6o THE BIOLOGY OF INSECTS 



arrangements the two wings of a side are coupled and present 

 an extensive surface to the air, which is compressed as they 

 are pulled downwards by means of the depressor muscles, 

 and turned backwards by the action of the flexors and 

 by the wing's elasticity. This atmospheric compression 

 leads to the resistance which is the mechanical agent in 

 supporting and propelling the insect during flight. Then 

 in the great order of the two-winged flies (Diptera), the 

 forewings alone are developed as organs of flight, while in 

 the beetles and earwigs the hindwings only are efficient, 

 the forewings being modified into firm sheaths (elytra). 



Our knowledge of the mechanism of insect flight is 

 largely due to the researches of E. J. Marey (1895), who by 

 obtaining tracings of the wing-tips of flying insects on the 

 smoked cylinder and fastening a spangle of gold leaf to the 

 tip of the wings, vibrating as in flight, of a wasp held by 

 forceps in bright sunlight, demonstrated that the path 

 (trajectory) of the wing- tip is a narrow and elongate " figure 

 of eight." Marey 's work has been supplemented by the 

 special studies of F. Stellwaag (1910) and W. Ritter (191 1) 

 on the flight of the hive-bee and the blow-fly respectively. 

 In the latter insect there are ten directly-acting muscles 

 attached to the sclerites of each wing-base ; these though not 

 working as the depressors and elevators of the wing, are of 

 much importance in effecting the suitable tension of the 

 wing, and possibly act in steering. 



Observation of insects during flight aflfords to the student 

 abundant opportunity of noticing how the details of wing 

 movement vary in diff"erent groups. How markedly, for 

 example, does the heavy flapping flight of many of our 

 larger butterflies contrast with the darting movement, 

 alternating with the apparently motionless poising in the air, 

 of a " Humming-bird " Hawk-moth or a hoverfly ! In the 

 former case the number of wing-strokes per second might be 

 roughly calculated by observation ; in the latter they can be 

 determined only by tracings on smoked paper compared with 

 those made by a tuning fork of standard vibration rate, or, 

 if the wing- vibrations produce an audible hum, by verifying 



