258 TRANSACTIONS OF THE NORTHERN 



Insects vary extremely in the method and the degree of their preva- 

 lence. Some, like the codling moth and the plum curculio, are always 

 with us, but varying more or less in abundcince from year to )''ear. 

 Others, like the Hessian fly, the chinch bug, and the army-worm, come 

 upon us at longer or shorter intervals, being swept away in the interims 

 either by adverse climatic influences, or by the agency of natural enemies, 

 or, in extreme cases, by our abandonment for a season or two of the 

 damaged crop. Some, like the oyster-shell bark-louse, seem to be 

 undergoing a slow but sure process of extinction ; whilst others, like 

 the grape-leaf and root-louse, may be insidiously increasing upon us, 

 as its representative in Europe has already done to a very serious ex- 

 tent. 



The astonishing multiplication of certain kinds of insects in particu- 

 lar seasons is well calculated to excite both our wonder and alarm. 

 Scarcely a year passes that some destructive onslaught of this kind does 

 not occur. Last season the famous army-worm overran various por- 

 tions of the country, especially northern Wisconsin, and several of the 

 south-eastern counties of Iowa. These insects are so called because 

 they march like an army of soldiers, all in one direction, turning aside 

 for no obstacle. At Peshtigo many wells were rendered useless by rea- 

 son of these worms falling into them by the thousand, when the varia- 

 tion of a foot or two from their straight line of march would have car- 

 ried them safely past. Three years ago the tent caterpillar of the for- 

 est swarmed in such profusion in southern Illinois and in Kentucky, 

 that in several instances railroad trains were reported to have been 

 stopped by the myriads of these insects swarming upon the road, and 

 greasing the rails with their crushed bodies. 



Both of the caterpillars just mentioned, however, are usually restrict- 

 ed to small sections of territory, sometimes involving but two or three 

 counties in their ravages. Far difi'erent is the case with the notorious 

 chinch bug, which, year before last, swept over forty thousand square 

 miles of territory, and was estimated to have destroyed thirty millions 

 of dollars worth of spring wheat and barley. You all know what chinch 

 bugs are, and most of you have probably seen them accumulate in par- 

 ticular localities, so that you could shovel them up by the half bushel. 



I have just said that the enormous multiplication of these and some 

 other insects is well calculated to excite our astonishment. The question 

 which naturally occurs to every one is, where do these legions of insects 

 come from so suddenly.^ for it sometimes seems as if the very dust under 

 our feet had become vivified into crawling vermin. The answer to this 

 question is to be found primarily in the enormous prolificacy of certain 

 species of insects, the females of some species being known to lay five hun- 

 dred eggs or upward. It would also undoubtedly be found, if we had traced 

 the matter carefully, that the increase has not been so sudden as it appear- 

 ed. They do not really spring out of the dust, but must all have come by 

 a regular process of generation. A certain species of insect may be pret- 

 ty numerous one year, but not sufficiently so to do serious damage, or 



