( cxx ) 



The ravages of locusts are proverbial, yet in spite of their 

 enormous consumption of green food there is still enough of 

 it left in the countries they frequent to maintain them in 

 countless millions.* 



On the whole I think it would be difficult to show that any 

 species of vegetable-feeding insect was ever wiped out or 

 turned from a common kind to a rare one as a consequence 

 merely of its food plant having been all — or nearly all — eaten 

 down by itself or its congeners ; so that, whatever may be 

 the cause of the remarkable numerical constancy we find in 

 the individuals of different species, this persistence cannot be 

 sufficiently accounted for by the relation between the food 

 supply and its insect consumers.! 



Climate and disease. 



Extreme climatic variations are often very destructive, but 

 these climatic causes can hardly be accountable for destruction 

 of such a continuous or frequently recurrent nature as to pro- 

 duce a permanent reduction in numbers, an established species 

 having become fairly adapted to its climate. 



There is another source of destruction — that which is classed 

 under the general name of " diseases " — and this, though 

 especially rife when insects are bred under artificial conditions, 

 certainly also operates in a state of nature, but not, I think, 

 frequently to any very great extent. Observations by 

 naturalists on this subject, however, are certainly desirable, 

 though probably it is only with the assistance of those trained 

 in the study of some of the lower forms of life that the nature 

 of these diseases can be investigated with precision. 



I conclude, therefore, that we must look to something more 

 than failure or consumption of food-supply, climate or disease, 



the instinct of which despatches the teeming hosts that are bred to "fresh 

 woods and pastures new." Pyrameis cardui and the Ceylon Catopsila 

 pyranthe are notorious for their wandering, "not single spies but in 

 battalions." See Major Neville Manders, Trans. Ent. Soc. Lend. 1904, 

 pp. 701-6. 



* In Cyprus more than live thousand millions of egg eases gathered after 

 several years of British occupation. — Sharp, " Cambridge Nat. Hist.," 

 Pt. i, p. 292. 



t I suggest that this conclusion may be important, as tending to 

 exclude from a large field of operation what would otherwise be the 



Eowerful influence of direct mutual competition for food between 

 erbivoroua insects. 



