ARTHROPODA. 291 



concentrated than in the preceding order. There are gen- 

 erally eight simple eyes, rarely six. They breathe both 

 by tracheae and lung-like sacs, from two to 

 four in number, situated under the abdomen. 

 All the species are carnivorous. 



The instincts of Spiders are of a high 

 order. They are, perhaps, the most wily of 

 Articulates. They display remarkable skill 

 and industry in the construction of their 

 webs; and some species (called "Mason Spi- piform organs, 

 ders") even excavate a subterranean pit, line it with their 

 silken tapestry, and close the entrance with a lid which 

 moves upon a hinge. 151 



CLASS IV. Insecta. 



Insects are distinguished by having head, thorax, and 

 abdomen distinct, three pairs of jointed legs, one pair of 

 antennae, and generally two pairs of wings. The number 

 of segments in the body never exceeds twenty. The head, 

 apparently one, is formed by the union of four segments. 

 The thorax consists of three the prothorax, mesothorax, 

 and metathorax each bearing a pair of legs; the wings, 

 if present, are carried by the last two segments. The ab- 

 domen is normally composed of ten segments, more or less 

 movable upon one another. The skin is hardened with 

 chitin, and to it, as in all Arthropods, the muscles are at- 

 tached. The organs of sense are confined to the cephalic 

 division of the body, the motor organs to the thoracic, and 

 the vegetative to the abdominal. All the appendages are 

 hollow. 



The antennae are inserted between or in front of the 

 eyes. There is a great variety of forms, but all are tubu- 

 lar and jointed. They are supposed to be organs of touch, 

 and also seem to be sensitive to sound. The eyes are 

 usually compound, composed of a large number of hexago- 

 nal corneae, or facets (from fifty in the Ant to many thou- 



