The arachnids are the largest class of animals (exclusive of the insects), with 65,000 known species worldwide, and undoubtedly there are another million or so yet detected species. Its members exist nearly everywhere—often in considerable numbers. Most authorities recognize 11 orders with some being relatively rare. This text will discuss several of these orders. Arachnids can be distinguished from other arthropods by a fused head and thorax (cephalothorax); they also have an abdomen and 4 pairs of legs. The first pair of appendages behind the mouth is the chelicerae (chelicera-singular), and the second pair is the pedipalps. The chelicerae and pedipalps vary considerably in structure and function in the different orders of this class. These differences are used to distinguish one order from another.

 Fang-like Chelicerae of a Tarantula and Short leg-like Pedipalps (immediately left of fang). Image Dr. Kaae.


Scorpions have been found in many fossil records, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber. The oldest known scorpions lived around 430 million years ago in the Silurian period. Though once believed to have lived on the bottom of shallow tropical seas, early scorpions are now believed to have been terrestrial and to have washed into marine settings together with plant matter. These first scorpions were believed to have had gills instead of the present forms' book lungs though this has subsequently been refuted.  The oldest Gondwanan scorpion (Gondwanascorpio) is considered the earliest known terrestrial animals from Gondwana.  The eurypterids, marine creatures that lived during the Paleozoic era, share several physical traits with scorpions and may be closely related to them. Various species of Eurypterida could grow to be anywhere from 3.9 inches to 8.2 feet in length.

Scorpions are well-known Arthropods that range in distribution as far north as Canada and to as far south as the southern tip of South America. These creatures are hardy and can survive in extreme conditions including temperatures as high as 115 F, being frozen solid for weeks, total submersion in water for up to 48 hours, irradiation levels many times the lethal limit to humans and lack of food for up to one year. These are amongst the oldest ancestral arthropods having crawled from the oceans for a terrestrial way of life over 350 million years ago. Because scorpions are greatly feared by many humans, one can only imagine if these, ancestral monsters existed today. Imagine the extreme reaction to encountering one of these several foot long scorpions! Their chelicerae are used to cut and chew food while the pedipalps are pincher-like, or chelate, and are used to subdue and hold their prey during feeding.

A California Desert Scorpion with Pincher-like (chelate) Pedipalps.  Note the Small Jaw-like Chelicerae Located in the Front of the Cephalothorax. Image Dr Kaae.

The cephalothorax bears one pair of simple eyes located dorsally near the mid-line of the cephalothorax and several along the lateral margins on each side thus allowing a scorpion to see in all directions at any one time. These eyes are comparatively simple and do not produce precise images.  However, they are quite sensitive to minor differences in brightness (dark versus light and shades) and therefore movements.  As a result, it is very difficult to approach a scorpion without being detected.  As with us, eyes can be dazzled or even harmed by bright sunlight. Scorpions use the same tool we use to prevent this from occurring, namely sunglasses if you will (amazing stuff).  Scorpion eyes contain pigments that migrate toward the surface when exposed to bright light (sunlight) which of course form a protective barrier.  As light dims, the pigments merely drain to the lower areas of the eye.

The abdomen terminates in a five-segmented tail-like structure bearing a bulbous stinger at the tip. The tip of the stinger is very sharp and quite strong with one fourth of its composition consisting of zinc, iron and manganese. Males can be distinguished from females by their larger pair of comb-like structures (pectins) which are located on the underside of the cephalothorax. The function of pectins is unknown, but is thought to be used both for detecting food and the presence of the opposite sex.

