150 



INTRODUCTION TO ZOOLOGT. 



Fig. 143. 

 Flba (magnified). 



IV. SucTORiA. — These insects may be 

 represented by the common Flea (Pulex 

 irritans. Fig. 143). The mouth of the 

 Flea is formed for suction, and the hind 

 legs for jumping. The length of its leap 

 has been measured, and found to be two 

 hundred times that of its body — an extra- 

 ordinary instance of muscular power. 



Class V.— ARACHNID A— SPIDERS, &c. 



Fig. 144. — Mygale. 



The present class includes Mites, Scoi-pions, and Spiders. 

 They exhibit a more concentrated state of the nervous system 

 than insects; they do not undergo similar transformations; 

 and in the larger tribes there is a higher condition of the 

 respiratory system ; for they breathe not by air tubes, but by 

 "air sacs, or lungs."* They differ from true insects, also, in 

 their having four pair instead of three pair of legs. 



The eyes vary in number and position, but are never 

 compound. Spiders have the sense of hearing, but neither 

 the organ nor its situation is known: the same may be said 

 of the sense of smell. 



Owen, pages 250, 251 257, 260. 



