164 



THE ANIMALS AND MAN 



segments composing each of these two parts are so fused that, 

 ej^cept in the scorpions, they are usually indistinguishable. 

 There are no antennae, the eyes are simple, the mouth-parts 

 fitted for biting, and there are four pairs of legs. In their 

 internal anatomy the arachnids show in some forms a peculiar 

 modification of the respiratory organs, the tracheae being 

 fiat and leaf-like and massed together in a few groups rather 

 than being tubular and ramifying through the body. 



The dorsal vessel or heart usually 

 has a few blood-vessels or arteries 

 running from it. This class is divid- 

 ed into three orders, the Arthrogastra, 

 or scorpions, the Acarina, or mites 

 and ticks, and the Araneina, or 

 spiders. 



The scorpions (fig. 73) have the 

 posterior six segments of the abdo- 

 men much narrower than the seven 

 anterior segments and forming a tail 

 which bears at its tip a poison-fang 

 or sting. This sting is used to kill 

 prey, such as insects and other small 

 animals. The tail can be darted 

 forwards over the body to strike prey 

 which has been previously seized by 

 the large pincer-like maxillary palpi. 

 Scorpions are common in warm 

 regions, about twenty species being known in south- 

 em North America. Their sting though painful is 

 not dangerous to man. The young are born alive and are 

 carried about by their mother for some time after 

 birth. 



The mites (fig. 74) and ticks (fig. 75) are mostly small 

 obscure animals, which live more or less parasitically. The 

 common red spider of house-plants as well as the sugar- and 



Fig. 73. A scorpion 

 trurus sp., from 

 fomia. (Natural 



Cen- 

 Cali- 

 size.) 