File:Scorpion Photograph By Shantanu Kuveskar.jpg

Left.  Five Segment Tail with Sharp Bulbous Sting.  Also Note Fine Receptive Hairs (also called setae) on Various Parts of Body. Image Courtesy of Shantanu Kuveskar  CC BY- SA 4.0.  Right. Comb-like Pectins on Underside of Scorpion.  Image Dr. Kaae     

These creatures have a unique method of locating their prey which consists of any living animal their size or smaller. These include insects, other scorpions and arachnids, birds and small mammals. Scorpions are nocturnal and lack well-developed eyes. Vision is of little use for this purpose. Instead, their legs are equipped with many fine erect hairs that are extremely sensitive to the movement of an approaching prey. When a beetle or any other prey approaches a scorpion, its movement creates two types of waves across the ground.  One type is fast moving and the other is slow. A hungry scorpion stands with its legs spread in an almost circular configuration. As a result their legs essentially point in all possible directions. They can easily detect the direction of a potential prey by detecting which leg is disturbed first by the fast moving wave pulses. Apparently the distance away from the prey can be computed by the scorpion by using the difference in time it takes the fast moving waves and slow moving waves to reach the scorpion's leg. The authors have no idea how a scorpion uses wave mechanics or the laws of physics to find their prey.

When scorpions are viewed under ultraviolet light (e.g. blacklight), they glow.  This is due to structures in their exoskeleton that reflect ultraviolet light.  The exact reason for this phenomenon is not known. However, we also do not know exactly how or what scorpions see at night.  It would make perfect biological sense that scorpions might be able to see ultraviolet wave light and thus another possible means of finding a mate in the dark.  Also many plants emit or reflect ultraviolet light to attract insect pollinator (e.g. bees, moths, flies and beetles).  Many other insects are attracted to ultraviolet light.  This is well illustrated by the fact that entomologists use ultraviolet lights to attract insects when night collecting.  Of course all this adds up to the distinct probability that scorpions reflect ultra-violet light in order to attract these types of insects.

Scorpion Viewed Under Ultraviolet Light. Image Courtesy Jonbeebe at English Wikipedia CC BY 3.0

As with many arthropods, scorpions exhibit external reproduction. When a female is ready to mate, she will deposit a sex pheromone (spermatophore) on the ground. She typically doesn't move far from this location. Any sexually mature male of the same species that randomly encounters this chemical attractant can detect its present by sensory (olfactory) receptors located on the previously described pectins. As a result, the male instinctively is aware that a female is nearby and that she is ready to mate. Actually scorpions and other arthropods are not really aware of anything but are programmed to react to given stimuli.  Once the male perceives this chemical, his mating reaction is activated. At this point, he exhibits a rather violent series of jerky movements-named juddering. As a result of this lurching, seismic waves are sent across the ground thus attracting the female. Mating can get a little rough at this point.  It is not uncommon for the male or female or both to club (not sting) each other with their tail (stinger). The male in some species will actually sting the female in the soft joint between the opposing claws of her pinchers.  This obviously does not kill the female but apparently tends to tranquilize her to point where she is less combative and more receptive to mate. It is well documented that scorpions are relatively immune to the venom of their species. Mating terminates in his grasping her pedipalps and dragging his mate over the stalked spermatophore.

Once mated, it generally takes a year before the female gives live birth to a dozen or more young. Typically the young scorpions crawl up on the back of the female until their first molt. If the young scorpions move from the mother’s back prior to their first molt, they will typically die due to the lack of sufficient moisture.  The mother secretes a liquid that prevents this dehydration.  It is not uncommon to purchase an already mated female scorpion from a pet shop. In captivity, an ill-fed or stressed female will often eat her newborn young. Even if they survive birth and the jaws of their mother, young captive scorpions rarely survive to adulthood because of their precise humidity requirement during molting.

Female Scorpion with Newly Born Young on Abdomen-Yum, Yum!  Public Domain.

It has been postulated that the sex pheromone of some scorpions is not only used to find a potential mate, but it is occasionally used to attract dinner. Apparently some larger species of scorpions will duplicate the sex pheromone of different smaller species.  When males of the smaller species begin juddering, the larger scorpion quickly finds and consumes the one-course meal (fresh scorpion, yum-yum!).

