216 ELEMENTS OF COMPARATIVE ZOOLOGY. 



nerve-cord passes on either side of the gullet, to unite in a 

 second enlargement (ganglion) behind. Thus, as will readily 

 be understood, the alimentary canal passes through the 

 nervous system, a condition which is totally different from 

 anything found in the vertebrates. Behind the infra- 

 cesophageal ganglion a double nerve-cord extends along the 

 floor of the body, connecting 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. 



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. 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 developed in the antennas 

 and the palpi of labium and maxillae. In some insects the 

 sense of smell is strongly developed, and there is reason to 

 believe that the olfactory organs are in the antennae. 



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 mandibulate group in which the 

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

 while in the Uaustellate 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. 



The second method of subdivision depends upon the 



