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PREFACE TO THE SECOND EDITION 



In preparing a second edition of this book two reasons have 

 led to the decision to confine its scope within the original limits. 

 Firstly, in any attempt to discuss recent advances in entomology, 

 at all exhaustively, several volumes, each the size of the present 

 one, would be needed. Even to include adequate extra chapters 

 the size of the book would need to be increased a good deal beyond 

 its prescribed limits. Secondly, those other aspects of entomology 

 that commend themselves for inclusion are, to a large extent, 

 provided for elsewhere. In this connection, insect physiology, 

 insects and climate, general morphology, insects and plant viruses, 

 for example, have already formed the subjects of recent books or 

 monographs. The writers of these several books have discussed 

 such subjects far more adequately than could be expressed within 

 the limits of single chapters. 



Certain aspects of entomology, naturally, have required more 

 revision than others. Under morphology, while current views on 

 Wing Venation have undergone but little change. Head 

 Segmentation and the Genitalia have needed considerable revision. 

 Also, recent studies of the musculature have led to revision of 

 views respecting the homologies of certain of the appendages 

 and their parts. As regards Metamorphosis, discoveries in 

 regard to the presence of hormones are discussed. Palaeontology 

 has developed so rapidly as to require very full revision of the 

 original chapter : several new illustrations, taken from recent 

 papers by Dr. F. M. Carpenter, have been included in this chapter 

 with the author's permission. In dealing with the Sense Organs 

 the subject of response to visual stimuli has been extended so as 

 to include the results of a good deal of recent investigation. In 

 this connection valuable help has been given by Dr. M. Hertz, 

 which is here gratefully acknowledged. The subject of Stimulatory 

 Organs has also been included. With regard to Biological Races, 



