THEIR RELATION TO WEATHER 139 



of rain, vegetation appears at all, insects will be found 

 on it. And again, some insects occur in midocean 

 among masses of seaweed, undergo their transforma- 

 tions and develop generation after generation without 

 ever coming within reach of land. 



So we find that there is no climate and almost no 

 earthly condition in or under which insects do not 

 exist; yet, on the other hand, insects are, as a rule, 

 extremely sensitive to changes in climatic conditions, 

 and some of them succumb easily to any extreme range 

 of temperature, even within their native home. Zo- 

 ologists have divided the world into faunal regions 

 based on cHmate, and have subdivided these into 

 smaller regions based on the geographical conforma- 

 tion of the country whose fauna is under considera- 

 tion; and we have found in our studies that a large 

 number of insect species have an extremely restricted 

 faunal range. Beyond that range they do not thrive at 

 all, and not infrequently, where no natural barrier 

 seems to exist, spread nevertheless does not take place. 

 The check in such cases is weather in the broad sense 

 of that term, or, more accurately, the meteorological 

 conditions. The mere fact that any species of insect 

 'is regionally distributed usually indicates that any 

 climatic condition not normal to such region would 

 be fatal to it. An apparent exception occurs when 

 insects are confined to one food plant — the occurrence 

 of such plant being then a condition precedent lo its 

 existence at all. But this is merely shifting factors 

 about a little, for usually the climatic conditions deter- 

 mine the distribution of the plant and, in consequence, 

 really of the insect as well. 



We know that occasionally we have abnormal 

 seasons, and sometimes several seasons of the same 

 kind may occur in succession. When that happens 



