Xll] SCORPIONIDA 269 



chelicerae may be clawed or chelate, like a lobster's claws (Fig. 121), 

 but they often take the form of piercing stylets and the gnathobases 

 of the pedipalpi may form a sheath to protect them. 



The number of species is very great ; amongst the commoner 

 forms may be mentioned Tetranychus telarius, often known as 

 the Red Spider, which spins webs under leaves in which whole 

 colonies shelter. This species is believed to do great damage in 

 hot-houses. Tyroglyphus siro, the Cheese-mite, which burrows in 

 decaying cheese, and the genus Pkytoptus, which causes the conical 

 galls on lime-trees, maples, etc., are also familiar. 



Order IV. Scorpionida. 



Scorpions are not found in Great Britain, though they are common 

 on the Continent of Europe around the Mediterranean basin and 

 generally in warm climates. They retain a more marked segmenta- 

 tion than is the case with the other Arachnids we have considered. 

 The abdomen is very long, distinctly segmented and differentiated 

 into two portions ; (a) the mesosoma, consisting of seven segments 

 of the same diameter as the prosoma, bearing the respiratory 

 appendages , (6) the metasoma, a much narrower part, consisting 

 of five segments and a curved spine like a tail at the apex of which 

 is the opening of a poison gland. The mesosoma has six pairs of 

 appendages. The first of these forms the genital operculum, a plate 

 bearing on its posterior aspect the genital pore in both sexes ; the 

 second are " pectines," curious comb-shaped structures, whose exact 

 function is not yet determined, but which are morphologically 

 reduced and thickened gill-books which project freely and are not 

 enclosed in respiratory chambers. The third, fourth, fifth and sixth 

 segments bear each a pair of lung-books, and it has already been 

 explained that the floors of these are formed of highly modified 

 plate-like appendages which in the adult have lost all trace of 

 their origin from limbs. The seventh segment of the mesosoma 

 shows no traces of limbs and tapers to join the first segment of 

 the metasoma. At the posterior end of the fifth metasomatic 

 segment, on the ventral surface, is situated the anus, and behind 

 this is a conical pointed joint which contains the poison glands 

 and which forms a very efficient and powerful sting. The whole of 

 this tail is very mobile and the sting can readily be directed to any 

 point. In life the tail is usually borne turned forward over the 

 body so that the sting threatens the head. 



