PRACTICAL PAPERS—COLORADO POTATO BUG. 835 
bug is in certain seasons comparatively quite scarce, therefore 
it is about to disappear and trouble them no more. This is a 
very fallacious mode of reasoning. There are many insects— 
for instance, the notorious Army-worm of the North (Leucania 
unipuncta , Haworth)—which only appear in noticeable num¬ 
bers in particular years, though there are enough of them left 
over from the crop of every year to keep up the breed for the 
succeeding year. There are other insects—for instance, the 
Car.kerworm (.Anisopteryx vernata , Peck)—which ordinarily 
occur in about the same numbers for a series of years, and 
then, in a particular season and in a particular locality, seem 
to be all at once swept from off the face of the earth. These 
phenomena are due to several different causes, but principally 
to the variation and irregularity in the action of cannibal and 
parasitic insects. We are apt to forget that the system of 
Nature is a very complicated one—parasite preying upon 
parasite, cannibal upon cannibal, parasite upon cannibal, and 
cannibal upon parasite—till there are often so many links in 
the chain that an occasional irregularity becomes almost inevit 
able. Every collector of insects knows that scarcely a single 
season elapses in which several insects, that are ordinarily 
quite rare, are not met with in prodigious abundance ; and 
this remark applies, not only to the plant-feeding species, but 
also to the cannibals and parasites. Now, it must be quite 
evident that if, in a particular season, the enemies of a par¬ 
ticular plant-feeder are unusually abundant the plant-feeder 
will be greatly diminished in numbers, and will not be able to 
expand to its ordinary proportions until the check that has 
hitherto controlled it is weakened in force. The same rule 
will hold with the enemies that prey upon the plant-feeder, 
and also with the enemies that prey upon those enemies, and 
so on ad infinitum. The real wonder is, not that there should 
be occasional irregularities in the numbers of particular species 
of insects from year to year, but that upon the whole the 
scheme of creation should be so admirably dove-tailed and 
fitted together, that tens of thousands of distinct species of 
