Class 4. Arachnida. Order 2. Aranema. 283 



Order 2. Aranema {Spiders). 



The Spiders may be distinguished from other Arachnidas by the 

 separation of the cephalothorax from the abdomen by a deep 

 constriction. Both regions are unsegmented, but newly-hatched 

 animals show indications of abdominal segmentation. Anteriorly the 

 cephalothorax bears a group of six to eight ocelli, arranged in various 

 ways. The chelicerae consist of a simple, strong basal joint, and a 

 claw-like terminal segment, at the tip of ,whioh opens a poison gland. 

 The pedipalpi are antenniform, with broad basal joints; the terminal 

 joint in the adult males is modified, hollowed out, etc., and thus 

 adapted for introducing the spermatozoa into the genital aperture of 

 the female; it is often very complicated in form. The legs are very 

 strong, often of considerable length. Anteriorly, on the ventral 

 surface of the abdomen, there are always two stigmata, each leading 

 into a lung-sac; in a few Spiders, posterior to these, there is a 

 second pair, which either (Mygale) lead into a similar pair of lung- 

 sacs, o r {Argyroneta) lead into tracheee. In most of the Spiders, 

 however, this second pair of stigmata is wanting, instead there is an 

 unpaired stigma posteriorly, just in front of the spinnerettes, and 

 this leads into a variously modified system of tracheae. The majority 

 of this group possess both lungs and tracheae; a smaller 

 number have only lungs, but in this case there are four. Posteriorly, 

 below the anus, there are four or six spinnerettes, fairly large 

 processes beset with a larger or smaller number of short, fine tubes (in 

 Epeira, altogether about 700) ; at the apex of each of these is the 

 aperture of a silk gland, which lies in the abdomen. These spinning 

 glands may vary considerably in structure, even in the same animal, 

 and may give rise to different secretions. As the substance is 

 pressed out through the tubules it hardens to form fine threads ; 

 in many Spiders which form webs some of these threads remain sticky . 

 By means of the feet the fine threads may be woven together into 

 coarser ones ; all Spiders spin cocoons for their eggs, many form 

 webs or tubes, in which they live. The genital aperture lies 

 anteriorly on the abdomen. The males are often smaller than the 

 females, sometimes the difference is so great that, although in other 

 respects the structure is normal, they have been termed dwarf 

 males. Spiders feed chiefly upon Insects which they kill with 

 their chelicerse. There are very many species, but the group 

 is very uniform, and is abundantly represented in temperate 

 countries. 



As examples may be mentioned : the Bird-spiders (My gale) , large tropical 

 forms, tMckly covered with, hairs, and possessing foiu- lung-sacs ; they will even 

 attack and consume small Yei-tehrata ; the common C r o s s - s p i de r (Epeira 

 diadema), which, like the House-spider (Tegenaria domesticd), spins webs, 

 and often lives in or near houses; the Water -spider {Argyroneta aquatica) 



