A RAMBLE UP BURNHOPE. 97 



and with them, the almost exactly similar Melanostoma, 

 whose males have normal fore-feet. How is it that two flies, 

 otherwise so similar in appearance and habits, should have 

 these joints so different? I suppose there must be some 

 advantage to the male Platycheirus in the dilated fore-feet or 

 they would not have been developed. But what can their 

 purpose be is a question that suggests itself to me every time 

 I look through the lens at their pretty little paws. Buzzing in 

 front of the flowers were some large bee-like flies belonging 

 to the genus Eristalis, all of whose members are wonderful 

 hoverers. It is this habit of almost constantly hovering which 

 has given to the members of this family of the Syrphidae their 

 EngUsh name of hoverer flies. I watched them as they 

 poised apparently without motion in front of the opened 

 flowers, some silently, some emitting a loud hum. How is 

 that motionless poise maintained ? And why are some species 

 silent, while others hum ? Is the humming due to vibration 

 of the wings alone, or is it otherwise produced ? If the former, 

 why is it not always emitted ? And why do some species 

 hum while hovering, but, apparently, are silent while flying? 

 An Eristalis was hovering in front of a flower ; in a moment it 

 was gone, and the humming had stopped. But only for a 

 moment ; there it is again, a yard or two away, and hovering 

 and humming as before. It seems to hum when it hovers, 

 but to be silent when it makes its darting flight. Yet if, on 

 the one hand, the hovering be due to the wings vibrating at 

 such a speed as to produce equilibrium; and if the sudden 

 darting flight be due to increased rapidity with an altered 

 angle; and if, on the other hand, the hum be solely due to 

 wing vibration, then we would expect, not cessation of sound 

 during flight, but alteration of pitch. But perhaps the flight 

 is due not to increased rapidity of wing vibration, but to a 

 more powerful wing-stroke, by which the point of the wing is 

 carried through a much larger arc in flying than in hovering, 

 while the rapidity of the wing-strokes may really be reduced. 

 But is the humming due to wing-vibration ? Certainly many 

 flies produce sound otherwise than with the wings. A quick 



