THE LEPIDOPTERA 297 



plants for its young must be found. There can be no doubt, in theory 

 at least, that there have been many cases in the past where failure to 

 succeed in this has resulted in the failure to establish themselves, of 

 many species which would have been serious pests. 



Once started, however, even in a small way, an increase in numbers 

 and in distribution becomes possible. If some of the insects, however, 

 were parasitized and the parasites escaped, as well as those not so affected, 

 the spread might be checked because of the small number of the pests 

 which would not be found by the parasites. 



The spreading of a species from the point where it starts, has been 

 aptly compared to that of a ripple caused by throwing a stone into water, 

 which passes out in every direction on its surface. Such a spread will 

 extend as far as the insect can find food on which it can live and a tem- 

 perature and humidity under which it can survive. It follows that for 

 many insects adapted to northern climatic conditions, a point will be 

 reached in its southward spread where the temperature and humidity are 

 such as to prevent its going farther. A lofty and continuous mountain 

 range may, by producing such conditions, also prove a barrier to farther 

 extension in that direction, even though beyond the range a favorable 

 climate may again be found. Absence of any food upon which an insect 

 can live will also put an end to distribution in that direction, and a pest 

 adapted to the moist climate of the Eastern States may find itself unable 

 to establish itself in arid regions. The rapidity with which it spreads 

 appears to be determined by its fecundity, power of flight in many cases, 

 and food supply, at least generally; an insect having a high rate of in- 

 crease, abundant food, and strong in flight sometimes spreading several 

 hundred miles in a year. The much larger area to the north and north- 

 east of Boston than to the south and west, now occupied by the Brown- 

 tail Moth appears to be due, in part at least, to strong southwesterly 

 winds while the moths are flying. 



Study of these and other factors involved, shows that northern insects 

 as they spread southward are found chiefly at least, on higher land. One 

 living at near the sea level in the Northern States will generally be found 

 in the mountains in the South, and if it extends into Mexico it will there 

 occur only on the higher Cordilleras, gaining by its elevation the lower 

 temperature it has lost by its change of latitude. 



Thus we find that with sufficient information at hand, the distribution 

 of many insects can be mapped, and that there is a division of the country 

 into regions, the insects of one region rarely spreading far beyond its 

 limits, and then only forming outposts of the species. 



It is true that some species are less affected than others by these 

 conditions. The Monarch Butterfly, the House Fly and many others 

 appear to be able to live under wide differences of temperature, humidity 

 and the other factors concerned. As a whole though, an insect will 