The sting of scorpions is used to immobilize prey and occasionally for defense. The effect of a sting on humans depends primarily on the species of scorpion involved. Typical symptoms vary tremendously ranging to a very mild tingling to some pain and swelling to possible death.  Worldwide, almost all scorpion species are not considered dangerous to humans. Of the more or less 1500 worldwide species of scorpions, only a comparatively few are considered potentially dangerous to humans. The venom of the most toxic species of scorpions is reportedly 100,000 times more toxic than cyanide.  Fortunately little is normally injected. In the United States, the sting of only 2 in the genus Centruroides, can be considered dangerous and rarely result in human death.  These occur across much of the Southern United States down into Mexico. In Arizona, the bark scorpion genus Centruroides is a dangerous species accounting for 75 human deaths from 1926 to 1996 and on a yearly basis far fewer since then.  Most of these casualties occur in children and babies.  This is undoubtedly due to more venom per body weight. There are twice as many deaths from scorpion stings than from the bites or stings of all other U.S. forms of venomous animals (except the honeybee) combined; this includes poisonous snakes. Actually the honeybee causes far more deaths.  One might think the Africanized bee might add significantly to this figure.  However, the regular honeybee is the main culprit.  It is a matter of more exposure to bees than other arthropods and the fact that some individuals are allergic to the sting of a honeybee.  In addition almost everybody has been stung by a bee at one time or another. How many people do you know that has been stung by a scorpion?

Centruroides scorpions have long and narrow pincers and tails and are yellow-reddish in color with two dark stripes running down the back.  The venom is neurotoxic and consequently affects nerve transmission.  The Durango or scorpion (Centruroides)) is found in Durango, Mexico.   Death from the sting of this species can occur in as little 3 hours.  If stung, the site should be iced immediately.  This should slow or prevent spread of the venom. A physician should be consulted immediately.  An effective antivenin is available and if quickly applied death can easily be avoided.  The sting of this scorpion will rarely kill an adult unless some type of allergic exists. On the other hand, stings to small children or babies can be extremely dangerous.

In Mexico, many deaths occur per year from the sting of this genus. This is due to the lack of adequate medical facilities and the openness of many of the houses. In northern Mexico, many of the houses are poorly constructed and lack window glass. These scorpions are climbers and can be found in the walls and ceilings. Homes with thatched roofs are attractive habitats and commonly harbor dozens of scorpions. This type of roof construction is prevalent throughout Mexico. The main victims are babies and young children under two years of age. If a young child accidentally rolls over on one of these scorpions in bed, the parents frequently are not aware of the sting until it is too late.


Centruroides scorpion. Courtesy Charles & Clint - Flickr. CC BY_SA 2.0


Although there are no dangerous species native to California, the Centruroides scorpion has been found here in a few isolated instances. A number of years ago, a species of this genus was found infesting several blocks in Anaheim Hills. Apparently, a pregnant female was accidentally introduced into a recreational vehicle that had been stored for several months along the Arizona side of the Colorado River. This population was eradicated by Orange County Vector Control.

Several species of scorpions are commonly available in pet stores. These include the emperor and imperial scorpions--the former being most commonly sold. The black emperor scorpion is an African species and is the largest species in the world (up to 10 inches or more total length). It is characterized by very large, roughly textured pedipalps.  The imperial scorpion occurs in SE Asia and is more aggressive than the emperor.  Recently, importation of the emperor scorpion has been curtailed in California.

African Emperor Scorpion with Large Rough Pedipalps. Image Dr. Kaae.

This docile species rarely stings and is somewhat reluctant to pinch with its pedipalps. One symptom of a sting is typically a mild, short-lived burning sensation. This symptom is typical very mild because this scorpion is slow moving and does not whip its stinger—this normally results in a shallow pin prick with very little toxin injected before the victim withdraws that part of the body being attacked. However, after handling hundreds of these scorpions with no sting, I recently received a rather deep penetrating sting (careless handling). The result was a painful burning sensation (worse than a bee sting) that lasted for two days. The difference in my reaction to both types of stings may be due to the fact that adult scorpions frequently do not inject their venom with all stings. 

