105 
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1 
THE FOOD RELATIONS OF PREDACEOUS BEETLES. 
No facts are of more fundamental importance to a correct under- 
ending of the general principles of economic entomology than those 
ating to the fluctuations of numbers among insects. While it is 
ibably true that all species fluctuate more or less, their numbers 
Tying considerably, one year with another, it is certainly also true 
it different species differ extremely in this particular, some re¬ 
dlining relatively constant, and others undergoing the greatest 
cremes of abundance and scarcity. 
Even without experience of the fact, we might easily see that the 
lely fluctuating species must be most injurious to agriculture, 
ainst the attacks of those insects which, appearing year after year 
|i the same numbers, produce a uniform and steady drain on their 
i r i ources, the plants infested by them have necessarily learned to 
|)tect themselves by producing a surplus of sap, of foliage, of 
om and of Iruit; and we consequently find it a general rule with 
| nts of all descriptions, both wild and cultivated, that they will 
' lure a considerable loss of numbers or of substance without ap- 
Lciable injury to the organism or species as a whole, or to its 
Toductive power. 
j! 
3ut against the overwhelming attack of those enemies which leave 
or a time unmolested, and then burst forth in innumerable, de¬ 
ling hosts, it is far less easy for the vegetable world to defend 
|'lf; and such insect outbreaks never fail to leave their traces for 
j onsiderable period. How greatly the damage to agriculture in¬ 
ked by insects of inconstant numbers, subject to uncontrollable 
breaks, exceeds everything done to our crops by those of the 
re constant class, a few comparisons of familiar species will make 
lent.- If we contrast the consequences of a visitation of the “rocky 
antain locust” with the effects on vegetation of even the corn¬ 
iest of our resident grasshoppers, or if we compare the damage 
j\e by the chinch-bug with that attributable to all other members 
’ ts order taken together, or the injuries of the army-worm with 
j se of the common “grass-worms” of our meadows, we shall have 
king but fair illustrations of the relative harmlessness of those 
Lets whose numbers vary but little from year to year. In short, 
| > not too much to say that if the oscillations of insects could be 
I pressed so that each species should be represented each year by 
identical number of individuals, by far the most important nroh- 
s of economic entomology would be solved. 
