page_title: Scorpion
section_title: Mesosoma
hierarchical_section_title: Scorpion / Morphology / Mesosoma
caption_attribution_description: Español: Vista ventral de un escorpión de especie sin identificar donde se puede observar los pectenes, el par de órganos sensitivos con forma de V invertida en la imagen. English: Ventral view of an unidentified scorpion species where the pectines can easily be observed. Pectines –the sensitive organs– have the inverted V shape in the image Português: Vista ventral de um escorpião de espécie sem indentificar onde pode-se observar as pectinas, o par de órgãos sensitivos com forma de V invertida na imagem. Català: Vista ventral d'un escorpí d'espècie sense identificar on es pot observar els pectenes, el parell d'òrgans sensitius amb forma de V invertida en la imatge.
context_page_description: Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognized by the pair of grasping pedipalps and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9–12 mm in Microtityus minimus to 23 cm in Heterometrus swammerdami.
  The evolutionary history of scorpions goes back to the Silurian period 435 million years ago. They have adapted to a wide range of environmental conditions, and they can now be found on all continents except Antarctica. There are about 1,750 described species, with 13 extant families recognised to date. Their taxonomy is being revised in the light of genomic studies.
  The vast majority do not represent a serious threat to humans, and healthy adults usually do not need medical treatment after being stung. Only about 25 species have venom capable of killing a human. In some parts of the world with highly venomous species, human fatalities regularly occur, primarily in areas with limited access to medical treatment.
  Scorpions with their powerful sting appear in art, folklore, mythology and numerous brands.
context_section_description: The mesosoma or preabdomen is the broad part of the opisthosoma. It consists of the anterior seven somites (segments) of the opisthosoma, each covered dorsally by a sclerotised plate, its tergite. Ventrally, somites 3 to 7 are armoured with matching plates called sternites. The ventral side of somites 1 has a pair of genital opercula covering the gonopore. Sternite 2 forms the basal plate bearing the pectines. Morphologically the pectines are a pair of limbs that function as sensory organs.
  The next four somites, 3 to 6, all bear pairs of spiracles. They serve as openings for the scorpion's respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical or oval according to the species. There are thus four pairs of book lungs; each consists of some 140 to 150 thin lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles hold the lamellae apart. A muscle opens the spiracle and widens the atrial chamber; dorsoventral muscles contract to compress the pulmonary chamber, forcing air out, and relax to allow the chamber to refill. The 7th and last somite do not bear appendages or any other significant external structures.
  The mesosoma contains the heart or "dorsal vessel" which is the center of the scorpion's open circulatory system. The heart is continuous with a deep arterial system which spreads throughout the body. Sinuses return deoxygenated hemolymph to the heart; the hemolymph is re-oxygenated by cardiac pores. The mesosoma also contains the reproductive system. The female gonads are made of three or four tubes that run parallel to each other and are connected by two to four transverse anastomoses. These tubes are the sites for both oocyte formation and embryonic development. They connect to two oviducts which connect to a single atrium leading to the genital orifice. Males have two gonads made of two cylindrical tubes with a ladder-like configuration; they contain cysts which produce spermatozoa. Both tubes end in a spermiduct, one on each side of the mesosoma. They connect to glandular symmetrical structures called paraxial organs, which end at the genital orifice. These secrete chitin-based structures which come together to form the spermatophore.
caption: Ventral view: the pectines have a comblike structure in an inverted V shape.
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