Insects and their world 



Fig. 55. A flea. Order Siphonaptera. This species is found on the wood- 

 mouse 



fore-wing. This is the final stage in making the fore- and hind-wing 

 beat as one. 



Yet another solution to the difficulties of managing two pairs of wings 

 is to get rid of one pair altogether. Mayflies (Fig. 4) have gone a long 

 way in this direction, reducing the hind-wings to small ovals, with an area 

 much below that of the broad fore-wings: occasionally the hind- wings 

 have gone altogether. The true flies (Diptera, Figs. 47, 49-5O5 52-54) 

 have converted the hind-wings into knobbed organs called halteres, 

 which beat during flight, and which are beheved to act hke gyroscopes, 

 enabhng the fly to balance better, and taking over some of the functions 

 of directional control from the wings. The fact that flies are among the 

 most efficient performers in the air suggests that it is better to have one 

 really good pair of wings than to try to co-ordinate two pairs. 



The frequency of wing-beat varies from 5 per second in the Swallowtail 

 Butterfly to about 1000 per second in a tiny biting midge (Pringle, I957)- 

 An aeroplane has a number of separate devices for controlling its speed, 

 rate of ascent or descent, change of direction, and stability in the air. 

 An insect does all these things by minute variations in the path followed 

 by the wings in each beat. 



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