210 



ELEMENTARY ZOOLOGY 



so fused that, except 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 arach- 

 nids show in some forms a pecu- 

 liar modification of the respiratory 

 organs, the tracheae being flat and 

 leaf-like and massed together in a 

 few groups rather than being tubular 

 and ramifying through the body. 



The dorsal vessel or heart usu- 

 ally has a few blood-vessels or 

 arteries running from it. This class 

 is divided into three orders, the 



FIG. 89. A scorpion Cen- Arthrogastra, or scorpions, the 



trurus sp., from Cali- A r ~ r : nQ 

 fornia. (From specimen.) 1 ld > 



o r mites 



and ticks, and the Araneina, or 

 spiders. 



The scorpions (fig. 89) have 

 the posterior six segments of 

 the abdomen much narrower 

 than the seven anterior seg- 

 ments and forming a tail which 

 bears at its tip a poison-fang or 

 sting. This sting is used to kill 

 prey, insects and other small 

 animals. The tail can be darted 

 forwards over the body to strike 

 prey which has been previously FlG . 

 seized by the large pincer-like 

 maxillary palpi. Scorpions are 

 common in warm regions, about twenty species being 



cheese-mite, Ty- 

 roglyphus siro, greatly en- 

 larged. (After rk-rlese.) 



