Diagnosis . Animalia arthropoda arachnoidea vivipara, pulmonata, abdomine sessile, segmentato, in parte posterior i caudiformiter attenuato acapice vesicula aculeata venenifera armato, mandibulis palpisque cheliferis, subtus pectinibus duobus.                                                                                         Description . Scorpiones is one of the orders of the class Arachnoidea (or chelicerota) which differs from other classes of Arthropoda by the absence of antennas on the cephalic part of the body; this order can be defined as follows: scorpions are arachnids, viviparous animals with a segmented body consisting of six cephalothoracic segments, covered dorsally by a common carapace , and twelve abdominal segments ,of which the five posterior segments narrow in the shape of a caudal appendage; with six pairs of limbs, of which the anterior two, the chelicera and pedipalpi, are armed with chelae; with a postanal appendage modified into a poison gland armed with a sting (aculeus); with a pair of comb-shaped appendages, located behind the genital aperture on the second abdominal segment; with four pairs of stigmata ,one pair each on the third, fourth, fifth and sixth abdominal sterna respectively. Apart from the marine forms –known as Xiphosura and sometimes regarded as a separate subclass (Delodranchiata or Merostomata) of Arachnoidea, and also two degenerate groups, Tardigrada and linguatulida, whose relation to Arachnoidea is unclear –the interrelationships of all other order of this class and their relation to the order Scorpiones can be seen from the following key:
                                                               KEY TO ORDERS OF THE CLASS ARACHNOIDEA                                                                                   1. Abdomen very long , its posterior half narrowed in the shape of a mobile caudal appendage; the postanal segment in the shape of a poison organ armed with a sting; behind the genital aperture there are two comb-shaped appendages ; large free-living arachnoidea ,breathing with lungs ……………..………………………………………………………………… Scorpiones. Abdomen not long and its posterior half not in the shape of a long, mobile caudal appendage; postanal segments either absent or filiform and wanting a poison organ; behind the genital aperture no comb-sha ped appendage…..……..………………… .2  2. postanal segments present and from a more or less long filiform caudal appendage………………………………………………………………………………………………3.   No postanal segments…………………………………………………………………………………………………………………………………………………………………………………….…. 4.  3. No free thoracic segments between cephalothorax and abdomen; chelicerae with a terminal claw; pedipalpi shout with chelae; large free-living Arachnoidea breathing with lungs ……………………………………………………………………………………………………………………………………………………………… Uropygi   Two free narrowed thoracic segments between the cephalothorax and abdomen; chelicera with chelae; pedipalpi thin, not differing from the legs ; very small free –living Arachnoidea devoid of special organs of respiration………………..…………………………………………………………………………………………Palpigradi.   4.Two free posterior thoracic segments, i.e., not fused with the cephalothorax; chelicera large, chelate; pedipalpi long, not much thickened, without terminal claws with a tactile organ; large free-living Arachnoidea breathing with trachea…………..……………………………………………..……………..…..Solifugse All the cephalothorax segments fused …………………………………………………………………………..…….………………………………………………………………….………………… 5. 5. The last pedipalpal segment froms a claw(chela); pedipalpi longer and stouter than the short legs; chelicera chelate; small free –living Arachnoidea breathing which tracheae……………………………………………………………………………………………………………………………………Pseudoscorpions The last pedipalpal segment does not from chelae……………………………………………………………………………..………………………………………………………………………………………………. 6. 6. Abdomen ventrally , in front of anus ,with spinnerets; chelicerae with a poison gland and a terminal claw; pedipalpi shorter than the legs, in the male with a copulatory organ; large and small free- living Arachnoidea breathing with lungs………………………………………………………………….………………Aranea. Abdomen without spinnerets………………………………………………………………………………………………………………………………………………………………………………… 7. 7. Abdomen not segmented , sometimes consisting of rings fused with the cephalothorax; mouth parts piercing and sucking ; genital aperture situated between coxae ;small Arachnoidea, some of microscopie size, often leading ,in adult or larval stages, an ectoparasitic mode of life ………………………………Acari Abdomen segmented…..……………………………………………..………………………………………………………………………………………………………………………………. 8.  8. Lateral eyes present , abdomen separated from the cephalothorax by a constriction ; first pair of legs flaglliform, multisegmented limbs; genital aperture on abdomen; pedipalpi longer than legs , thickened; large free – living Arachnoidea breathing with lungs…………………………..…………………………Amblypygi No lateral eyes ,abdomen not separated from cephalothorax by a deep constriction ,legs generally long ,not significantly different from one another ; pedipalpi short ,not thickened; genital aperture between coxae ; free -living Arachnoidea of medium and small size ,breathing with tracheae……………………………….……………………………..…………opiliones.
