26 AN INTRODUCTION TO ENTOMOLOGY 



Class HEXAPODA 

 The Insects 



The members of this class are air-breathing arthropods, with distinct 

 head, thorax, and abdomen. They have one pair of antennae, three pairs 

 of legs, and usually one or two pairs of wings in the adult state. The 

 opening of the reproductive organs is near the caudal end of the body. 



We have now reached in our hasty review of the classes of arthro- 

 pods the class of animals to which this book is chiefly devoted, the 

 Hexapoda,* or Insects, the study of which is termed entomology. 



The number of species of insects now known is around 600,000, 

 perhaps more rather than less. The number of species yet undescribed 

 is purely problematical. Probably there are hundreds of thousands 

 of unknown forms distributed over the tropical portions of the earth. 



Insects vary greatly in size. Folsom says that some insects 

 are smaller than the largest protozoans, while some are larger than 

 the smallest vertebrates. A beetle, Dynastes hercules from Vene- 

 zuela, which may be 155mm. long, and a Venezuelan grasshopper, 

 Tropidacris latreillei, which may attain a length of i66mm., are 

 among the largest insects. Some moths of the genus Attacus may 

 have a wing expanse of from 240 to 2 5 5mm. while a BraziHan noctuid, 

 Erebus agrippina, is said to have a wing expanse of 280mm. On the 

 other hand, certain beetles of the family Trichopterygidae may be 

 but .25mm. in length, and some hymenopterous egg parasites are 

 even smaller. 



Insects are essentially terrestrial ; and in the struggle for existence 

 they are the most successful of all terrestrial animals, outnumbering 

 both in species and individuals all others together. On the land they 

 abound under the greatest variety of conditions, special forms having 

 been evolved fitted to live in each of the various situations where 

 other animals and plants can live; but insects are not restricted to 

 dry land, for many aquatic forms have been developed. 



The aquatic insects are almost entirely restricted to small bodies 

 of fresh water, as streams and ponds, where they exist in great num- 

 bers. Larger bodies of fresh water and the seas are nearly destitute 

 of them except at the shores. 



*Hexapoda: hex (?|), six', pons (ttoi/s), a foot. 



