14.9 



CHAPTER IV. 



FOREST INSECTS. GENERAL ACCOUNT. 



Section I. — Classification. 



Entomology is the science dealing with iilsects, which belong 

 to the division of the animal kingdom termed Arthropoda, 

 including all animals the bodies of which are bilaterally sym- 

 metrical and composed of segments, that is, of successive 

 transverse divisions which present a more or less complete 

 recurrence of structural features, and which have articulated 

 appendages. The body and its appendages possess a hardened 

 exterior, formed of a substance termed cldtin, 'similar in 

 character to, but not identical with, horn, and the muscles 

 are internal, and attached to the external skeleton. The sym- 

 metry of the successive segmients is not complete from end to 

 end of the body, and is more evident in the body- walls and 

 their appendages, than in the viscera. The segments are 

 aggregated into deiinite groups, the components of which are 

 more nearly related to each other, particularly in the structure 

 of their appendages than to the segments of the other groups. 

 This form of segmentation is termed lictcronomous. The 

 nervous system consists of a double chain of ganglia placed 

 along the ventral surface of the body, connected with each 

 other longitudinally and transversely by nerve-commissures, 

 and traversed anteriorly by the digestive system ; the vascular 

 system is dorsal. Eespiration is effected in various ways. 



The four great Classes of Arthropoda are : — 



1. Crustacea: respiring by branchiae or gills, or by the 

 general surface of the body ; with two pairs of antennae and 

 more than eight locomotive appendages, the latter forked or 

 biramous. Crabs, lobsters, shrimps, woodlice, etc. 



2. Avachnida : respiring in various ways, usually air- 

 breathing ; head and thorax united ; with two pairs of jaws 



