<P> The cuticle of scorpions is covered with hairs in some places that act like balance organs . An outer layer that makes them fluorescent green under ultraviolet light is called the hyaline layer . Newly molted scorpions do not glow until after their cuticle has hardened . The fluorescent hyaline layer can be intact in fossil rocks that are hundreds of millions of years old . </P> <P> Scorpions are opportunistic predators of small arthropods and insects . They use their chelae (pincers) to catch the prey initially . Depending on the toxicity of their venom and size of their claws, they will then either crush the prey or inject it with neurotoxic venom . The neurotoxins consist of a variety of small proteins as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim . Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general, it is fast acting, allowing for effective prey capture . Scorpion venoms are optimised for action on other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). A few scorpion species, however, mostly in the family Buthidae, can be dangerous to humans . The scorpion that is responsible for the most human deaths is Androctonus australis, or fat - tailed scorpion of North Africa . The toxicity of A. australis's venom is roughly half that of the deathstalker (Leiurus quinquestriatus), but since A. australis injects quite a bit more venom into its prey, it is the most deadly to humans . Human deaths normally occur in the young, elderly, or infirm; scorpions are generally unable to deliver enough venom to kill healthy adults . Some people, however, may be allergic to the venom of some species, in which case the scorpion's sting can more likely kill . A primary symptom of a scorpion sting is numbing at the injection site, sometimes lasting for several days . It has been found that scorpions have two types of venom: a translucent, weaker venom designed to stun only, and an opaque, more potent venom designed to kill heavier threats . </P> <P> Unlike the majority of Arachnida species, 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 undergone at least one moult . The young generally resemble their parents, requiring between five and seven moults to reach maturity . Scorpions have quite variable lifespans and the lifespan of most species is not known . The age range appears to be approximately 4--25 years (25 years being the maximum reported life span in the giant desert hairy scorpion, Hadrurus arizonensis). They are nocturnal and fossorial, finding shelter during the day in the relative cool of underground holes or undersides of rocks and coming out at night to hunt and feed . Scorpions prefer to live in areas where the temperature is 20--37 ° C (68--99 ° F), but may survive in the temperature range of 14--45 ° C (57--113 ° F). </P> <P> Scorpions have been found in many fossil records, including coal deposits from the Carboniferous Period and in marine Silurian deposits . They are thought to have existed in some form since about 450 to 425 million years ago . They are believed to have an oceanic origin, with gills and a claw - like appendage that enabled them to hold onto rocky shores or seaweed . </P>

An animal that is not a member of the class arachnida is