ADAPTATION 



227 



formed of feathers in birds. Both tail and wings arc inchidcd, the 

 latter alone as propelling organs. In other flying animals, such as 

 the bats, similar organs are formed of folds of skin extending from 

 limb to limb along the sides of the body or stretched between 

 elongated bones (Fig. 134). The fins of flying fishes express the 

 same adaptive tendency as fins in general, but carried to an ex- 

 treme correlated with the Ughtness of the air (Fig. 135). Insect 

 wings are entirely different; since the exoskeleton provides rigidity 



Fig. 134.— Bats. 



A, Vesper lilio noctula, an insectivorous species; B, Pteropus 

 sp., frugivorous. (From Lull.) 



throughout the body, the wings are simple broad flat evaginations 

 of the body wall, stiffened by local thickenings, the veins, between 

 which the cuticula is extremely thin and fight (Fig. 136). 



Power and endurance are closely correlated. Enlargement of 

 the muscles that move the organs of flight is necessary, and is ex- 

 treme in birds. The large pectoral muscles, attached along the 

 keel of the sternum, make up the masses of breast meat with which 

 everyone is familiar. The rhythmic contraction of these muscles 

 during long flights demands an abundant supply of food and oxy- 

 gen, and rapid removal of wastes. These needs are met by modi- 

 fications of the alimentary tract, providing for the storage of a 



