ENTOMOLOGY 



CHAPTER I 



CLASSIFICATION 



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At the outset it is essential to know where insects stand in relation to 

 other animals. 



Arthropoda. Comparing an insect, a centipede and a crayfish 

 with one another, they are found to have certain fundamental characters 

 in common. All are bilaterally symmetrical, are composed of a linear 

 series of rings, or segments, bearing paired, jointed appendages, and have 

 an external skeleton, consisting largely of a peculiar substance known as 

 chitin. 



If the necessary dissections are made, it can be seen that in each of 

 these types the alimentary canal is axial in position; above it extends the 



FIG. i. Diagram to express the fundamental structure of an arthropod, a. Antenna; al, 

 alimentary canal; b, brain; d, dorsal vessel; ex, exoskeleton; /, limb; n, nerve chain; s, 

 suboesophageal ganglion. After SCHMEIL. 



dorsal blood vessel and below lies the ventral ladder-like series of seg- 

 mental ganglia and paired nerve cords, or commissures; between the 

 commissures that connect the brain and the subcesophageal ganglion 

 passes the oesophagus. These relations appear in Figs, i and 163. 

 Furthermore, the sexes are almost invariably separate and the primary 

 sexual organs consist of a single pair. 



No animals but arthropods have all these characters, though the 

 segmented worms, or annelids, have some of them for example the 

 segmentation, dorsal heart and ventral nervous chain. On account of 

 these correspondences and for other weighty reasons it is believed that 



