14 EXTERNAL STRUCTURE OF INSECTS 



tory pits in and on papillae or the antennae. The function of taste 

 is located in some of the mouth parts, and minute hairs on the body 

 are connected with nerve endings within and form part of a mechan- 

 ism affording a delicate sense of feeling. The sense of touch is 

 doubtless most highly developed in the antennae, hence these are 

 often called the "feelers." 



Reproduction in Insects. The two primary forces directing 

 animal life are the need of nutrition and the instinct of reproduc- 

 tion. Normally we find among insects, as in many other animals, 

 mating and egg-laying, the egg hatching and producing the larva 

 which later gives rise to the pupa. 



Agamic Reproduction. In a few forms, however, notably 

 among the plant lice, we have agamic reproduction, that is, 

 several generations produced without mating, and the phenomenon 

 is so striking and at such variance to the rule that we give here a 

 brief outline of reproduction of plant lice, which, in a general way, 

 is similar in practically all of the genera. 



From an egg which has passed the winter on the favorite food 

 plant of a species, there hatches in the spring a female called the 

 "stem-mother." This insect in a few days gives birth directly to 

 other females; each one in its turn does the same, averaging five 

 or six young a day for several days before the mother's activity is 

 ended by death. This process, known as parthenogenesis, endures 

 throughout the summer. During the summer and particularly 

 towards fall winged females are born called "migrants," who fly 

 to other parts of the tree or plant, or to other trees and plants, and 

 start new colonies. In the fall, true sexual males and females ap- 

 pear, mate, and the eggs are laid which produce the stem-mothers 

 in the following spring. The enormous number of descendants 

 resulting in the autumn from even the one stem-mother can be 

 estimated approximately, and one can readily realize how destruc- 

 tive plant lice may be and how difficult to exterminate. 



ORDERS OF INSECTS 



As already stated, the class Insecta is divided into a number of 

 main divisions called orders. Entomologists recognize nineteen 

 of the main orders, besides several sub-orders. These orders 

 are here briefly described. 



Order Thysanura. Wingless insects without metamorphosis, 

 the larval form being retained by the adult. True compound eyes 

 rarely present. Examples, fish moth and spring tails the former 



