342 ARTICULATA: INSECTA. 



formed by a membranous fold. The tracheae communi- 

 cate with the air through openings called stigmata. 



The nervous system consists of a small brain and a mass 

 of thoracic ganglia which connect with more or less de- 

 veloped ganglia in the abdomen. 



Most kinds of spiders have eight ocelli ; but some kinds 

 have only six, some only two, and certain species which 

 inhabit caves are regarded as blind. 



One of the most wonderful parts of the structure of 

 spiders is their silk-spinning apparatus. On the abdomi- 

 nal extremity there are from four to six protuberances, 

 each of which is perforated with a great number of minute 

 holes in some species as many as a thousand in each pro- 

 tuberance. From these minute holes passes the adhesive 

 fluid or liquid silk, which has its origin in internal reser- 

 voirs ; and as soon as the fine streams of this material 

 come to the air they harden into silk. 



The eggs of Spiders are enclosed in cocoons spun from 

 the same kind of material of which they construct their 

 webs, and the form of the egg-oases or cocoons varies 

 according to the species. The young remain in the CO- 

 FIG. 466. 



Egg-case or Nest of a Spider " Vase-marker " found on a grape-vine. 



coon for a long time, and grow there to double the size 

 which they have when hatched, apparently without food. 

 Some kinds (Myyale) of spiders have four lung-sacs 

 and four stigmata, and hence Latreille called them Te- 



