106 WILLIAM F. FISKE — THE BIONOMICS OF GLOSSINA ; 



a result, or else parasites or disease, or predatory foes would so increase as to 

 threaten extermination. The species by migration avoids both disasters. By 

 the aid of its migratory instincts it is enabled to maintain itself in greater average 

 abundance, permanently, than would be possible otherwise. 



Only one notable instance came to the writer's personal observation in which 

 the migratory instinct of Dendroctonus frontalis lay dormant for more than two 

 generations. This was a colony on a mountain, in a spruce forest, and the 

 conditions were unfavourable. For three years this colony remained in one 

 locality without notable increase or decrease. This exception is very sig- 

 nificant. When struggling under disadvantage at home, to have tempted the 

 clangers of migration would perhaps have been fatal. From an unfavourable 

 locality it did not, therefore, fly, but conserved all its forces to maintain its 

 existence against unusual odds. This is a very necessary concomitant to 

 migration from favourable localities, because in exceptional seasons when climatic 

 conditions are bad all localities may be unfavourable. 



It is also to be noted that even under the most favourable conditions the 

 migratory instinct lay dormant for one or two generations. Its existence would 

 never be guessed by anyone studying the species for a short period in any 

 locality, nor for an indefinite period in a locality unfavourable to the insect. 

 Neither would it be at all likely to be observed were it not for the fact 

 that the species is notably gregarious and thus much more easily followed in 

 its movements. 



Migration as a factor in Natural Control. 



Since all insects (or other animals) possess powers of reproduction in excess of 

 what is actually necessary to enable the species to maintain itself at a constant 

 average abundance, it is essential for the good of each species that their powers 

 of reproduction be curtailed, or that the superabundant individuals be destroyed. 

 It is also essential for its greatest good, that its actual, prevailing abundance in 

 any particular locality should be maintained as nearly as possible at the average 

 permitted by the laws governing the natural equilibrium between co-existent 

 species. Otherwise, if its numbers fluctuate over a wide range, it is liable, 

 during years of unusual abundance, to destroy permanently its food supply,* or 

 during years of extreme rarity, to become extinct. In order that this constant 

 average may be maintained uniformly, some element in control of the sort which 

 has been termed " facultative " is necessaiy. This, by the terms of its definition, 

 must become relatively more efficient when the insect increases above its average, 

 and relatively less efficient when its numbers diminish below this average. Some 



* This rather frequently happens when an insect subjected to efficient natural control in one 

 faunistic region is introduced into a new and favourable faunistic region where such control 

 does not exist. Coccids, for example, may live for indefinite periods in large numbers on 

 certain kinds of trees, without appreciably injuring the health of these trees, provided their 

 increase is efficiently controlled. But without efficient control they quickly become so abundant 

 as to cause the death of the particular varieties of plants on which they are absolutely 

 dependent ; these plants are replaced by other varieties, and the insect is for ever deprived oi 

 its means to exist in that locality. This has actually taken place in America, following the 

 introduction of the San Jose 8cale, the Elm Leaf Beetle, the Gipsy Moth, etc. 



