48 A BOOK OF INSECTS 



chute-like outgrowths from the sides of the body. Such 

 outgrowths would act as gliders, enabling the insect to 

 prolong its leaps ; and it is interesting to note that an 

 Australian spider actually possesses folds or flaps, one on 

 each side of the abdominal region, which spread out when 

 the creature launches itself into the air. Analogous con- 

 trivances are seen in other animals, such as the Australian 

 flying phalanger — the so-called " sugar squirrel," the flying 

 squirrels of the Northern Hemisphere, a few lizards and 

 at least one frog; while the flying-fish rushes at high 

 speed through the water, hurls itself into the air, and 

 spreading its huge, wing-like fins, glides rapidly forward 

 until its momentum is exhausted. Professor J. R. Ains- 

 worth Davis has pointed out that when once the ancestors 

 of insects had developed parachute-folds, useful for 

 gliding from one twig or grass-stem to another, " the 

 establishment of joints between the folds and the thorax 

 would be of service in guiding even parachute movements, 

 and from this stage on it is not difficult to imagine the 

 gradual modification of the folds into wings." 



The manner in which insects fly has been investigated 

 by Professor E. J. Marey, the chief authority on animal 

 locomotion. Briefly, the wing may be regarded as a lever, 

 the fulcrum of which is situated at a point where the base 

 of the wing projects into the thoracic cavity. To the 

 short arm of the lever a muscle is attached by which the 

 wing is raised ; while a stronger muscle, by means of which 

 the powerful down-stroke is effected, is attached to the 

 long arm just beyond the fulcrum — i.e. just outside 

 the wing joint. Other muscles, which are not attached 

 directly to the wing, assist flight by altering the shape of 

 the thorax as the wings rise and fall ; but by an elaborate 

 series of experiments Professor Marey showed that, owing 

 to the structure of the wing itself, simple up and down 



