OF THE CLASS ARACHNIDA. 463 



of small birds, and even man himself is obliged to make an 

 effort to break through them. The manner in which spiders 

 arrange their silk is no less varied : some confine themselves 

 to stretch irregular threads, and others weave a web the 

 messes of which are of extreme regularity. Sometimes we, 

 see them immovable in the middle of their web, watching 

 their prey ; at other times, they conceal themselves in a 

 retreat which they construct at hand, and which sometimes 

 has the appearance of a silky tube, sometimes that of a small 

 cup or cupola. 



559. The araclmida are divided into two orders, accord- 

 ing to the structure of the organs of respiration and of circu- 

 lation. 



The arachnida pulmonaria are chiefly characterized by the 

 presence of pulmonary pouches and of a well developed vas- 

 cular apparatus ; but they may also be recognised by other 

 peculiarities of structure. Thus, their 

 eyes are six, eight, or even more in 

 number, and under the abdomen may 

 be seen two, four, or eight stigmata. 

 Moreover, the general form of these 

 animals varies. Sometimes they have 

 the abdomen globular, winders at its ex- 

 tremity, and small mandibular feelers ; 

 at other times the abdomen is elongated 

 and composed of several rings ; their 

 mandibular feelers advance like arms, 

 and terminate like forceps ; or finally, 

 there exist no winders at the extremity 

 of the body, but in general a venomous 

 apparatus. Spiders properly so called 

 (Fig. 174), that is to say, the mygale 

 (Fig. 495), the epeira, the lycosse or tarantulse, the theridions 

 (Fig. 502), present us with the first of these two modes of 

 conformation ; the scorpion (Fig. 499), the second. 



560. The arachnida trachearia have no pulmonary 

 pouches, but respire by tracheae, like insects, and have 

 only a rudimentary vascular apparatus for the circulation of 

 the blood. Some are without eyes, and in those which have 

 them, we never find more than two or four. Some of these 

 animals, known by the name of faucheurs, greatly resemble 

 spiders, and are remarkable for the length of their limbs ; 

 others have the mouth formed for suction, and constitute the 



