76 



THE BIOLOGY OF BIRDS 



During coasting flight, in gliding or in sailing, there is 

 sometimes an interesting interlocking of the primary 

 feathers, as Trowbridge (1906) demonstrated in hawks. 

 He found that the edges of many of the primaries were 



Fig. 18. — The so-called " Flying Dragons," Droco volatis, parachut- 

 ing arboreal lizards of the Far East. From a specimen. The skin is 

 extended on about half-a-dozen elongated and movable ribs. As figure 

 A. shows, the parachute is folded close to the side of the body when the 

 animal is resting. Figure b. shows the parachute expanded as the 

 animal leaps. 



firmly interlocked. This makes the outer portion of the 

 wing more rigid when the bird is using it as a vol-plane and 

 it keeps the primaries partly extended without muscular 

 exertion. 



