294 FOREST ENTOMOLOGY. 



aphis weighs toVo f a grain avoirdupois, and a stout man not more 

 than two million grains, about twenty stones, then the tenth brood 

 formed (exclusive of all the preceding broods, supposing that the 

 multiplication had been altogether unchecked by the various causes 

 which generally influence it) will exceed in weight five hundred 

 million men, or about one-third the entire population of the globe. 

 This calculation is made on the assumption that each individual 

 member could turn the scales at 280 lb., and also that each aphis is 

 capable of producing only twenty young, whereas, according to 

 Latreille, the average rate of production of a viviparous aphis during 

 the summer months is twenty-five per day, but this is greatly in 

 excess, as far as this country goes. 



Fortunately for vegetation, the aphis has several natural enemies 

 to keep the enormous reproduction within bounds. Heavy rains act 

 as a check, but, entomologically considered, the small Chalcididce, 

 Hymenopterous parasites which pass their metamorphic stages in the 

 interior of the aphidian bodies, are of inestimable value in keeping 

 them down. Throughout the summer months the dead bodies of the 

 Aphididse may be found adhering to the food-plant. These bodies are 

 mere chitinous shells, with a small hole whence the tiny Hymenop- 

 terous parasites have emerged. Ladybirds, horse-flies, Chrysops, and 

 birds are natural enemies. 



At the end of the season the last brood for the year is generally 

 composed of males and females. In certain species males have not 

 been discovered for a few years. But investigation seems to show 

 that sooner or later males will appear -verifying what Andrew 

 Knight said in 1799 regarding self-fertilisation, viz., that it is a "law 

 of nature that organic beings shall not fertilise themselves in per- 

 petuity." Subsequent research has borne out this statement in other 

 animals. It is well known to every entomologist, collector, or econ- 

 omist that the number of insects depends very considerably on the 

 conditions of the weather and the supply of food. The Aphididae are 

 certainly more susceptible to those external influences than any other 

 class of insects. Should the weather suddenly become colder, a 

 great number of winged specimens will appear, as they do also in the 

 event of the food-supply becoming scarce. 



It is neither by chance nor latent ability that apparently apterous 

 individuals suddenly become winged, but apterous specimens give 

 birth to young which develop wings. Hence the change from wing- 



