ARACHNIDA. 455 



the surface of the water. Many of them load one side of 

 the case with heavier pieces, so as to keep that side down- 

 ward. 



The Genus Neuronia contains the Half-banded Cad- 

 dice-Fly, N. fasciata, Say, which is about an inch long, 

 and expands more than an inch and a half, with the gen- 

 eral color tawny. 



SUB-SECTION VIII. 



THE ORDER OF ARACHNIDA, OR SPIDERS. 



THIS Order embraces insects which have the body 

 divided into only two well-marked regions, the head 

 and the hind body ; the head and thorax being closely 

 united into one piece. They are wholly destitute of 

 wings, have simple eyes, eight legs, and are subject to 

 no changes in form in coming to maturity, which they 

 reach after moulting their skins six times. Their legs 

 are attached to the forward region, and the hind body 

 is soft or little protected, and with some exceptions is 

 very large in comparison with the head. Most of them 

 feed on insects proper. Some are parasitic on verte- 

 brated animals. They naturally divide into two groups, 

 Tracheary and Pulmonary Arachnida. 



The Tracheary Arachnida perform their respiration by 

 means of tracheae, which divide near their origin into 

 various branches, not, as in insects proper, forming 

 two trunks which run parallel to each other through the 

 whole length of the body, receiving air through numer- 

 ous stigmata, and receive air through only two stig- 

 mata, and these situated near the base of the abdomen ; 

 the number of their simple eyes is never more than four. 

 Such are the Pseudo-Scorpions and their allies. 



The Pulmonary Arachnida perform their respiration 

 by means of pulmonary sacs, situated in the under part 

 of the abdomen, into which the air is admitted by means 