Even though it is very difficult to receive a painful sting from the emperor scorpion, it is unwise to allow the uninformed to handle any scorpion. The human mind is unpredictable and fear can result in real symptoms that are not the result of the scorpion's toxin (e.g. psychosomatic symptoms). The authors recently observed such a reaction at a county fair. A graduate entomology student was handling an emperor scorpion and received a rather minor sting, the type that should result in nothing more than a mild tingling. It became immediately apparent that he was not familiar with the relatively harmless effect of such a sting and panicked. He immediately wrapped a T-shirt around his arm forming a tight tourniquet in attempt to shut off most of the blood flow to his wounded hand. He requested medical attention resulting in fair officials sending for paramedics. He reported pounding of his heart and feeling faint. Being concerned about the possibility of his going into shock prior to the arrival of an ambulance, I decided to try to calm him down. After a short discussion about the nontoxic nature of this scorpion's sting, I allowed the scorpion to sting me several times with no obvious symptoms. I am sure he thought I was crazy but this seemed to give him some relief and by the time the ambulance arrived he was back to normal. Clipping off the very tip of a stinger can eliminate the possibility of being stung by a pet scorpion. This does not hurt the animal and prevents its ability to penetrate the skin.

Warning-Do not let scorpions sting you on purpose—like many other entomologists, I am a little nuts.

Pet scorpions may be kept in small aquariums on a variety of substrates; however, sand is not desirable. Sand is abrasive (sand paper!) and can wear of the waxy outer layer of an arthropod’s exoskeleton-thus causing an increased loss of water from the body and possible death. A water source is necessary, and weekly feedings of a cricket or two will insure survival. Most species takes several years to reach adulthood and generally live a year as an adult. Because scorpions sold in pet stores are collected as adults, they generally live less than a year in captivity.  Emperor scorpions are quite different than almost all other species, and more than one can be kept in the same cage.

A Pile of Emperor Scorpions.

The Imperial scorpion is an Asian species that is also black, smaller than the emperor and has smoother, smaller pedipalps. Imperial scorpions are more aggressive than emperors but still are reluctant to sting; they will pinch when agitated. Generally, scorpions with large pedipalps do not rely on their sting to immobilize prey.  As a result, their venom is not generally very toxic. I had always thought scorpions fed almost exclusively on other arthropods, but one day in Thailand I saw a small chick being dragged down into a wood-pile.  On close inspection, an imperial scorpion had it by the leg and was going to have Colonel Sanders for lunch.

Scorpions and other arthropods are a source of human food in many areas of the world. A friend of ours was conducting a business transaction in a village in northern China. The transaction was going to bring considerable wealth to the village and a feast was prepared in his honor. The main dish was boiled scorpion served in the center of a wooden platter rimmed with bee larvae and fried cicada nymphs.

Scorpion on a Stick. Yum Yum.  Image Courtesy istolethetv CC BY 2.0 International

Scorpions and Modern Medicine. Scorpion venoms are possibly finding some use in modern medicine. The venom of the dangerous death stalker scorpion acts like a smart bomb that seeks out cancer cells.  In laboratory tests scorpion venom, armed with radiation or anticancer drugs may kill cancer cells one at a time, preserving the rest of the healthy tissues.

Scientists at University of California, Irvine (UCI) have isolated and synthesized a chemical (name: TRAM) in the arachnid’s venom that eventually could become a drug to treat autoimmune diseases -- disorders in which the immune system attacks itself.  It has great potential for the treatment of rheumatoid arthritis, multiple sclerosis and lupus. In principle, it should also work on about 60 other autoimmune disorders, but that won’t be known without extensive testing. The compound also holds promise for organ transplant patients, whose immune systems must be prevented from rejecting the new body part. During tests, the UCI researchers found the compound suppresses the immune system's T-cells, which trigger any fight against invaders. (The T-cells are the same cells the AIDS virus destroys.) Like scorpion venom the synthetic compound blocks a cell membrane channel called IKCa1.

