scorpions that tolerate gentle handling.

The Scorpionidae family is represented by one genus in Southern Africa, the endemic Opistophthalmus genus. These are burrowing scorpions and use their powerful pincers to crush their prey. When harassed these scorpions make a hissing sound by “rubbing the stiff bristles on their mouthparts against the underside of their carapace”. [x] These scorpions can deliver painful stings, but are considered harmless. Some species in the genus are very colourful.

 

Opistophthalmus glabrifrons, this large female was moving around at night. She was gravid and very defensive.

Opistophthalmus capensis, this scorpion is in a heightened state of agitation, its pincers are ready to grasp at anything within reach and its venom gland has already begun to secrete a droplet of venom.

Scorpions are one of the few arthropods that actively care for their young. They give birth to live young which emerge from the mothers body one at time and shortly after being born they climb onto the mothers back. The young scorpions stay on their mother’s backs until their first moult, once they have moulted they move away from the mother and make their way into the world on their own, ready to fend for themselves.

Opistophthalmus pugnax with young on her back. It is fascinating to see these animals with their young on their backs. The young are tiny replicas of the adults.

Due to their attractive appearance scorpions have become very popular and many people have developed a keen interest in them. There are special UV torches that can be bought to find scorpions at night and there is a very good reference book available on scorpions called Scorpions of Southern Africa by Jonathan Leeming. There is an initiative that has been created by the Animal Demography Unit at the University of Cape Town called Scorpion Map. This map allows everyday people to upload images of scorpions along with other data onto a database which is beginning to catalogue and map the scorpions of Africa. I recommend that anyone with an interest in scorpions gets involved in this project (see http://vmus.adu.org.za).

These trombidiidae mites are parasites on this scorpion. They attach themselves in a way analogous to ticks on bigger animals.

The next family of arachnids that I’d like to look at are mites and ticks, sub-class Acari. Mites are minute arachnids and 40 000 species have been described globally. It is said that this is probably one tenth of the actual species number so there could be 400 000 species of mites out there. [xi] Mites are everywhere, they are said to exist in densities of millions per square meter “all over the inhabited earth” and “even on our skins there are permanent populations of mites”. [xii]

This large free living mite was photographed in Johannesburg. I have not found a satisfactory identification for it. I assume it is predatory due to its size.

Mites come in so many different forms. There are predatory mites, parasitic mites, mites that feed on plants, and mites that are decomposers. Mites cause some medically important conditions, most notably scabies in humans and mange in animals. The mites pictured here are parasitic mites of the family Trombidiidae. The adult is a large free-living mite.

Trombidiidae or Velvet Mite has attached itself to this gecko. Photograph courtesy of Francis Burger.

Ticks (order Ixodida) are well known because of their ability to infect people and animals with disease. They are blood-sucking parasites and have been around for about 120 million years. [xiii] Ticks have an interesting life cycle, the female lays up to 3 000 eggs. When the larva hatch they climb up the nearest vegetation and wait for passing hosts. I have been host to this stage of tick development and one day pulled over forty tiny ticks from my body. These larva ticks are commonly known as ‘pepper ticks’ due to their small size. Once they have fed and are ready to grow the ticks moult into nymphs and will either reattach to the same host or find a new host. At the next moult they will drop off or reattach, depending on the species. It is through this dropping-off and reattaching to a new host that zoonotic diseases are spread. In southern Africa many diseases are spread by ticks, these include the livestock diseases of Heartwater, Redwater, East-coast fever, Gall fever, African swine pest, Billary and the human diseases Congo fever and Tick-bite fever. 

This Amblyomma tick has attached itself to this tortoise. Big reptiles such as monitors, tortoises and snakes are often host to ticks, sometimes they can become quite infested and their conditions deteriorate.

Although ticks are disliked because of their disease spreading qualities, it must be kept in mind that disease plays an important role in the environment. It strengthens the gene pool by eliminating weaker animals and also kills weaker competitors in times of environmental stress (drought) thus favoring the stronger individuals and thereby strengthening the population.

Diurnal solifuge, this animal paused for a moment long enough for me to get a photograph and was off again. The colours on this species are very striking.

The last group of arachnids I would like to look at, and these are perhaps my favourite group, is the order Solifugae, These are known by various popular names: sun spiders, red romans and even camel spiders (this is a popular name given to these animals by Americans in Iraq, there is a myth that these spiders burrow underground and feed on sleeping camels from below). There are some interesting Afrikaans names for solifuges, the ones that stand out for me are vetvreters (fat eaters) and baardskeerders (beard cutters). [xiv]

This nocturnal solifuge was photographed while consuming a bolus of chewed up invertebrate. One can see the abdomen beginning to become distended, solifuges will gorge themselves until they cannot move, this is the source of the Afrikaans name vetvreter (fat eater).

When looking at a solifuge one is instantly drawn to the size of the jaws. There is a jaw on each side and “each jaw consists of two parts, a solid upper jaw and movable lower jaw, moving past each other like the blades of shears. The large lower jaws house the powerful muscles… Both jaws are equipped with strong chitin teeth”. [xv] 

Solifuges bear a superficial resemblance to spiders, but, amongst other differences, they lack venom glands and the ability to produce silk.

Solifuges move fast and run with their front legs outstretched feeling their way around. They are predatory (although they are known to be scavengers) and gorge themselves when they eat. Solifuges are very active and they are always doing something, making them interesting to watch. I have sat watching them dig holes for a long time, moving like little bulldozers until the hole is very deep. I have also followed hunting solifuges and they seem to never rest, checking every little space for prey items.  Solifuges can also bite humans and the bite can be likened to a powerful pinch, but they are completely harmless to people and their livestock.

Solifuges are covered in sensitive sensory hairs, they use these to detect fluctuations in their environment, especially the movements of potential prey items and the bigger vibrations of predators.

According to Holm and Dippenaar-Schoeman there are 240 species in Southern Africa. There is not much information on these animals available at a popular level, but Holm and Dippenaar-Schoeman (2010) give a good overview of the families that occur locally.  I try to photograph every solifuge I can find and have found them to be incredibly diverse. This is definitely a group I will be looking at more closely in the future.

Solifuges come in a variety of different shapes. This species has a smaller, slimmer build with long legs, perhaps this is an adaption to an arboreal lifestyle.

With this brief overview of arachnids I hope to have inspired some of you to take a closer look at these interesting animals we share this world with. When you see a spider moving across a wall or a tick on your pet, think about how old these creatures are and how long they have been on the earth and how long they will continue here compared to our fleeting stay.

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[i] Brusca, R. C and Brusca G. J.  2003. Invertebrates. Sinauer Associates, Inc.: Sunderland (653-654).

[ii] http://en.wikipedia.org/wiki/Chelicerata

[iii] Leroy, A and J. Leroy. 2000. Spiderwatch in Southern Africa. Struik: Cape Town.

[iv] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (158).

[v] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (159).

[vi] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (77).

[vii] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (77).

[viii] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (81-83).

[ix] Leeming, J. 2003. Scorpions of Southern Africa. Struik: Cape Town (42).

[x]  Leeming, J. 2003. Scorpions of Southern Africa. Struik: Cape Town (66).

[xi] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (85).

[xii] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (85).

[xiii] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (103).

[xiv] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (142).

[xv] Holm, E and A. Dippenaar-Schoeman. 2010. Goggo Guide: the Arthropods of Southern Africa. Lapa Publishers: Pretoria (144).