74 THE COCKROACH [CH. Ill 



is possible, but it is a very uncommon occurrence to meet 

 a specimen on the wing. In flight the wing-covers (anterior 

 wings) are held almost at right angles to the body on the 

 right and left sides and apparently make but slight active 

 vibrations, acting chiefly as aero-planes. The posterior (true) 

 wings are the organs of propulsion; their front edge is fairly 

 strong and rigid and offers a cutting edge to the air and 

 is not easily deflected, on the other hand the bellying 

 hinder margin is far more pliant, and at each stroke of 

 the wing is thrust upward by the pressure of the air so 

 that its under surface slants obliquely and thus gives 

 the insect a forward push. This action can be imitated 

 by folding one edge of a half-sheet of ordinary note- 

 paper so as to stiffen it for about half an inch all 

 along and then, holding the sheet horizontally a few 

 feet above the ground, release it and allow it to fall; 

 the unfolded, more pliant portion of the paper will bend 

 upwards a little and the sheet will not fall straight to 

 the ground but will glide obliquely downwards, travelling 

 steadily in the direction of the stiffened edge. In an 

 experiment conducted with such a simple model a distance 

 of about 9 feet was traversed horizontally by the sheet of 

 paper in falling from a height of 7 feet. The propelling 

 power of all wings whether of birds or of bats or of insects 

 depends more or less upon a similar pliant hinder portion. 

 The precise effect upon the flight of the varying arrange- 

 ment of " veins " in insect wings is entirely unknown and 

 presents a series of problems in aerostatics that would tax 

 the highest mathematical ability. 



When at rest the flight-wings of the cockroach are 



