No. 4.] INJURIOUS INSECTS. 153 



A few years ago I attempted to determine the nmnber 

 of young a wingless female of our common apple louse 

 {Aphis mali) would give rise to, and how many generations 

 there would be in a year. For this purpose I selected a 

 branch of an apple tree suflSciently near the ground to be 

 easily observed, removed all the minute, polished, black 

 plant-lice eggs to be found in the crevices of the bark 

 except those reserved for the purpose in question, and 

 marked the branch so that I should not fail to recognize it. 

 These eggs hatched in due time, and the young lice crawled 

 to the nearest leaves, where they established themselves 

 and began to grow very rapidly. I carefully removed all 

 but one louse from each leaf, so that the chances of error 

 in the observation would be very small. Just at that time, 

 when everything was going on to my satisfaction, a cold rain 

 storm came on, which lasted twenty-four hours or more ; 

 and after it cleared off, much to my astonishment and dis- 

 may, there was not a plant louse to be found on that entire 

 tree, and yet before the storm they were swarming over all 

 parts of it except on the branch which I had cleared for my 

 observations. This led me to conclude that cold and 

 abundant rains were destructive to plant lice, and in wet 

 seasons we might expect a dearth of them on our fruit 

 and other trees. The experience of last summer, however, 

 which was the most rainy season we have had for years, 

 and at the same time one in which the plant lice were more 

 abundant all over the country than I have ever before 

 known them, leads me to the conclusion that I know very 

 little about what climatic or meteorological conditions are 

 favorable or unfavorable to the life of these insects. 



Some very careful observations were made in Europe 

 many years ago on the woolly aphis (Schizoneiira lanigera) 

 of the apple tree, which is quite as common in this country 

 as in Europe. It was found that the wingless female hatch- 

 ing from the Qgg in the spring gave birth to about one hun- 

 dred young in the course of her life, and that each of these 

 also gave rise to about the same number, and so on for ten 

 generations. This led me to make some mathematical cal- 

 culations bearing on the possible progeny of a single })lant 

 louse hatched from an Q^y^g in the spring. If, as in the case 



