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Scorpions were evolutioning from the Paleozoic Era. They're complex than any other arachnids and they have more segments. Besides, scorpions are strongly segmented in the design of the heart and nervous system. The possession of the book lungs, rather than a trachea, for respiration is also primitive.


-EXTERNAL FEATURES-

The three major regions that form the body of a scorpion are:



  • the anterior prosoma


  • the medial mesosoma


  • the posterior metasoma




Each parts

The chelate is used to masticate food, formed by three-segmented chelicerae that arise from the first segment. The pedipals originate in the second segment and terminate in chelate pincers(chelae). Scorpions use pedipals for a variety of purposes(including prey capture, defense, courtship, and burrow excavation). Four pair of legs arise from segments three through six. Both pedipals and legs are composed of segments:
  • (basal)coxa

  • trochanter

  • femur

  • patella

  • tibia

  • basitarsus

  • tarsus.

 Legs end in tarsal claws and are used to grasp the surface during walking. The coxae of the pedipals and the first two pairs of legs are expanded ventrally to form a gnathobase that encloses a preoral cavity where food is masticated, squeezed, and transferred from chelicerae to the mouth. One medial and from zero to five lateral pairs of eyes are set into the dorsal carapace that covers the prosoma.

Anatomy of underside of scorpion
About the mesosoma

The mesosoma has seven segments:

  • The first(pregenital) segment is reduced and bears the sternum;

  • The second(genital) bears the ventral gonopore covered by a pair of small genital opercula.The unique comblike pectines arise from the genital segment.

  • Four pairs of book lungs are on the ventral side of mesosomal segments three through six.

  • The seventh mesosomal segment, marks the end of the "body". The mesosoma is covered dorsally by seven tergites and ventrally by five sternites; these plates are separated from each other and laterally by a flexible pleural membrane.

  • The tail is composed of the five segmented cylindrical metasoma and a posterior telson bearing the stinger. The anus exits at the end of the fifth metasomal segment.
Cuticle change

The exoskeleton is composed of chitinous cuticle overlain by impermeable epicutilar waxes. Growth is accompained by ecdysis. Scorpions increase in weight until the exoskeleton becomes too small to allow further growth and a new exoskeleton is secreted by the epidermis under the old. Scorpions, like other arachnids, probably increase blood pressure just before molting. The cuticle ruptures at the side and front margins of the carapace. The chelicerae, pedipals, legs and body are withdrawn from the old cuticle over a period of 12 hours. They probably change blood pressure to expand body volume temporarily while the new cuticle hardens. This allows room for future growth in mass. As the cuticle tans, it hardens, darkens, and gradually acquires the ability to fluoresce under ultraviolet light.