SCORPIONS

 

Figure 1

Figure 2 Different Species of Scorpions

 

Context:

  1. Introduction
  2. Scientific Classification
  3. Anatomy
  4. Life Cycle
  5. Venom
  6. Bibliography

 

Introduction:

Arachnids are found throughout the world in nearly every habitat and are the most diverse group of animals. They include the scorpions, spiders, mites, ticks, and other eight-legged land invertebrates. Scorpions are venomous arthropods that belong to the class Arachnida and are considered relatives of the spiders, mites, and ticks. There are approximately 1,100 species of scorpions known worldwide.

Most scorpions are nocturnal, hiding under rocks, in cracks, or within burrows during the day, and coming out after sunset. Although they are normally associated with hot, dry areas like deserts, scorpions are found in a variety of habitats including grasslands, savannas, deciduous forests, mountain pine forests, rain forest, and caves. Scorpions have even been found under snow-covered rocks at elevations of over 12,000 feet in the Andes Mountains of South America and the Himalayas of Asia.

Fossils suggest that arachnids were among the first animals to live on land, perhaps in the early Devonian Period, nearly 400 million years ago. About 60,000 species are known, although many remain undiscovered. Scorpions are the oldest arachnids for which fossils are known, and they were the first arachnid fossils to be found in Paleozoic strata.

1 2 3

Figure 3 Example of Scorpion families: 1) Buthidae 2) Scorpionidae 3) Chactidae

 

Scientific classification

Arachnids constitute the class Arachnida, in the phylum Arthropoda. The class is divided into 11 orders: the Acarina (mites and ticks), Amblypygi (tailless whip scorpions), Araneae (spiders), Opiliones (daddy longlegs), Palpigradi (palpigrades), Pseudoscorpiones (false scorpions), Ricinulei (ricinuleids), Schizomida (micro whip scorpions), Scorpionida (true scorpions), Solpugida (wind scorpions), and Uropygi (whip scorpions).

Figure 4 Centruroides gracilis male & female Figure 5 Centruroides keysi (new species) male & female

 

Anatomy

Scorpions are easily distinguished from other arachnids by their large, well developed claws (pedipalps) and distinct division of the abdomen (opisthosoma) into a broad preabdomen (mesosoma) seven segments long and narrow, tail-like postabdomen (metasoma) five segments long.

All scorpions possess a poisonous sting (telson) that contains a pair of poison glands that can paralyze prey or may deliver a painful sting to incautious persons. While all scorpions are venomous, only about twenty species worldwide possess venom of sufficient toxicity to kill humans.

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. However in scorpions, they are large and powerful pincers that are used to grasp and subdue prey. Their prey includes a variety of arthropods and other invertebrates, and the larger species are known to prey on small vertebrates, such as smaller lizards, snakes, and mice.

On its dorsal side, the scorpions have a unique pair of sense organs called pectines. These are usually larger and contain more "teeth" in the male than the females. They are used to sense textures of surfaces they walk across and serve as chemoreceptors to detect their own species of scorpions.

Prey is located primarily by sensing vibrations. The pedipalps have an array of fine sensory hairs called trichobothria that sense air-borne vibrations; the tips of the legs have small organs that detect vibrations in the ground. The surfaces of the legs, pedipalps, and body are also covered with thicker hairs (setae) that are sensitive to direct touch. Even though they are equipped with venom to defend them, scorpions fall prey to many types of creatures, such as centipedes, tarantulas, insectivorous lizards, birds, and mammals

 

Figure 6 Ventral View of a Scorpion Figure 7 1) Telson 2) Post Abdomen Tail 3) Pedipalp

4) Cephalothorax 5) Pre Abdomen 6) Walking Legs

 

Life Cycle

The scorpions mating ritual is enacted when the male uses his pedipalps to grasp the female's pedipalps in order to lead her on a courtship dance. The details of courtship vary from species to species, with some even exhibiting a deliberate and prolonged "sexual sting" by the male. The male sweeps his pectines over the ground surface to help locate a suitable place to deposit his spermatophore, which contains the sperm. The male pulls the female over the surface where she draws the sperm into her genital pore that is located near the front on the underside of her abdomen.

Scorpions have a long gestation period varying from several months to a year and a half in which the young develop as embryos in the female's ovariuterus. During this time, nutrients are transferred from the mother's digestive gland to the embryos. The young are born live and use the motherís legs to ascend to her back.

On the average, a female gives birth to about 25-35 young. They remain on her back until they molt for the first time, which is usually within a week or two after birth. Once they climb down, they assume an independent existence, and periodically molt to reach adulthood. Typically five or six molts over two to six years are required for the scorpion to reach maturity. Adult scorpions range in size from 1.5 cm to 21 cm in length.

The average scorpion lives three to five years, but some species can live up to 10-15 years. A few scorpions exhibit social behaviors beyond the mother-young association, such as forming over wintering aggregations, colonial burrowing, and even living in extended family groups that share burrows and food.

 

Figure 8: Mother Scorpion carrying her children Figure 9: The Courtship dance of the Scorpions

 

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Scorpion Venom

Scorpion venoms are complex mixtures of neurotoxins (toxins which affect the victim's nervous system) and other substances; each species has a unique mixture. Despite their bad reputation, only one species in the U.S. and about 20 others worldwide have venom potent enough to be considered dangerous to humans.

The US species, Centruroides exilicauda, is found over southwestern region. The venom of this scorpion may produce severe pain and swelling at the site of the sting; numbness; frothing at the mouth; difficulties in breathing, including respiratory paralysis; muscle twitching; and convulsions. Death is rare since an antivenin is available for severe cases. Death by scorpion sting, if it occurs, is the result of heart or respiratory failure some hours after the incident.

The world's most dangerous scorpions live in North Africa and the Middle East (species of Androctonus, Buthus, Hottentotta, Leiurus), South America (Tityus), India (Mesobuthus), and Mexico (Centruroides).

 

 

Figure 10 Scorpion hunting Figure 11 Vejovis carolinianus Female

 

Figure 12 Tityus floridanus Female

 

 

Bibliography

"Arachnid," Microsoft® Encarta® Online Encyclopedia 2000, http://encarta.msn.com © 1997-2000 Microsoft Corporation. All rights reserved.

Campbell, Neil A., Reece Jane B., and Lawrence G. Mitchell., Biology, 5th ed. Addison Wesley Longman, Inc., Menlo Park, CA. 1999.

Cloudsley-Thompson, J. L., M.A, PH.D. (CANTAB.), F.R.E.S., F.L.S., Spiders, Scorpions, Centipedes and Mites, Pergamon Press, NY, New York, 1958.

Muma, Martin H., Scorpions, Whip Scorpions and Wind Scorpions of Florida, Florida Department of Agriculture, 1967.

Royo, A.R Scorpions, Scorpions, http://www.desertUSA.com/oct96/du_html. © 1996-2000, Digital West Media Inc. All rights reserved

Van De Graaff, Kent M. and John L. Crawley, A Photograpfic Atlas for the Biology Laboratory, 3rd ed, Morton Publishing Co. Englwood, CO. 1996.