BRANCH ARTHROPODA; CLASS INSECT A : THE INSECTS 187 



marked sex dimorphism ; in numerous species the males 

 are winged while the females are wingless, and in a few 

 cases this condition is reversed. Where there is a 

 difference in size between male and female, the females 



FIG. 51. The auditory organ of a locust (Melanoplus sp.). The large clear 

 part in centre of the figure is the thin tympanum, with the auditory 

 vesicle (small black pear-shaped spot) and auditory ganglion (at left of 

 vesicle and connected with it by a nerve) on its inner surface. (Photo- 

 micrograph by Geo. O. Mitchell.) 



are usually the larger. Fertilization of the egg takes 

 place in the body of the female and, strangely, this fertil- 

 ization is effected after the eggshell has been formed. In 

 all insect eggs there is a minute opening in one pole of 

 the eggshell called the micropyle through which the 

 sperm-cells enter. In a few cases the young are born 

 alive, but such a viviparous condition is exceptional. In 



