CHAPTER 3
Arachnids including Scorpions, Sunspiders, Vinegaroons and Harvestmen
The
arachnids are the largest class of animals (exclusive of the insects), with
65,000 known species worldwide and, undoubtedly, another million or so undescribed
species. Its members exist nearly everywhere—often in considerable numbers.
Most authorities recognize 11 orders with some being relatively rare. This text
will discuss seven 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, while the second pair is the pedipalps (Figure 3A). The
chelicerae and pedipalps vary considerably in structure and function in the
orders of this class and these differences are used to distinguish one order
from another.
Figure 3A.
Fang-like chelicerae of a tarantula and short leg-like pedipalps (immediately left
of fang).
Scorpions. Scorpions are well known animals that
range in distribution as far north as Canada and 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 three foot
long scorpions! Scorpion chelicerae are used to cut and chew food while the
pedipalps are pincher-like, or chelate, and used to subdue and hold the prey
during feeding (Figure 3B).
Figure 3B. A California desert scorpion
with pincher-like (chelate) pedipalps. Note
the small jaw-like chelicerae located in the front of the cephalothorax.
The cephalothorax bears one pair of eyes near the
midline and several along the lateral margins on each side 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 very sensitive to minor differences in brightness and
therefore movements. As a result it is
very difficult to approach a scorpion without being detected. It might be expected that these sensitive
eyes might be dazzled or even harmed by bright sunlight; however, the scorpion
has a solution to that, namely sunglasses if you will. Its eyes contain pigments that migrate toward
the surface when exposed to sunlight forming a protective screening
barrier. With reduction in light the
pigments merely drain to the lower areas of the eye.
The abdomen terminates in a five segmented tail-like
structure with a bulbous stinger at the tip. The tip of the stinger is very
sharp and quite strong (one fourth of its composition is metal including iron,
zinc 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 (Figure 3C). The function of pectins is unknown but they are
thought to be used both for detecting food and smelling the presence of the
opposite sex.
Figure 3C. Comb-like pectins on underside of scorpion.
These creatures have a unique method of locating their
prey which consists of any living animal their size or smaller, including
insects, other scorpions and arachnids, birds and small mammals. Because they
are nocturnal and do not have well-developed eyes, vision is of little use for
this activity. 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 sends out 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. With their legs essentially pointing 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. The distance of the prey
from the scorpion is computed 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, but insects and their relatives are nothing more than small
computers that have been programmed by nature.
When scorpions are placed under ultraviolet light (e.g.
a blacklight), they glow (Figure 3D). The reason for this is due to
certain 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 lengths and this is another possible means of
finding a mate in the dark of night. Also many plants emit or reflect
ultraviolet light and, in doing so, attract pollinators, which are often insects
(e.g. bees, flies and beetles). Many
other insects are positively phototrophic (attracted to light) and are even more
attracted to ultraviolet light than other light wave lengths. This is well illustrated by the fact that
entomologists use ultra violet 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
insects, their main source of food.
Figure 3D. Two scorpions (one
eating the other) viewed
under ultraviolet light. Image compliments of Bob Spencer.
Scorpions exhibit external reproduction. When a female
scorpion is ready to mate, she will deposit a chemical or sex pheromone on the
ground. She normally does not travel far from this release. Any male of the
same species happening to venture over this chemical, will sense its presence
by olfactory receptors located on the pectins. At this point the male
instinctively realizes that a female is nearby and that she is receptive for
mating. Subsequently, his mating drive kicks in and he exhibits a rather violent
series of jerky movements that scorpion experts have named juddering. His
lurching sends seismic waves across the sand, which in turn are perceived by
the female. The waves alert her to his presence and indicate that he is
receptive to mating. Typically the male will then choose a flat surface to
attach a sperm packet that is located on an elongated stalk. He subsequently
goes through a series of mating movements to attract a 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 is some
species will actually sting the female in the soft joint between the opposing
claws of her pincher. This obviously
does not kill the female but apparently tend to tranquilize her to point where
she is less combative and more receptive. 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 mother until the first molt. If the young scorpions
are removed from the mother’s back prior to their first molt they will
typically die due to the lack of sufficient moisture. Apparently the mother secrets a liquid that
prevents this dehydrating. 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.
It has been postulated by scorpion experts that the
sex pheromone of some scorpions is not only used to secure a potential mate but
is commonly used to attract dinner. Apparently some larger species of scorpions
will duplicate the sex pheromone of different smaller species. Then, when males
of the smaller species begin juddering, the larger scorpion quickly finds and
consumes the one-course meal (fresh scorpion, yum-yum!).
