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Order Scorpiones - Scorpions

Centruroides vittatus - female Centruroides gracilis - female Centruroides gracilis - female Centruroides gracilis - female Striped Bark Scorpion - Centruroides vittatus ? Bark scorpion - Centruroides sculpturatus ?  - Centruroides sculpturatus Vaejovis spinigerus pectines - Vaejovis spinigerus - female Brown SC scorpion
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Chelicerata (Chelicerates)
Class Arachnida (Arachnids)
Order Scorpiones (Scorpions)
Numbers
19 families
9 subfamilies
66 genera
>1520 species
Size
Adults range from 9 mm (Typhlochactas mitchelli, Mexico) to over 210 mm (Hadogenes troglodytes, South Africa).
In America, adults range from 18 mm (Pseudouroctonus andreas) to over 100 mm (Hadrurus spp. and Centruroides gracilis).
Identification
Scorpions have two large pedipalps which end in a chela (hand or "pincer"). They are well-known for the long five-segmented metasoma (tail) with an additional telson that bears the stinger/aculeus (pseudoscorpions lack a metasoma).
Range
In the United States, scorpions are most abundant in the semiarid regions of the Southwest. Scorpion diversity increases dramatically west of the 100 degree meridian.
The greatest individual "spots" of diversity in the US occur in several localities of Trans-Pecos Texas (e.g., Langtry, Rio Grande Village, Lajitas/Terlingua) and Anza-Borrego State Park, California, with 9 species occurring within a square mile.
Arizona and California have the greatest scorpion diversity with about 60 species (includes undescribed forms).
They do not occur in the Great Lakes states (except extreme SW Illinois) or New England, or Alaska.
Habitat
In the US, scorpions are found in a wide range of habitats, from coastal sand dunes in Florida, Texas, and California, to below sea level along the Salton Sea and in Death Valley, to over 9,000 feet in mountains of Arizona, New Mexico, and California, including in the ancient bristlecone pine forest.
Scorpions are found under rocks and other surface debris, in road cuts, cliff faces, under bark of live and dead trees, in houses, lumber yards, and deeply buried to depths of 20 feet in arroyos!
Food
Predatory. They eat mostly soft-bodied insects and other arachnids, including other scorpions. They generally avoid malacostracans (pill/sow bugs, wood lice) except in some species where it is an exclusive food item. Some larger species are known to prey on vertebrates such as lizards, snakes, and small mammals (shrews, rodents).
Life Cycle
Scorpions live from two years to as long as 25 years in captivity. Most species mature within a year or two. They go through ecdysis/molting and there are no significant changes except when males reach maturity their pectines (combs, sensory appendages) increase in size and the pedipalp chelae (hands) also become more robust in many species. The stages between each molt, beginning with immediately after birth, are called instars (i.e., a newborn scorpion is in its first instar). There are no more molts upon reaching maturity, which is usually in the 6th-9th instar depending on the species.

They undergo a complex courtship which has been called "promenade à deux". Gestation lasts from about 2-5 months (mostly buthids) to as long as 18 months. The scorpions are born alive, are truly vivparous, but oddly enough, there are two developmental types: apoikogenic and katoikogenic. Similar to ovoviviparity, apoikogenic scorpions are born with an outer covering and are gestationally nourished with yolk (Iuridae and Vaejovidae have very small amounts), but also receive some nutrients from the mother through the membrane and are therefore not truly ovoviviparous. Katoikogenic scorpions are born without a covering and lack yolk. Katoikogenic scorpions are unique in all invertebrates in that the embryos are nourished directly from the mother via an oral feeding apparatus called a diverticulum.
Remarks
The sting of most scorpions is not serious and usually causes only localized pain, some swelling, tenderness and some discoloration. Systemic reactions to scorpion stings are rare.
The sting of one of our scorpions, however, Centruroides sculpturatus(until recently thought to be the same as Centruroides exilicauda), the Arizona Bark Scorpion, can be fatal. Most healthy adults are not at significant risk- only children, with their smaller body size, are in danger (treatment with antivenom has pretty much put a stop to deaths where available, but bark-scorpion stings should still be taken very seriously). The site of the sting does not become discolored.
Another scorpion known to have an intense sting is Centruroides vittatus, but no deaths have been attributed to it directly. All but one of the 25 or so dangerous scorpions in the world are in the family Buthidae. The exception is Hemiscorpius lepturus (Iran, Iraq, Pakistan, and Yemen) in the family Hemiscorpiidae, which has been shown to have an unusually toxic hemolytic venom.

Scorpion fluorescence under ultraviolet light has been been intensely studied and debated since its discovery by Arizona geologists in the 1940s. It enables researchers to observe scorpions in the wild for nocturnal ecological studies and to find new species and distributions. In the 1970s the total number of known scorpion species was about 600, today there are over 1520, due mostly to UV detection. Although the chemicals in the scorpion exoskeleton that cause fluorescence have been identified, the reason behind it has not.
See Also
Pseudoscorpions look like miniature scorpions, but don't have the stinging tail.
Several other minor Arachnid orders are also like scorpions, but can't sting, either:
Male Scorpionflies are insects with a scorpionlike (non-stinging) abdomen, but have wings and otherwise look very different