6o ECONOMIC ENTOMOLOGY 



metabolic activities of insects are related to both temperature and 

 humidity. He says: "Apparently there is a degree of atmospheric 

 humidity which being the most favorable to the maximum speed of 

 insect metabolism should be designated as the optimum; that this opti- 

 mum varies for each species, for each stage of each species, and for 

 each stage of each individual." 



The codling moth is an example of a common insect whose develop- 

 ment is greatly influenced by weather conditions. Even within the 

 limits of a single state or province the rate of its development and the 

 time of its stages are influenced by latitude, by early and late seasons, 

 by cool and warm seasons, and by wet and dry seasons. The student 

 will find in the observations of Simpson in Idaho, Pettit in Michigan, 

 Sanderson in New Hampshire, Hammar in Pennsylvania and Michigan, 

 Jenne in Arkansas, Caesar in Ontario, Headlee in Kansas, Siegler and 

 Simanton in Maine, Brooks and Blakeslee in Virginia, and Forbes 

 in Illinois much valuable data for investigations on the relation of 

 insects to climatic factors. 



The Distribution of Insects 



On account of the large increase of international trade many economic 

 forms of importance have been introduced into Canada and the United 

 States from other countries, and as it is a matter of pubHc interest 

 to know if such imported forms are likely to become injurious con- 

 siderable attention has been given of late years to the investigation 

 of this problem. The problem is not yet completely solved, but 

 progress may be reported. 



The common natural means of dispersal are flight, wind, animals, 

 and railways, etc. Many insects are able to fly long distances, many 

 are carried by wind currents and many are transported on materials 

 of commerce. A few examples of such dispersal may be cited. The 

 brown-tail moth is a good flyer, and is thus able to spread rapidly. 

 On the other hand, the female of the gipsy moth cannot fly, consequently 

 the spread of this insect follows the lines of trade. It has been observed 

 that the Hessian fly spreads most readily in the direction of the pre- 

 vailing winds at the time of the emergence of adults, and that the larvae 

 of the San Jose scale are carried by the winds. Again, warble and bot 

 flies are transferred from one district to another by their hosts. Man 



