STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 6 1 



himself is responsible for the carrying of lice from district to district, 

 often resulting in war time in serious epidemics. 



The Colorado potato beetle migrated eastward from its home in 

 Colorado, preferring the cultivated potato to the wild solanums, and 

 the asparagus beetles followed the lines of travel westward from the 

 Atlantic. 



On the other hand, there are many barriers to the spread of insects: 

 oceans, seas, mountain ranges and deserts are natural barriers and 

 tend to prevent wide distribution. Climatic conditions, such as 

 temperature and moisture, are also very important factors in insect 

 distribution, often indirectly due to the absence of suitable food plants. 

 The chinch bug and the Rocky Mountain locust, for example, do 

 not thrive under moist conditions. 



As the habits of insects are very variable, some being limited, 

 others almost cosmopolitan in their distribution; some sensitive to 

 •emperature and moisture extremes, others more or less indifferent 

 to these factors; some feeding only on one or two hosts, others more 

 or less general feeders and therefore not so likely to be restricted in 

 their range, the problem of the determination of probable insect 

 distribution is not an easy one to solve. Moreover, the presence or 

 absence of parasites complicates the problem. 



Dr. Merriam's map of the Life Zones of North America shows 

 the distribution of birds and mammals and plants into seven zones 

 running east and west, viz.: Arctic, Hudsonian and Canadian oi the 

 Boreal region; the Transition, Upper Austral and Lower Austral of 

 the Austral region; and Tropical (see Map).^ It must be understood, 

 however, that the boundaries of these zones are not hard and fixed, 

 for there is necessarily an overlapping to some extent. It is interesting 

 to note that the great agricultural areas of Canada and the United 

 States lie mainly in the Transition and Upper Austral zones. Recent 

 studies go to show that in general insects tend to conform to the same 

 zonal distribution, although several forms, such as the house-fly, 

 mosquito, chinch bug and army-worm, range through several zones. 



^ Merriam states the laws of temperature control as follows: 

 I. "Animals and plants are restricted in northward distribution by the total 

 quantity of heat during the season of growth and reproduction; and 2. "Animals 

 and plants are restricted in southward distribution by the mean temperature 

 of a brief period during the hottest part of the year." 



