105 
THE FOOD RELATIONS OF PREDACEKOUS BEETLES. 
No facts are of more fundamental importance to a correct under- 
standing of the general principles of economic entomology than those 
relating to the fluctuations of numbers among insects. While it is 
probably true that all species fluctuate more or less, their numbers 
varying considerably, one year with another, it is certainly also true 
that different species differ extremely in this particular, some re- 
maining relatively constant, and others undergoing the greatest 
extremes of abundance and scarcity. 
Even without experience of the fact, we might easily see that the 
widely fluctuating species must be most injurious to agriculture. 
Against the attacks of those insects which, appearing year after year 
in the same numbers, produce a uniform and steady drain on their 
~ resources, the plants infested by them have necessarily learned to 
protect themselves by producing a«surplus of sap, of foliage, of 
bloom and of fruit; and we consequently find it a general rule with 
plants of all descriptions, both wild and cultivated, that they will 
endure a considerable loss of numbers or of substance without ap- 
preciable injury to the organism or species as a whole, or to its 
reproductive power. 
But against the overwhelming attack of those enemies which leave 
it for a time unmolested, and then burst forth in innumerable, de- 
vouring hosts, it is far less easy for the vegetable world to defend 
itself; and such insect outbreaks never fail to leave their traces for 
a considerable period. How greatly the damage to agriculture in- 
flicted by insects of inconstant numbers, subject to uncontrollable 
outbreaks, exceeds everything done to our crops by those of the 
more constant class, a few comparisons of familiar species will make 
evident. If we contrast the consequences of a visitation of the “rocky 
mountain locust” with the effects on vegetation of even the com- 
monest of our resident grasshoppers, or if we compare the damage 
done by the chinch-bug with that attributable to all other members 
of its order taken together, or the injuries of the army-worm. with 
those of the common ‘‘grass-worms”’ of our meadows, we shall have 
striking but fair illustrations of the relative harmlessness of those 
insects whose numbers vary but little from year to year. In short, 
it is not too much to say that if the oscillations of insects could be 
suppressed so that each species should be represented each year by 
an identical number of individuals, by far the most important prob- 
Jems of economic entomology would be solved. 