External structure . Scorpions are among the largest terrestrial representatives of the class Arachnoidea; the largest species among the Scorpionidae reach 20 cm in length;on the other hand,no scorpion species is known shorter than 2.5 cm in the adult stage;the usual length of scorpions is 6 to 10 cm.Their entire body is covered with smooth,hard or finely torose,granulated or shagreen–like chitinous integument.On different parts of the body,mainly on the dorsal side of the pedipalpal chelae and the caudal,narrowed posterior part of the body,this instegument also has a sculpture,sometimes transverse and oblique,sometimes longitudinal, occasionally comb–shaped or ridge–like elevated rows of granular tubercles or denticles.The body segmentation and the shape of its individual parts is very uniform in the whole order;the body of a scorpion is divided into two main sections,cephalothorax and abdomen.The cephalothorax (prosoma) is the anterior part of the body carrying the limbs;it consists of six primary segments which are partly fused but still distinct,although very modified in the adult stage;the fully fused terga from a continuous carapace(scutum cephalothoracis) convering all six sterna;the sterna are more distinctly separate from each other.Tha carapace (cph.) is in general trapezoidal,distinctly widening posterior;its four corners are more or less rounded;the anterior margin is usually slightly convex but often with a more or less deep median notch;lateral margins slightly sinuous and the posterior margin is mostly straight or also sinuous,its surface is not smooth because there are depressions and protuberances in addition to the usually protruding ocular protuberance;thus,the posterior third of the carapace is fairly distinctly separated by two transverse lateral grooves converging medially;near the anterior angles of the carapace there are two elevation,on the external margin of which the lateral eyes are situated; these elevations are called frontal protuberances.(convexitates fronatles),and are separated from each other by a longitudinal groove ,sometimes by a groove running from the ocular protuberances towards the anterior margin;the anterior margin of the carapace is usually rather flat but sometimes the front is sharply depressed anteriorly (in the genera B u t h e o l u s, O r t h o c h I r u s and M I c r o b u t h u s ).the ocular protuberance (tuberculum oculiferum)carries a pair of principal eyes and is usually situated somewhat anterior to the middle,but in some cases (for example, C a l c h a s and C h a e r I l u s) it almost reach is the anterior magin;on the other hand;in the south African genus O p I s t h o p h t h a l m u s, it is situated near the posterior margin of the carapace.In addithion to the anterior longitudinal groove or depression (in family B u t h I d a e ),there is more or less distinct groove which runs to the ocular protuberance from the posterior margin of the carapace.Sometimes the groove is also marked on the protuberance itself,extending across it and connecting with the anteriorgrppve;the anterior longitudinal groove sometimes (in genus O p I s t h o p h t h a l m u s)represents a real furrow which bifurcates befor reaching the anterior margin,thus forming a triangular area (triangulus anteocularis). The eyes situated on it are called principal eyes(oculi principals);they are round or longitudinally elliptical,situated rather close to each other;the distance between them is in some species longer and in others shorter than their diameter.near the anterior angles of the carapace are set the lateral eyes (oculi laterales );their number varies in different genera from two to five;in the case of genus B e l I s a r I u s,there are no eyes at all,either lateral or principal;some genera belonging to different families have two lateral eyes (for example,U r o d a c u s and H e t e r o s c o r p I o n of the family scorpionidae,or O e c l u s of the family Diplocentridae,also all the genera of the family chactidae).Most scorpions have three lateral eyes;in the family B u t h I d a e, the number of lateral eyes increases in many genera up to five on each side because in addition to the three principle lateral eyes of varying size and situated in one line parallel to the margin of the carapace,there are two additional small accessory eyes (ocelli accessorii),set above the posterior end of the row of lateral eyes;sometimes the accessory eyes are hardly visible among the sorrunding portuberances.principal and lateral eyes of scorpions are essentially distinguished by their structure;whereas the lateral eyes are like the simple eyes (ocelli)of insects,the principal eyes represent an intermediate type toward compound eyes;under the common chitinous lens they have a nonpigmented layer of cells forming a vitreous body,and underneath it a layer of terminal nerve cells of the retina bundled into separate groups (retinulae) consisting of five cells each,connected by common five–segmented rod (rhabdom).