VIII INTRODUCTION. 



may be arranged according to their function under 

 two heads. 



1. Muscles that move the segments. 



2. Muscles that move the appendages. 



These of the first class consist of numerous, " dis- 

 tinct, isolated, straight fibres, which are not gathered 

 into bundles united by common tendons, or covered 

 by tendinous .sheaths to form distinct muscles, as in 

 the Vertebrata, but remain .separate from each other." 

 (16, II., 935). The form of these muscles is deter- 

 mined by that of the parts to which they are attached. 

 They have, however, this character, — their sides are- 

 straight. 



Tho.se of the second class usually differ from the 

 preceding in being united at the distal e.xtremity by 

 tendons. 



The strength of the muscles of insects is very great. 

 (16, II., 942). The rapidity of action is also great. 

 The House-fly vibrates its wings 352 times a second ; 

 the Honey-bee 440 times a second, (i, 362). 



Digestive system. — The alimentary canal in its sim- 

 plest form, as seen in the larvae of insects, is a straight 

 tube passing from one end of the body to the other. 

 In the larva of Stylops and the young of bees it ends 

 in a blind sac. In the adult insects it becomes devel- 

 oped into a long convuluted tube, having different 

 parts performing different functions. (Examine mod- 

 els). 



The appendages to the alimentary canal are the 

 salivary glands, the Malpighian, or biliary tubes, and 

 the so-called uriniferous organs. 



Circulatory system. — " Abdominal dorsal vessel or 

 heart ; eight chambers, separated by very complete 



