ENTOMOLOGY 



CHAPTER I 



CLASSIFICATION 



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 sym- 

 metrical, 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 : 





Diagram to express the fundamental structure of an arthropod, a, antenna; a/, 

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

 s, subcesophageal ganglion. After SCHMEIL. 



above it extends the dorsal blood vessel and below lies the 

 ventral ladder-like series of segmental 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. 



