SAFETY DEVICES IN BIEDS WINGS GRAHAM 



285 



their normal development. The hooks are designed to engage with 

 the sj)iny barbules on the next barb toward the tip of the feather. 

 Both hooks and spines are very flexible, and that is why a feather 

 can be made to return to its proper tidy state after one has withdrawn 

 the hooks from their hold on the spines by rubbing it up the wrong 

 way, as, for instance, when using the feather as a pipe cleaner — 

 unless, of course, the pipe is a very foul one; then, nothing will 

 avail. 



Spines 



.R»°dii 



20. — Section 

 barbs outside 



Figure 20. — Section through adja- 

 cent barbs outside the friction 

 area 



Figure 21. — Section through adja- 

 cent barbs in the friction aroa 



Figure 20 is a sectional view of two such barbs with the hooked 

 barbicels branching downwards off the tip-side barbules. In the 

 friction area of a slot-forming feather, however, the tip-side bar- 

 bules do not terminate at the point where the last barbicels branch 

 off downward, but go on, with a sharp upward bend, as shown in 

 Figure 21, and bear several more hooked barbicels.*' These give the 

 friction area its typical rough appearance, and their purpose is to 

 hook on to the next overlapping feather and prevent overspreading. 



3 



Figure 22. — Section through the 

 unemarginated parts of adja- 

 cent slot-forming feathers with 

 the friction area not engaged 



Figure 23. — As in Figure 22, but 

 here the friction area of the 

 lower feather is just coming into 

 operation at Z. AB is the total 

 breadth of the friction area 



During the earlier stages of the spreading process the protruding 

 underside of the shaft of an overlapping feather rides over the fric- 

 tion area of the lower feather and prevents its engaging (this phase 

 is shown in Figure 22) ; but at the critical moment, when the slot is 

 approaching the fully open position, the sharply curved-down lead- 

 ing edge of the upper feather arrives at the forward margin of the 

 friction area of the lower one, and the hooks engage, gradually locking 

 the feathers together, except for a certain amount of " give " due to 

 the springy nature of the barbules and barbicels. Figure 23 shows a 



• Only a few wings have been examined for this peculiarity, but it is suspected that all 

 slot-forming feathers possess it to a greater or less degree. 



