344 THE ZOOLOGIST. 



only that the thunder-clouds extend over ahout two-thirds of the 

 whole expanse of sky as measured from the horizon to the zenith. 

 The insects vanish as suddenly as they appear, so that hardly 

 one of them is discoverable on the following morning. It is not 

 known whether they proceed further west, though this is probably 

 the case. It is certain, at any rate, that they do not remain here, 

 otherwise one would undoubtedly find them lying about dead after 

 the heavy rain of a thunder-storm." 



Herr Gatke has expressed his conviction (op. cit. p. 116) that 

 Lepidoptera during their more extended flights are subject to the 

 same meteorological influences as birds, and it is curious that in 

 regard also t© Dragon-flies it should have been observed by him- 

 self (p. 89) and by Mr. Hudson (op. cit. p. 131), of different species 

 in two very different quarters of the globe, that these swarms of 

 insects suddenly make their appearance immediately before some 

 atmospheric disturbance, such as a heavy thunder-storm or a 

 high wind. 



It is observable also that while superabundance of life, in the 

 case of the Lemming and Locust, requires to be checked by the 

 intervention of certain natural enemies — the former being attacked 

 by Owls, Hawks, and Buzzards, the latter by Pratincoles and 

 Storks* — no such interference has been observed in the case of 

 Butterflies and Dragonflies, more lowly organized creatures, 

 which go to their destruction unharassed, yet driven as it were 

 by some inherited impulse which they are apparently unable to 

 withstand. 



From the incomplete nature of the statistics at hand it is not 

 possible to draw any definite conclusions as to the cause of the 

 migrations observed ; but taking the case of Vanessa cardui as 

 furnishing the greatest number of recorded observations, the 

 following statements may be taken as fairly correct, or at least as 

 warranted by the evidence : — 



(1) The migration does not take place annually, but at 

 irregular intervals, and only in those years in which there has 

 been an abnormal abundance of the species, or failure of the 

 food plant ; when there is, so to say, a surplus population to be 

 got rid of. 



:; ' rratincola nordmanni and Ciconia alba, known respectively to African 

 colonists as the " Small" and " Great Locust-birds." 



