Scorpions are ancient animals and fossil records indicate that they were already in existence about 425-450 million years ago during the Silurian period and evolved from an amphibious ancestor. They occur in habitats ranging from forest to deserts but it is in the arid areas that they are most common.
Scorpions are the oldest arachnids for which fossils are known, and they were the first arachnid fossils to be
found in Paleozoic strata. The Silurian scorpions appear to have lived in the
water, since their fossils have gills, but by the Carboniferous scorpions
with such features are no longer found -- fossils from the Pennsylvanian
age Mazon Creek beds have book lungs covered by protective plates,
and so were probably land-dwellers. The best scorpion fossils come from the
Devonian and the Oligocene; there is a severe lack of fossils known from
the intervening period.
These earliest scorpiones are considered to be Protoscorpions, since they
possess many traits which are plesiomorphic for scorpions. For example,
in all scorpions the thick front portion on the abdomen is made up of
seven segments, but the number of sternite plates which cover this
region varies among the earliest fossils, while all living species have five.
All scorpions have an additional
five segments after the initial seven, ending in a sharp sting.
This sting contains a pair of poison glands which can paralyze prey, usually
insects or small rodents, or may deliver a painful sting to incautious
persons. Most scorpion stings are merely painful, leading to swelling in
the immediate region of the sting, but some scorpions of northern Africa and
the American southwest can be deadly. In the US, the deadliest scorpions
are to be found in Arizona, where it is a good idea to shake out shoes
before putting them on in the morning!
Like the insects and spiders, scorpions belong to the phylum Anthropoda and like the spiders they belong to the class Arachidna but belong to a different order, Scorpiones.
Besides their unusually long and dangerous tails, scorpions also differ from
other arachnids in having large pedipalps. These are the second
pair of appendages on the body, and are usually rather inconspicuous in
arachnids, but in scorpions, they are large and powerful pincers which may
be used to grasp and subdue prey. Scorpions may also have more eyes than other
arachnids, some species possessing as many as six pairs, though most do not
have this many. They have three joints in their chelicerae, or the
first pair of appendages,
located next to the mouth.
Most scorpions are nocturnal, hiding under rocks, in crevices, or
within burrows during the day, and
coming out after sunset. Because of this, and because of their painful stings,
it can be dangerous to travel at night in scorpion territory without shoes,
even inside homes. (I have nearly stepped on a scorpion in my parents' home
near Dallas.) One unusual feature of scorpions that has helped many field
biologists is the UV fluorescence of scorpion bodies. Biologists hunting
for scorpions wave an ultraviolet light near the ground as they walk along,
watching for an eerie greenish light to be reflected back. The UV light is
absorbed by the scorpion's armor and is reflected back as visible light.
Unlike the majority of Arachnida, scorpions are viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have under gone at least one moult. Before the first moult scorplings cannot survive naturally without the mother, depending on her for protection and to regulate their moisture levels. In some species the young/mother association can continue for an extended period of time especially in species which display more advanced sociability (e.g Pandinus spp.). The size of the litter depends on the species and can range from 3 to 100+ scorplings .
The young in a general way resemble their parents. Growth is accomplished by periodical shedding of the exoskeleton (ecdysis). A scorpion's developmental progress is measured in instars (how many moults it has undergone). Generally scorpions require between 5-7 moults to reach maturity. Moulting is effected by means of a split in the old exoskeleton which takes place just below the edge of the carapace (at the front of the prosoma). The scorpion then emerges from this split, the pedipalps and legs are first removed from the old exoskeleton, followed eventually by the metasoma. When it emerges the scorpion’s new exoskeleton is soft, making the scorpion highly vulnerable to attack. The scorpion must constantly stretch while the new exoskeleton hardens to ensure that it can move when the hardening is complete. The process of hardening is called sclerotization. The new exoskeleton does not fluoresce, however as sclerotization occurs the fluorescence gradually returns.
Scorpions have very variable lifespans and the actual lifespan of most species is not known, however the range is approximately 4-25 years (25 years being the maximum reported life span of the species H. arizonensis)
