240 SYSTEMATIC ZOOLOGY. 



through the nervous system, a condition which is totally 

 different from anything found in the vertebrates. Be- 

 hind this second or infraoesophageal ganglion a double 

 nerve-cord extends along the floor of the body, connect- 

 ing a series of similar ganglia. In the lower insects there 

 is a ganglion in each segment, but in the higher these tend 

 to move forward and to unite with each other into a few 

 masses or compound ganglia. It will thus be seen that 

 this nervous system is strikingly like that of an annelid 

 or crustacean. 



The eyes are always on the head. In the adult insects 

 compound eyes are usually present, and besides these 

 there may also be simple eyes or ocelli. In the latter 

 there is but a single lens, while the compound eyes are 

 composed of many distinct visual structures, each with its 

 own lens. Organs, which are regarded as ears, occur in 

 various forms. In the grasshoppers these organs are on 

 the base of the abdomen; in the crickets, on the legs; in 

 many groups the antennae are supposed to have auditory 

 powers. Taste resides chiefly in the lower lip, while 

 touch, though found all over the body, is especially devel- 

 oped in the antenna? and the palpi of labium and maxillse. 

 In some insects the sense* of smell is strongly developed, 

 and there is reason to believe that the olfactory organs 

 are in the antenna?. 



The group of Insecta may be subdivided in two ways, 

 accordingly as different characters are employed. If we 

 follow one method the mouth-parts form the basis of 

 division, and we have a mandibulatc group in which the 

 jaws are fitted for biting, as in the grasshopper and beetle; 

 while in the haustellate group the mouth-parts are no 

 longer fitted for biting, but form a tube through which 

 liquid food is sucked, as in the bugs and butterflies. 



