INSECTS. 



93 



The body is deeply cut into segments, a peculiarity which 

 explains the origin of the word insect.* In the lower trihes 

 the segments of the body are numerous, and in some cases so 

 many as sixty or eighty pairs of legs may be counted on one 

 individual. From this circumstance the term " Mj^riapoda " 

 has been applied to the Centipede [Scolopmdra, Fig. 65), and 

 others of similar organization [Fig. 37). 



In the true insects, the body consists of three portions 

 {Fig. (j'o)', the head, with the " horns" or antennas, and the 

 organs of sensation ; the thorax or chest, with the organs of 



Antenniet 



\tt pair of Leg* 



Head. 



1st pair of JVingt -—\^.-= — == 

 id pair nf Legs 



2d pair (tf IVtngs - 

 3d pair of Legs ■■ 



Tibia - 



Tamit 



Aniioiir..!, 



Fig. C6. — External Anatomy or an Insect. 



locomotion, whether wings or legs; and the abdomen, includ- 

 ing the organs needful for nutrition and reproduction. 



The heart is an elongated muscular tube, situated alonor 

 tlie middle of the back, and hence called the dorsal vessel. 

 The circulating fluid is cold, transparent, and nearly colour- 

 less.t " The action of the heart is accelerated, as in other 



• Latin inseclus. cut or notchod. 



^ West wood, lut. to GlasMtication of Insects, page 15, vol. L 



