Human experience in the past has often been crystallized into homely saws, of which the one that states that “ Prevention is better than cure ” should be regarded as one of the foundation stones of all work in economic entomology. We see this put into practice in such routine measures as the destruction of mosquito and fly larvae, ploughing fields in the dry weather to destroy grasshopper egg-masses, the destruction of weeds and stubble, the collection of egg-masses, caterpillars and hoppers, the enforced removal of cotton-plants after harvest, legislation against the introduction of pests into new countries, and similar measures which are so familiar that they need not be enlarged on here. Such measures are directed to and, if applied consistently, are largely effective in keeping within bounds insects which, if not thus checked, normally attain to destructive numbers, and by “ normally ” I mean that on the average they occur in such numbers every year, or for a series of years. There are, of course, fluctuations, due to natural causes, in the pest-status of many insects, which may be insignificant in normal years and occasionally rank as bad pests, or vice versa ; of such causes we know as yet very little, beyond the pious ascription of them to the effects of parasitic control or climate. But, after eliminating these fluctuations, we still often find our expectations upset by outbreaks for which it is very difficult to invoke any adequate explanation other than that they are the effect of a large influx into the area affected of the insect concerned. Such a theory may appear far-fetched at first sight, especially as it is very difficult of proof in most cases, but we find in India an accumulation of cases for which such an explanation seems the only one possible.