Insects and their world 



and over certain sensitive hairs on the face. Other experiments with 

 locusts have suggested that the apparent movement of the ground is 

 used as a guide. 



The speed at which insects can fly has been much exaggerated. As we 

 have seen, the damselflies and scorpionflies flit about quite slowly, and 

 their speed is said to be 1-3 m.p.h. — a walking-pace. Bumblebees and 

 hoverflies, though their wings move too quickly to be seen and give 

 them the appearance of great speeds, in fact move at only about 6-10 

 m.p.h. Horseflies follow cars and trains at speeds up to about 25-30 

 m.p.h. Hocking (1953) calculates an absolute maximum speed of 36 

 m.p.h., and a sustained speed of not more than 24 m.p.h. for one of the 

 biggest Australian dragonflies, and says that he doubts if any insect can 

 go much beyond this. 



The legend that a certain fly can reach 800 m.p.h. is based on an 

 imsound observation. In 1926 an entomologist reported that he had 

 seen botflies cover a distance of 400 yd. in one second, which works out 

 at 818 m.p.h. Both time and distance were estimated, without using any 

 instruments. In later papers he claimed to have recognised the species 

 of the fly, and its sex, and yet said that it was 'of course totally invisible 

 at top speed*, and became visible only when it slowed down to avoid 

 him ! It seems incredible that anyone ever took such a claim seriously, 

 but instead of questioning the soundness of the original observation, 

 people made elaborate calculations to prove or disprove that it was 

 possible to push such a hairy insect through the air at supersonic speeds, 

 and if it were, whether the energy necessary could be generated in so 

 small a body. 



Soaring Flight 

 We have already seen how insects may be supported on the surface of 

 water, if they have a surface area that is large in proportion to their 

 weight. A similar process occurs in the air. Very small insects fall 

 through the air extremely slowly, because they generate so much 

 frictional air resistance in proportion to their weight. This is true if they 

 are very soft-bodied and light in weight, like the aphids; or have hairy 

 wings which enormously increase the air-resistance, like the thrips 

 (Order Thysanoptera); or are exceedingly minute, like many parasitic 

 Hymenoptera: the fairy-flies (Mymaridae), parasitic in eggs of other 

 insects, may be less than one-quarter of a millimetre long. 



Once these insects have taken wing they are like a feeble swimmer in 

 a strong current, and are carried away bodily, not only horizontally with 

 the wind, but vertically by convection currents and atmospheric turbu- 



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