Whipscorpions-Vinegaroons. The whipscorpions are mainly tropical and live in the Southern United States. They are less common in California but prevalent from Arizona to Texas. In California, they commonly occur along the California-Arizona border.  The chelicerae are opposing and jaw-like, and the pedipalps are weakly chelate (pincher-like) but quite powerful. The body is elongated, slightly flattened and characteristically bears an elongate whip-like tail. These arachnids are also called vinegaroons as they readily squirt vinegar (a defensive secretion), or acetic acid from their tails if disturbed. A common misconception is that the bite from a vinegaroon will result in a vinegar taste in your mouth. They are easily kept as pets and are handy since you can shake them over your dinner salad-a quick source of vinegar- if I haven’t told you yet, some entomologists are a little unusual-not me of course. I should also mention that my wife Pat does not belong to the former group. I guess she is unusual in that she likes bug, especially spiders.

The secretion from the tail of the whipscorpion has a high content of acetic acid (the main component of vinegar) and low level of caprylic acid. The later reportedly functions to increase the defensive activity of acetic acid.  Acetic acid is water soluble and will “bead up” when on a waxy surface such as the exoskeleton of an arthropod prey. Since the most important predators of whiptailed scorpions are other arthropods, a secretion of acetic acid alone would likewise “bead up” on the exoskeleton thus reducing its effectiveness.  Apparently caprylic acid is more attractive to lipids and actually serves to spread both components over the prey’s body, and enhances penetration into the body.

Left. Image Dr. Kaae Whiptailed Scorpion from Arizona, Mastigoproctus giganteus. Right. Same Species Carrying Egg Sac. Image Courtesy Acrocynus CC BY-SA 3.0

The whiptailed scorpion most often encountered in Arizona is Mastigoproctus giganteus, the largest species in the world.  It measures up to three inches in length not including the tail. These nocturnal animals hide in short burrows in the soil and feed on insects and small animals. Whiptails are long-lived (up to 12 years), harmless to humans and quite docile. They can be easily kept in an aquarium but require a water source and feeding about once every two weeks.

Sunspiders-Windscorpions. This is a moderately-sized group of arachnids with over 120 species in North America. In the United States, most species are found in the desert areas, but some can be found throughout the southern section of the western states. They are one to two inches long, usually pale in color, hairy and have a waist or constriction (petiole) connecting the  cephalothorax to the  abdomen. The chelicerae are characteristically very large and fang-like and can be as big as the cephalothorax. These arthropods are commonly mistaken for vinegaroons.

A Sunspider or Windscorpion from Southern California. Image Dr. Kaae

Although sunspiders can be quite aggressive, they lack venom and are not considered dangerous to humans. The pedipalps or second pair of appendages behind the mouth are leg-like, quite elongated and used as feelers. These nocturnal arachnids have been called by a variety of names including sunspiders, camelspiders, sunscorpions and windscorpions. The latter name refers to their ability to "run like the wind." As one might expect, they are predatory on a variety of smaller animals including lizards, mice and insects.

These arthropods are difficult to keep in captivity and typically die a few months after capture. A very large Egyptian species occasionally is available in pet shops. This species can live in captivity for several months. They require daily feeding and considerable space to move.

There are a number of camel spider stories that have recently spread on the internet.  These began to appear during the 1990-91 Gulf War and have now reemerged and become even more widespread with the return of U.S. troops in Iraq.  Almost everything on the web (emerging from Iraq) about the size, ferocity and danger of these arthropods is untrue.  Some of the fallacies are listed below:

1. Camelspiders can move at speeds over 30 MPH, screaming while they run.

2. Camelspiders can be as large as a Frisbee.

3. Camelspider venom is an anesthetic that numbs their prey.

4. Camelspiders can jump three feet high.

5. Camelspiders get their name because they crawl into the stomach of camels and eat them.

The common camel spider in Iraq is somewhat larger than the species in southwestern U.S. (approximately three inches in length).  The picture commonly found on the web is a gross exaggeration of this species. A friend of mine worked in the medical corps during the Iraq War.  He indicated they pose no danger to the troops or camels but there have been a few troops were bitten which was basically small puncture wounds with no venom.  These bites commonly occurred when the troops attempted to set up bet on fights between these arachnids with true scorpions.  For some reason, most of the troops that participated in this came for the Deep South of the U.S. They don’t jump (the camel spiders not the troops) and can possibly run a short distance at a speed of a mile or two an hour.  And of course they don’t eat camel stomachs.

