CHAPTER X 



THE INSECTS: HEXAPODA 



Though numberless these insect tribes of air, 

 Though numberless each tribe and species fair, 

 Who wing the moon, and brighten in the blaze, 

 Innumerous as the sands which bend the seas ; 

 These have their organs, arts, and arms, and tools, 

 And functions exercised by various rules. 



H. Brooke, Universal Beauty 



Hexapoda (Gr. hex, "six"; pons (pod-), "foot"). The 

 previous chapters have been devoted to entomology, that 

 branch of zoology which treats of insects. The insects be- 

 long to the class Hexap'oda, the most numerous of all classes 

 of animals, comprising four fifths of the animal kingdom. In- 

 sects are built externally upon the plan of a series of somites, 

 grouped in three regions and with jointed appendages on 

 two of them, — the head and thorax. Except in very few 

 cases, where this number is reduced, the imagoes have six 

 legs. Hexapods are found in every variety of situation, 

 though they are, as a whole, adapted to life on the land and 

 in the air. A system of tracheae is universally present in the 

 imagoes, though the young of some species have gills for 

 breathing in the water. These gills are usually supplied with 

 tracheae, but in some species blood gills are found. 



The hard, chitinous covering (exoskeleton) necessitates 

 frequent molts to provide for increase in size. Among many 

 of the lower orders of insects the young gradually grow into 

 the adult form without any change more sudden or conspic- 

 uous than the appearance of the functional wings at the last 



molt. This type of gradual change to the adult form, or 



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