142 INSECTS 



stead of a higher temperature extending northward, 

 a low temperature extends southward. This is rarely 

 followed or accompanied by a southward migration, 

 but is very frequently attended by a great mortahty 

 among the southern insects which are unable to with- 

 stand the drop in temperature. Some species, indeed, 

 are so sensitive to cold that a drop of io° or even 5" 

 below the normal winter temperature causes a serious 

 mortality. 



Besides temperature, the amount of moisture has 

 a very decided effect on insect life. Some species do 

 best in dry weather, others flourish only when there 

 is an abundance of moisture, and sometimes a sudden 

 change from one condition to another will produce a 

 complete change in insect conditions within twenty- 

 four hours. Thripids as a rule require dry weather, 

 and after a period of drought and heat, the air may 

 be full of the little creatures not over an eighth of an 

 inch in length and so slender as to be almost invisible. 

 A cold rain lasting a few hours may reduce them to 

 so small a number as to make them practically undis- 

 coverable. Agriculturists sometimes take advantage 

 of this pecuHarity by spraying infested plants with cold 

 water, and that is really about as satisfactory a method 

 of control as we have. 



Every one who observes nature at all, has probably 

 noticed that in some seasons insects are much more 

 abundant than they are in others, and, more specifi- 

 cally, certain kinds may be almost completely absent 

 or on the other hand frightfully abundant. Now in 

 most cases these differences are largely and in some 

 even exclusively due to climatic conditions. There 

 is no greater check to insect life than adverse weather, 

 and many of the differences in abundance attributed 

 to other causes are really due to climate. 