Camelspider from Iraq.  Size is Greatly Exaggerated (really about three inches) by Angle of Camera.

Harvestmen-Daddy Longlegs. This group of fairly large arachnids is commonly confused with the daddy longleg spider.  Harvestmen differ from this spider in that the cephalothorax and abdomen are broadly joined.  In the spiders there is a narrow waist or petiole between these two areas; also, unlike this spider, the abdomen is segmented in harvestmen. The most notable features of many harvestmen are the long, slender legs. Their eggs are spherical, about 0.4 mm diameter with a smooth surface and a color change from off-white to dark gray-brown as they mature. Eggs are laid in leaf litter and other suitable locations and in clusters ranging from ten to several hundred.

The daddy longleg spider is typically found in most homes in the United States. Of course, these two can be easily differentiated by a close inspection of their anatomical differences, but more simply by the presence of webbing. Harvestmen do not spin webs. The chelicerae of harvestmen are fang-like, but these arthropods are not strictly predators; instead they are scavengers feeding on dead and living plants, dead and living insects and a variety of other materials.

It is not uncommon to find large numbers of daddy longlegs congregated in a relatively small space. It is possible that these congregations are associated with mating. It has been reported that when a large group is contained in a closed space, they have a fatal effect on one another. This may be associated with a chemical that they release to repel predators. Perhaps in close quarters, a build-up of this chemical is too much for their systems to bear.

It is commonly believed that the bite of a harvestman is toxic to humans. Actually, the chelicerae of harvestmen are quite small, and an attempt to bite would not penetrate the human skin. Also there are no records of this venom being toxic to humans. It is possible that this belief stems from the fact that some harvestmen prey on black widows, and because the black widow is known to possess one of the more toxic spider venoms, it would logically (or illogically) follow that a daddy longlegs would need toxic venom to subdue their notorious prey. 20100626.jpg

Various Harvestmen Including an Aggregation. Images from left to Right Courtesy jovengandalf CC BY-SA 2.0,  Luis Fernández García Creative Commons BY-SA 3.0.  Dschwen Creative Commons SA 2.5. Left species is a red devil harvestmen from South America, a Predatory Species

Whipless Whiptailed Scorpions. These arachnid belong to the order Amblypygi. They are active predators that are found in caves, deep leaf litter and similar situations, especially in tropical locations. As seen in the provided image, their pedipalps are modified for capturing and retaining prey-note large spines. The first pair of legs function as sensory organs and are not used for walking. The sensory legs are very thin and elongate, have numerous sensory receptors, and can extend several times the length of body. Amblypygi have eight legs, but use only six for walking, often in a crab-like, sideways fashion. The front pair of legs is greatly elongated and serve as “feelers” for finding their prey as they lack compound eyes. When a suitable prey is located with these elongated front legs, they grasp its victim with the long spines on the grasping pedipalps. They then chew up and consume the captured prey with their pincer-like chelicerae.  Depending on species they vary considerably in size with larger specimens reaching a 28 inch leg span. As gruesome as they look, they are considered harmless to humans

Courtship involves the male depositing stalked spermatophores, which have one or more sperm masses at the tip, onto the ground, and using his pedipalps to guide the female over them. She gathers the sperm and lays fertilized eggs into a sac carried under the abdomen. As with true scorpions, once the young hatch, they climb up onto the mother's back; any which fall off before their first molt will not survive. These predators mostly feed o a variety of arthropods, but larger specimens are capable and do on occasion fed on vertebrates such as small mice

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Whipless Whiptail Scorpions. Left Image Courtesy Graham Wise Creative Commons BYSA 2.0,


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