HEXAPODA (INSECTS). 



The group of Insects contains more species than all the 

 rest of the animal kingdom together, a conservative esti- 

 mate placing the number of distinct forms at over half a 

 million. Yet all of these agree in certain essential points. 

 Thus, in all, the hody is divided into three regions, head, 

 thorax, and abdomen, and of these the thorax alone bears 

 organs of locomotion. Three pairs of legs are always pres- 

 ent (whence the name Hexapoda six-footed given to the 

 group). Of wings there may be one or two pairs. The 

 head bears four pairs of appendages, one pair (the antennae) 

 being sensory ; the others (mouth-parts) being used in 

 eating. Breathing is by means of tubes (tracheae) which 

 open on the sides of the body and which penetrate to all 

 parts of the interior. The sexes are always separate, and 

 the reproductive organs open at the hinder end of the body 

 just beneath the vent. 



In the head no evidence of segments is seen, except as 

 shown by the appendages. The antennae, of which there 

 are only a single pair, are sensory in function. In many 

 cases they clearly bear organs of smell, and in some they 

 may also be hearing-organs. In the primitive condition 

 the mouth-parts are fitted for biting and eating hard sub- 

 stances, the mandibles being strong jaws, while the maxillae 

 and labium serve to hold the food in place. These latter 

 bear jointed prolongations the palpi which are sensory. 

 In other insects these mouth-parts are modified and united 



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