7O ECONOMIC ZOOLOGY 



both in number of species and, possibly, in the number of individuals, 

 with the probable exception of the Protozoa. In some ways the insects 

 are the most highly organized of the invertebrates, and exhibit an in- 

 telligence far superior to that of many of the lower vertebrates. 



It is a very distinctive group, without the wide variations in struc- 

 ture seen, for example, among the Crustacea; their name "Hexapod" 



FIG. 51. Acorn barnacle, Balanus sp. Xi. 



indicates one of their most marked characteristics, the invariable pos- 

 session of six legs, born on the thorax. There are usually two pairs of 

 wings, also born on the thorax. The body is distinctly divided into 

 three regions, head, thorax and abdomen. The respiratory organs are 

 branching air-tubes, or trachea, extending all through the body. The 



FIG. 52. Peripatus capensis, drawn from life. Somewhat enlarged. (From 

 Hegner, College Zoology, after Sedgwick.) 



eggs usually hatch into a grub or caterpillar, and undergo a marked 

 metamorphosis in development to the adult condition. The adult is 

 terrestrial or aerial, less often aquatic, but the larval stages of many 

 forms are strictly aquatic. Some forms, bees, ants, etc.; form complex 

 communities, with marked polymorphism. 



V. Arachnida, Figs. 59 to 63. This rather heterogeneous class 

 includes the spiders, scorpions, mites, ticks, king-crabs, etc.; though the 



