The Bionomics of South African Insects. 465 



on the same mountain and on the tableland of St. Armand 

 (" Insect Life," English translation, London, 1901, p. 103). 

 A valuable account of a large number of observations will 

 be found in Mr. J. W, Tutt's numerous papers on " The 

 Migration and Dispersal of Insects" (Ent. Record, 1898- 

 1902). The autlior recognizes the dangers of over- 

 multiplication as a cause of migration (1. c. vol. xii, 1900, 

 p. 238: see also vol. xiii, 1901, p. 200). Numerous 

 examples quoted by him prove that movement in vast 

 bands, often at great distances from land, has been 

 observed again and again in those very species which 

 are remarkable for their wide geographical distribution 

 and occurrence upon oceanic islands. The appearance 

 of two species of Hybcrnia, H. defoliaria and H. anranti- 

 aria, observed in large numbers in Heligoland by Gatke, 

 cannot be explained on the hypothesis here suggested 

 because, the females being flightless, males only appeared. 

 In several other instances recorded by Mr. Tutt the 

 presence of both sexes is either specially affirmed or 

 implied. In the great majority of cases, however, no 

 observations of sex were made, and it is to be hoped 

 that careful attention may be paid to this point in the 

 future. The proportion of females to males would also 

 be deserving of careful investigation. 



The limits of this memoir are perhaps too wide already, 

 and it is impossible to attempt any discussion of these 

 observations in detail, but I have taken the opportunity of 

 making a suggestion as to the possible essential meaning 

 of the instinct. 



I have deliberately used the word " emigration " of 

 insects, because this term probably expresses the exact 

 state of the case. In response to some stimulus connected 

 with undue increase, immense masses of individuals move 

 out of an overcrowded area. The line of movement may 

 carry them to destruction or to plenty, in both cases 

 benefits are probably gained, although they are of course 

 much greater in the latter. True " migration " as of birds 

 and perhaps of fishes implies different and far higher 

 faculties — the memory of the individual summed up by 

 tradition into what may be called the collective memory 

 of the species. 



TEANS. ENT, BOC. LOND. 1902. — PART III. (NOV.) 31 