In some species the sting is used
to immobilize prey; most also use it for defense. The effect of the sting on
humans depends primarily on the species involved. Typical symptoms include some
pain and swelling; although, worldwide, most scorpion species are not
considered dangerous to humans. Of the approximately 1500 species of scorpions
that have been identified worldwide, only about 25 are considered potentially
lethal to humans. The venom of the more toxic species is reportedly 100,000
times more toxic than cyanide. In the United States, the sting of only two, in
the genus Centruroides, can result in
death. In Arizona, Centruroides
sculpteratus is a dangerous species accounting for 75 human deaths from
1926 to 1965, mostly children and babies.
This is twice as many deaths from the bite or sting of all other forms
of US venomous animals (except the honeybee) combined, including poisonous
snakes.
These scorpions
have long and narrow pincers and tail segments and are yellow-to-reddish in
color, with two dark stripes down the back. Centruroides scultperatus
has neurotoxic venoms that have a marked effect on nerve transmission. C. suffusus, the Durango scorpion, is
found in Durango, Mexico. Symptoms of this sting are sharp pain,
numbness, drowsiness, itching of the mucous membranes, sneezing, excess saliva
production with swallowing, sluggish tongue, muscle contractions, reaction to
strong light and hemorrhaging of the stomach and lungs. Death can occur
in about three hours. A sting site should be iced to prevent spread of
the venom and treatment must be sought immediately from a physician. There is
an effective antivenin available and, if quickly treated, death can easily be
avoided. In adults, death from the sting
of one of these scorpions is unlikely. Stings in small children or babies can
result in consequences that are not only serious but also sometimes fatal.
In Mexico, hundreds of deaths per year occur from the
sting of this genus, mainly because of the unavailability of adequate medical
facilities and the openness of many of the houses. In northern Mexico many of
the houses are poorly constructed and, in some cases, lack window glass. These
scorpions are climbers rather that living on the ground and as a result, indoors
they typically can be found in the walls and ceilings. Homes with thatched
roofs are especially attractive habitats and commonly harbor 40 to 50
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.
Although there are no dangerous species native to
California, the Centruroides scorpion has been found there in a few
isolated instances. Recently, a species of this genus was found infesting several
blocks in Anaheim Hills. Apparently a pregnant female was accidentally
introduced in 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 emperor scorpion is an African species, black in color
and the largest species in the world (up to 10 inches or more total length). It
is characterized by very large, roughly textured pedipalps (Figure 3E). The imperial scorpion occurs in SE Asia and
is more aggressive than the emperor.
Figure 3E. African emperor scorpion with
large rough pedipalps.
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 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 the author, after handling
hundreds of these scorpions with no sting symptoms worse than a mild tingling,
recently received a rather deep penetrating sting (careless handling). The
result was a painful burning sensation (worse than a bee sting) lasting on one
finger 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. Scorpion venom is high
in protein and any excess use of this valuable material can be detrimental to
the health of the scorpion.
Even though it is very difficult to receive a painful
sting from this species, it is unwise to allow the uninformed to handle one.
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, if
that. 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; thus shutting 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 can 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
the emperor 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,
namely more than one can be kept in the same cage (Figure 3F).
Figure 3F. A pile of
emperor scorpions.
The Imperial scorpion is an Asian species, 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; therefore, their venom is not 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 Cornel Sanders for
lunch.
Scorpions and other arthropods are a source of human
food in many areas of the world. A friend of the authors 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 VENOM 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 (
Whipscorpions-Vinegaroons. The
whipscorpions are mainly tropical and live in the southern United States. They
are uncommon in California but prevalent from Arizona to Texas. The chelicerae
are opposing and jaw-like, while 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 often 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 (Figure 3G).
The defensive secretion from the tail of the
whipscorpion is around 84% acetic acid (the main component of vinegar) and 5 %
caprylic acid. The reason for the small
concentration of the later has to do with increasing the defensive activity of
acetic acid. Acetic acid is water soluble
and tends to “bead up” when applied to a lipid or waxy surface. As previously discussed the outside surface
of insects and arachnids is covered by a waxy layer. Since the most important predators of
whiptailed scorpions are other arthropods (e.g. ants, spiders) a secretion of
acetic acid alone would likewise “bead up” on the exoskeleton of an attacking
predator reducing its effectiveness.
Apparently caprylic acid is more attractive to lipids and actually
serves to spread both components over the attacker’s body and enhances
penetration into the body.
Figure 3G. A whiptailed scorpion
from Arizona, Mastigoproctus giganteus.
The
whiptailed scorpion most often encountered in Arizona is Mastigoproctus
giganteus, the largest species in the world (measuring 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, although 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, between the cephalothorax and abdomen. The chelicerae are
characteristically very large and fang-like, frequently being as big as the
cephalothorax. These arthropods are commonly mistaken for vinegaroons (Figure
3H).
Figure 3H. A sunspider or windscorpion
from Southern California.
