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Plant-lice

(Family Aphididae.)

The plant-lice are very numerous not only in point of numbers of individuals but also of numbers of species. In many respects these insects are abnormal, especially in their method of development, and they have attracted great attention from naturalists since Bonnet discovered, 150 years ago, the peculiar phenomenon known as parthenogenesis, which is almost invariably the rule amongst these creatures. Their life history is very surprising and their relations with other insects, especially with ants, are most interesting. They are very destructive insects, preying upon practically all cultivated plants, and their rate of increase is so enormous that if it were not for their innumerable natural enemies they would probably annihilate the greater part of plant life. The classic computation of Huxley that the uninterrupted breeding of ten generations of plant-lice from a single ancestor would produce a mass of organic matter equivalent to the bulk of five hundred millions of human beings (about the population of the Chinese Empire) is by no means an over-estimate, but if anything an under-estimate. This rapidity in breeding is not due to extreme prolificacy since the number of offspring of a single female is rather small, but rather to the early age at which the offspring begin themselves to reproduce. Parthenogenesis, which means the virgin birth, i. e., the birth of individuals from a virgin female, and the fact that with most species and during a large part of the year the young are born alive, account for this rapidity. Generation after generation is produced in this way but in all cases sooner or later there comes a true sexual generation composed of both males and females which pair and these females as a rule lay eggs instead of giving birth to living young. In many cases it is in this egg stage that plant-lice pass the winter. Thus it will be seen that true males make their appearance only a single time in a number of generations. Another curious feature in the life of plant-lice is the fact that while the majority of the generations in the course of a summer are composed of wingless females there comes once or twice during each season a generation of winged females, thus providing for the dispersal of the species and not only for the dispersal of the species but for the return to a perennial plant in the autumn from the weeds, grasses and other annual plants upon which the summer generations may have fed.

Plant-lice are attacked by a host of natural enemies. The little Braconid parasites of the subfamily Aphidiinae are practically exclusively parasites of the plant-lice. Their eggs are laid in the body of the louse, the young live within the body until they reach full growth, and the little four-winged parasite eventually emerges as an adult. Plant-lice parasitized in this way become swollen and dark in color and can readily be distinguished. These parasites themselves breed with extraordinary rapidity and are frequently responsible for the practical extermination of the hosts of lice which are frequently found in the wheat fields in the spring. The lady-birds and the larvae of the syrphus flies and the golden-eyed lace-wing flies as well as the maggots of certain midges of the genus Diplosis and others breed upon plant-lice and in fact derive the greater part of their food from these creatures. Extreme prolificacy is the means which nature adopts to continue the existence of many otherwise unprotected and much harassed species. In this instance, however, it has taken the form of extreme rapidity of development instead of great prolificacy.

The relationship between plant-lice and ants has often been described and is one of the most fascinating topics in the whole field of nature study. The honey dew which is secreted by plant-lice both from the anus and from two tubes upon the back of the abdomen is greatly enjoyed by ants which in a surprisingly intelligent way care for the plant-lice, drive away their natural enemies, carry them bodily to better feeding grounds, recognize the necessity for migrations at times and assist in these migrations and actually prepare locations in advance of transportation. The term which is frequently applied to Aphids, viz, "the milch cows of the ants," is a very appropriate one.

Probably the most famous of the plant-lice in the economic sense are the grape-vine Phylloxera (Phylloxera vastatrix), a species which is said to have cost the French nation more than the indemnity paid to the Germans after the Franco-Prussian war, the woolly root-louse of the apple (Shizoneura lanigera) erroneously known in most parts of the world as the American blight, and the hop-plant louse (Phorodon humuli Schrank), a species which is fast becoming cosmopolitan and which for years has been the greatest enemy to the hop-growing industry in England and parts of this country.