vi PREFACE TO THE SECOND EDITION 



the incorporation of new material has amphfied this section, 

 wliile Locusts and the Phase Problem form a subject which has 

 been rewritten and expanded. Finally, the two chapters on 

 Biological Control have required extensive revision in order to 

 bring them more nearly up to date. Minor additions and 

 alterations, which are numerous, do not require individual 

 mention. 



It will become apparent, on comparing the present edition with 

 its predecessor, that certain sections of the book are reprinted 

 in toto or with only slight modifications. This does not necessarily 

 imply that no recent work has aj^peared with reference to the 

 sections in question. On the other hand, it may be taken that the 

 results of such work do not materially alter the conclusions or 

 better exemplify the facts or phenomena concerned. 



As compared with the first edition, this book has been enlarged 

 to the extent of 56 pages. Of the illustrations, twenty-seven are 

 new, while eighteen of the earlier figures no longer appear. It 

 needs to be reiterated that the book is for students and teachers, 

 and does not cater for specialists in the particular fields that 

 are covered. The necessity for a second edition is regarded as an 

 indication that the book has, on the whole, fulfilled its purpose. 



A. D. IMMS. 



Zoological Laboratory, 

 Cambridge. 