Although sunspiders can be quite aggressive, they have
no venom and are not considered dangerous to humans. The pedipalps are leg-like,
quite elongated, and used as feelers. These nocturnal arachnids have been
called by a variety of names, including sunspiders, sunscorpions, camelspiders
and windscorpions. The latter name refers to their ability to "run like
the wind." As one might expect, being so fast, 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 days 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 camelspider 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 to 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. Camel
spiders can move at speeds over 30
2. Camel
spiders can be as large as a Frisbee.
3. Camel
spider venom is an anesthetic that numbs their prey.
4. Camel
spiders can jump three feet high.
5. Camel
spiders 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 US
(approximately three inches in length).
The picture (Figure 3I) commonly found on the web is a gross
exaggeration of this species. They pose
no danger to the troops or camels although there have been a few bites which
are basically small puncture wounds with no venom. These bites commonly come from the troops
attempting to fight these with true scorpions.
For some reason most of the troops that fight camelspiders and scorpions
come for the Deep South of the US. They don’t jump (the camelspiders 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
stomach.
Figure 3I. A camel spider from Iraq. Size is greatly exaggerated (really about
three inches) by angle of camera.
Whipless Whipscorpions. This is a
group of mostly tropical arthropods with a flattened body ranging from one to
two inches in length (Figure 3J). Their
pedipalps are terminally armed with large strong spines that aid them in
capturing prey. The front legs are
slender, feeler-like and in large species can reach over 12 inches in length. These arachnids are typically found in caves,
under loose bark and in forest litter.
Since their simple eyes do not form distinct images they rely on their
tactile greatly lengthened front legs to detect food, namely insects and other
arthropods.
Figure 3J.
A large whipless whipscorpion,
Harvestmen-Daddy Longlegs. This group
of fairly large arachnids is commonly confused with spiders. Harvestmen differ
from spiders in that the cephalothorax and abdomen are broadly joined (Figure
3K), while in spiders there is a narrow waist or petiole between these two
areas; also, unlike spiders, the abdomen is segmented in harvestmen. The most
notable features of P. opilio and many other harvestmen are the long,
slender legs and short, globular body. Adult body length is approximately 3.5–9
mm, with males generally smaller than females. The upper surface of the body is
colored with an indistinct and variable light gray or brown pattern, and the
lower surface is typically light cream. Immatures are similar to adults, only
smaller and with legs shorter relative to the body size. Eggs are spherical,
about 0.4 mm diameter, with a smooth surface and color changing from off-white
to dark gray-brown as they mature. They are laid in clusters of around ten to
several hundred.
There is a daddy longleg spider that is commonly
confused with havestmen and which is 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 these arthropods are fang-like,
but harvestmen 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 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 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 follow
that a daddy longlegs would need toxic venom to subdue its notorious prey.
In addition to releasing defensive chemicals and
possessing fang-like chelicerae daddy longlegs possess additional means of
defense-namely their long legs. When
threatened by a potential predator a number of species will retreat, not
necessarily with a great deal of speed but their extended legs allow them to
bridge large gaps in the environment. For example on a plant, ants (a common
enemy of harvestmen) may not be able to travel from leaf to leaf as easily as
the long legged harvestmen. In addition
many species of daddy longlegs when confronted by a potential predator will
stand stilt-legged thus removing their body from the reach of the
attacker. Finally if one legs is grasped
by an attacker or an inquisitive child for that matter, that leg will
autoamputate (fancy word for break off).
Much like the tail of a lizard the detached leg continues to twitch,
possibly diverting the attacker’s attention, thus allowing the harvestmen to
escape.
Figure 3K.
Harvestmen or Daddy Longleg.
Images courtesy John Moore
Harvestmen
are often common in crops such as corn, alfalfa, small grains, potatoes,
cabbage, strawberries, and apple in most temperate regions of the world.
Harvestmen will feed on many soft bodied arthropods in crops, including aphids,
caterpillars, leafhoppers, beetle larvae, mites, and small slugs. In Europe P.
opilio has a single generation per year and overwinters as eggs. In parts
of North America two or more generations may occur, and eggs, immatures, or
adults may overwinter. Eggs are laid in moist areas under rocks, in cracks in
the soil, or between the soil and the crowns or recumbent leaves of plants. The
eggs hatch in three weeks to five months or more, depending on temperature, and
the immatures undergo several molts and reach maturity in two to three months,
again depending on temperature.
Although P.
opilio by itself appears unable to keep populations of any pest under
control, it serves as one member of a complex of generalist predators that
exist in many crops and that together are able to help keep pest densities low.
In addition to pest arthropods, P. opilio also may feed on dead insects
and other decaying material, as well as earthworms, other harvestmen, spiders
and other beneficial invertebrates. Although its generalist feeding habits and
tendency for cannibalism may appear to reduce its value in some situations, they
may also allow it to persist in the crop during periods of low pest density and
help suppress outbreaks of pests in their early stages. P. opilio is
highly susceptible to at least some broad spectrum insecticides, while some
more specific products, such as Bts, appear to be less harmful.