points are as follows: In the fall both males and egg-laying females develop. After mating the males soon die; and the females, after depositing one or more winter eggs, likewise perish. The eggs may often be found on the terminal buds of trees, for example, on the terminal buds of the apple-tree three or four minute black eggs are laid. In the spring the eggs hatch and produce wingless females known as stem-mothers. Instead of laying eggs the stem-mothers by budding give birth to living young. The second and many succeeding generations consist, like the first, wholly of females, which may be wingless, but winged females may appear at any time and fly away to new plants. This is the spring migration. The tender upper growing shoots of trees are likely to he infested first, and later the lower branches. Many plant-lice have the singular habit of abruptly changing their food-plants. During the spring they feed on one kind of plant; after the spring migration during the summer on another kind; and in the fall, winged females again reappearing, they migrate back to the first kind of host-plant. This return in the fall to the spring food plant is known as the fall migration. The rosy-apple aphis passes the summer on the leaves of plantains, returning to the apple in the fall. Reproduction without pairing may continue for eight or more generations; but shortly after the fall migration both males and egg-laying females again develop. In warm climates plant-lice are believed to reproduce exclusively by parthenogenesis, years of collecting in tropical regions having failed to disclose the male form.
Plant-lice multiply with extreme rapidity, and it has been estimated that the offspring of each plant-louse, if all survived, would in 100 days amount to over 3,200,000 individuals, and at the end of 200 days to ten trillions. Fortunately they are held in cheek by a vast number of parasitic insects, such as syrphid flies, lady-beetles, and plant-lice lions, or they would threaten the destruction of all vegetation and drown the world in a flood of honey-dew. Their development is greatly influenced by climatic conditions. Occasionally there comes a year when plant-lice and scale-insects appear in hosts, and there is, consequently, a great abundance of honey-dew, as in 1884 and 1909 in this country, and 1898 and 1907 in Great Britain. In 1909 there was in eastern North America an unprecedented amount of honey-dew, while the crop of white clover and basswood was almost a complete failure. Most of the honey-dew came from the leaves of hickory and oak. While gathering it the bees were exceptionally cross. Since it became alternately partially liquid in the forenoon, and gumlike in the afternoon, they were able to collect it only in the morning hours; the moisture in the air softened it at night, but by noon the sun had again dried it to a viscous state. Honey-dew honey is often stored by the ton, and in certain localities, as in the Sacramento Valley, California, a crop is gathered almost every year.
The dew is forcibly ejected or flipped from the end of the abdomen; and when there are many aphids it falls in a spray of minute globules. If the dew were not thrown a little distance from their bodies they would soon be glued together. As they usually feed on the under side of leaves, the sweet liquid naturally drops on the foliage beneath them. As it is gumlike it may dry and remain on the leaves for a long time, so that the absence of plant-lice is no proof that it is of vegetable origin. If it is very abundant it may drip from the leaves to the ground. In 1891, Busgen observed that a single plant-louse on a maple leaf produced 48 drops in 24 hours (the drops were 1-25 of an inch in diameter) ; on a basswood leaf 19 drops, and on a rose leaf only 6 drops. The production of honey-dew has been found to be most active in the middle of the day when the temperature is highest.
Hemipterous insects of the families described live wholly on plant sap. The mouth-parts form a jointed beak consisting of four slender bristles enclosed in a jointed sheath, which is a prolongation of the upper lip. With this pointed beak