358 ELEMENTS OF BIOLOGY 



Insect Types. The number of species of insects is far greater 

 than that of all other animals taken together. Various authorities 

 recognize nineteen different orders of insects, the order characters 

 being wing differences, differences in mouth parts, and differences 

 in number of stages in their life cycles, that is, in metamorphosis. 

 According to the number of stages in their development, insects are 

 divided into two groups, insects of incomplete metamorphosis, and 



FIG. 213. FIG. 214. 



Figs. 213 and 214. — A common bug and a common beede, showing the difference 

 in the position of the wings and wing covers when at rest. 



insects of complete metamorphosis. Insects of incomplete meta- 

 morphosis pass through three stages in development, namely, egg, 

 nymph, and adult. Insects of complete metamorphosis pass through 

 four stages, egg, larva, pupa, and adult. In insects of incomplete 

 metamorphosis, the nymph and adult, and in insects of complete 

 metamorphosis the larva and usually the adult, feed. Nymphs, 

 larvae, and adults are therefore the stages that may be economically 

 important because of damage to crops, trees, and other property of 

 value. 



Uninformed observers frequently regard any insect as a bug, but 

 bugs are in fact only one order of insects. The bugs, order Hemip- 



