110 INSECT PHYSIOLOGY 



stroke. In strong fliers like the dragon-fly Aeschna the flight muscles 

 comprise 24 per cent, of the total body weight; in the honey-bee they 

 make up 13 per cent. 



In the more primitive insects, Orthoptera, Neuroptera, Isoptera, 

 Odonata, the fore- and hind-wings are moved independently during 

 flight, but in Hymenoptera, Lepidoptera, Hemiptera, &c, fore- and 

 hind-wings are linked together to form a functional unit, while in 

 Diptera only the forewings remain. At the base of each wing there is 

 an elaborate articulation which causes the wing to twist in its long 

 axis during its upward and downward movement. This reversal of 

 inclination during the elevation and depression of the wing produces 

 the same mechanical effect as the revolution of a propeller blade. 

 Each wing, in fact, acts as a propeller which draws air from above 

 and in front and drives it backwards in a concentrated stream. The 

 flying insect thus creates a zone of low pressure above and in front, 

 and a zone of high pressure directly behind it. 



Except in the very smallest insects, which have wings made of 

 bristles instead of a continuous membrane, the flight of insects is 

 based on conventional aerodynamic principles. The wings act as 

 aerofoils and the variation in lift is controlled by differences in wing 

 twisting, by the frequency of wing beat, and by the total angle over 

 which the wing is moved. It might be thought that the repeated re- 

 versal of movement of the wings would be very wasteful of energy ; 

 but much of the energy utilized in depressing the wing is in fact 

 stored as elastic energy in the walls of the thorax and is released 

 again to help in raising the wing. The flight muscles are thus aided 

 by an elastic oscillation. Elastic energy is stored and released in a 

 slightly different manner by the click mechanism already described 

 (p. 108). 



The insect in flight is faced with the problem of maintaining its 

 equilibrium. In such insects as Tipulids with a long abdomen and 

 long legs, stability may perhaps be maintained mechanically. But 

 many of the best fliers, such as Muscid flies, are inherently unstable. 

 They maintain their equilibrium by the active control of the wing 

 movements. They derive the necessary information from several 

 sense organs such as the eyes, as well as the antennae which perceive 

 air movements. But perhaps the most important sense organs are the 



