How insects move 



sequently the effect of both indirect and direct wing muscles is more 

 complex than the simple picture given above. It is analysed in detail 

 by Pringle (i957)- 



Like most other structures, the wings have tended to become simpler 

 during evolution. No insect is ever known to have had three pairs of 

 wings, and most insects find two pairs difficult to manage without 

 some modification. The damselfiies and the scorpionflies (Mecoptera) 

 have the two pairs of wings almost identical. They fly well, but rather 

 slowly, with a twinkling motion of the delicate wings, more like heli- 

 copters than true aeroplanes. The two pairs of wings do not beat strictly 

 together: the hind pair is usually ahead on the down-beat, with" the 

 result that it gets a grip on the air before it is disturbed by the fore- 

 wing. 



Insects that use their wings in a businesslike way, to get them quickly 

 from place to place, seem to have found it necessary to do one of two 

 things : either to lock together the two wings on each side so that they 

 function as one; or to use one pair of wings to do most of the work of 

 flying, and either reduce the others to a small size, or to adapt them for 

 some other purpose. 



This process has started in grasshoppers, where the fore-wing is 

 narrower and more leathery than the hind one. It still beats like a wing, 

 but at rest it forms a cover, hiding the bright colour of the hind- wing and 

 camouflaging the insect. The hind- wing, on the other hand, is elabor- 

 ately folded like a fan. Beetles have gone further, with the fore-wings 

 hardened into stiff, horny elytra, which are held up at an angle during 

 flight. They do not beat, and their function is more that of fixed aerofoil 

 surfaces like the wings of an aeroplane. The hind- wings of beetles, in 

 contrast, have an exceptionally broad sweep, and may move through 

 nearly i8o°, from straight up to straight down. 



In the Hemiptera-Homoptera (plant-Uce and related forms) the fore- 

 wings are narrower and tougher than the hind pair, and in the Hemip- 

 tera-Heteroptera they are divided into a stiff part and a membranous 

 part, from which the name Hemiptera, or 'half- wings' is derived. On 

 the other hand, according to Pringle (1957), Hemiptera tend to rely 

 for flight mainly on the fore- wings, and 'even a cicada can fly satis- 

 factorily without the hind-wings'. 



Another solution, that of locking the wings of each side together, is 

 seen in Lepidoptera (butterflies and moths), where there is a tendency 

 for the insect to become a tiny, narrow body supported between the 

 enormous wings. Hymenoptera have filmy, membranous wings (hence 

 their name), which are firmly held together by the engagement of a tiny 

 row of hooks {hamuli) on the hind- wing with the trailing edge of the 



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