INTRODUCTION.* 



Entomologists are realizing more clearly every year the tre- 

 mendous importance to agriculture and human economy of the 

 groups of insects having the predaceous or the parasitic habit. 

 There has long been a strong suspicion that without these 

 natural enemies of insects man would be helpless in combating 

 destructive crop-pests. The recent great destructiveness of 

 certain insects introduced from foreign countries, followed by 

 the successful repression of these pests in certain instances 

 with their natural insect enemies (likewise introduced from 

 foreign lands), has emphatically confirmed this opinion. 



There seems to be not the slightest doubt that predaceous and 

 parasitic insects are man's greatest ally in keeping destructive 

 insects in control. They work without the necessity of any 

 attention on the part of man and for the most part so unob- 

 trusively, that most of us are unaware of their existence. It 

 is only when some combination of conditions checks their 

 numbers that we experience for a year or two a serious out- 

 break of the pests which these beneficial insects have been 

 holding in submission and which would otherwise be seriously 

 destructive every year. 



It therefore seems desirable that we learn as much as pos- 

 sible about the life-histories, habits and biological relation- 

 ships of such of these insects as occur in our own country. Not 

 only because of the scientific interest which such facts may 

 have, or so that we may learn to appreciate their value, but 

 also in the hope that we may be able to conserve these natural 

 resources, and possibly artificially foster, encourage, increase 

 and transport, or extend the distribution of, the most useful 

 ones. 



One of the most universally distributed and destructive of 

 insect groups is the Homopterous family Aphididae, or as they 

 are commonly called, plant-lice or aphids. This group owes 



*The numbers in parentheses in the text refer to papers in the 

 bibliography, which are serially numbered. 



