DISTRIBUTION OF INSECTS. 153 



may be killed by other insects, which are thus indirectly useful 

 or injurious. 



The vast importance to foresters of forest insects, the 

 enormous amount of damage which these small but mighty 

 members of Nature's household can effect, combined with the 

 fact that, owing to their small size and obscure mode of life, 

 they escape observation much more readily than injurious 

 vertebrates, render it necessary to spend more time on their 

 study. 



A full account of the anatomy of insects will not be attempted 

 here, and the works of Altum, Eatzeburg, etc., may be referred 

 to, the most comprehensive work on forest insects being the 

 revision of the 8th edition of Eatzeburg's book by Judeich 

 and Nitsche.* 



The following works also merit attention : 5th Eeport of the 

 United States Entomological Commission, "Forest Insects," 

 by Dr. A. S. Packard, Washington, 1890; " Manual of Injurious 

 Insects," by Miss Ormerod, London, Messrs. Simpkin, Marshall 

 & Co., 1890; "Indian Forest Zoology," by E. C. Cotes, 

 Calcutta, 1893; and "Injurious Insects in Indian Forests," 

 by E. P. Stebbing, now being published in parts. " Leitfaden 

 der Forstinsectenkunde," by Dr. Nusslin (Paul Parey, Berlin), 

 an excellent work for students. 1905. Mr. A. T. Gillanders 

 is now bringing out a comprehensive work on Forest 

 Entomology. 



Section II. — Distribution of Insects. 



The geographical distribution of insects may be considered 

 both hurizontally and vertically. 



As regards the ]iurizo)ital distribution of insects, the local 

 mobility or the wandering nature of many species precludes 

 the possibility of defining zones similar to those laid down for 

 plants. It may be stated roughly that there are, in Central 

 Europe, fewer species in the north and east than in the south 

 and west. Beetles, however, form an exception to this rule, 

 being more abundant in the north and east. As instances of 

 special areas for certain insects may be quoted : The ash-cicada, 



• " Lebrbuch (ler Mitteleuropaischen Forstinsektenkunde " (alsacbte Auflage 

 \ou Ratzeburg's Die Waklvenlerber und ihre Feindc) berausgcgebcu von 

 Dr. J. F. Judeich u. Dr. H. Nitsche. Vienna : Eduard Hlilzel. 1S8'J— 95. 



