ARANEIDA OE SPIDEKS. 103 



iii. Aearina or Mites (fig. 41) are characterised by having the 

 head, thorax, and abdomen all fused into one piece, and the legs 

 either eight or four. 



Ararmda or Spiders. 



The Spiders may be considered of economic importance, as 

 they are all more or less of a beneficial nature. The food of 

 spiders consists almost entirely of insects, and thus they do a 

 considerable amount of good in keeping down an access of insect 

 life which may be noxious to us. Some spiders hunt for their 

 prey ; others, and those perhaps best known to us, have the habit 

 of spinning webs, in which their prey is trapped. Spiders have 

 either two, six, or eight eyes or ocelli (fig. 39, b, /), which vary 

 in position and size, and which supply important characters of 

 systematic value. The jaws with which the spider seizes its 

 prey are inserted immediately under the anterior margin of the 

 cephalothorax, and have, as a rule, on the extremity of their 

 inner surface a groove, with sharp teeth at the sides, into which 

 fits the fang when in a state of repose (b, n, and o, m). The fang 

 or last joint of the so-called "falsis'' is perforated by a minute 

 hole at the tip, through which a colourless poisonous liquid 

 exudes, secreted by the poison-glands within. 



Perhaps the most important structural feature to notice in the 

 spider is the presence of the so-called "spinnerets." These 

 structures are placed at the end of the body, and consist of two, 

 three, or four pairs of appendages. The spinnerets vary in 

 form : some are cylindrical, others round, and some conical. 

 The tip and under-side of these structures are perforated with 

 minute holes. Through these minute apertures the liquid sub- 

 stance that will harden and form the silk of the web is passed. 

 Thus it will be seen that numbers of minute threads of this 

 glue-like liquid are passed out, and these unite into one firm yet 

 elastic line of extreme fineness, but nevertheless of sufficient 

 strength to bear a heavy-bodied spider. This line is also covered 

 with little drops of a clear liquid (fig. 40, c), so that the spider's 



