PARASITISM AND DEGENERATION 197 



feelers. Each becomes a mere motionless sac capable only 

 of sucking up sap and of laying eggs. The young males, 

 however, lose their sucking beak and can no longer take 

 food, but they gain a pair of wings and an additional pair 

 of eyes. They fly about and fertilize the sac-like females, 

 which then molt again and secrete the thin wax scale over 

 them. 



Throughout the animal kingdom loss of the need of 

 movement is followed by the loss of the power to move, and 

 of all structures related to it. 



103. Degeneration through other causes. Loss of certain 

 organs may occur through other causes than parasitism and 

 a fixed life. Many insects live but a short time in their 

 adult stage. May-flies live for but a few hours or, at most, 

 a few days. They do not need to take food to sustain life 

 for so short a time, and so their mouth parts have become 

 rudimentary and functionless or are entirely lost. This is 

 true of some moths and numerous other specially short- 

 lived insects. Among the social insects the workers of the 

 termites and of the true ants are wingless, although they 

 are born of winged parents, and are descendants of winged 

 ancestors. The modification of structure dependent upon 

 the division of labor among the individuals of the com- 

 munity has taken the form, in the case of the workers, of a 

 degeneration in the loss of the wings. Insects that live 

 in caves are mostly blind ; they have lost the eyes, whose 

 function could not be exercised in the darkness of the cave. 

 Certain island-inhabiting insects have lost their wings, 

 flight being attended with too much danger. The strong 

 sea-breezes may at any time carry a flying insect oft 3 the 

 small island to sea. Only those which do not fly much sur- 

 vive, and by natural selection wingless breeds or species are 

 produced. Finally, we may mention the great modifications 

 of structure, often resulting in the loss of certain organs, 

 which take place to produce protective resemblances (see 

 Chapter XII). In such cases the body may be modified in 



