12 



AFFECTING THE LEAVES. 



7. The Apple-tree Plant-Louse (Aphis Mali x Fabr). — Every one who cultivates a. 

 single foot of land, or even grows a house-plant in a pot, must know what a Plant louse is 

 like, for they are to be found at one time or another, we may safely say, upon every kind 

 of ordinary plant that exists in this country. The good wife who tends with anxious 

 care her geranium or fuchsia in the cottage window, knows full well how mysteriously the 

 little green pests come back on her plants, in spite of frequent washings with soap-suds 

 or f mokings with the old man's pipe ; the gardener knows how the same minute creatures 

 suck the juices of the majority of his vegetables and plants, and what a very plague they 

 oftentimes are in the conservatory ; and the fruit grower must assuredly have noticed the 

 curled up leaves of his currant-bushes all alive beneath with a loathsome mass of these 

 insects, or the blackened tips of the young shoots of his apple-trees, whose shrivelled 1 

 leaves swarm on the underside with myriads of tiny greenish lice. But few, perhaps, can 

 tell why it is that they are so numerous, and appear in such thousands on a plant that a 

 few days before seemed perfectly free from their attack. The reason is because they are 

 so astonishingly productive. From a single female plant-louse, of an ordinary species, 

 Mr. Curtis has calculated that there may be produced in seven generations the tremendous, 

 number of 720 millions of descendants, each one of whom possesses, a similar fecundity. 

 In the case of the grain-louse, Dr. Fitch states that " a single one produces four daily, and 

 these become equally prolific when they are three days old; thus her .descendants in 

 twenty days will number upwards of two millions, and will increase at the rate of a 

 million daily ! " No wonder, then, that they appear as if by magic where none were 

 noticed before. 



The aphis or plant-louse of the apple belongs to the same order of insects (Homoptera) 

 as the bark-louse that we have already considered. Early in the spring, as soon as the 

 buds begin to expand, this tiny insect, with multitudes of its fellows, emerges from the 

 almost microscopically minute egg that has remained all winter in some crevice of the 

 bark. It at once attaches itself to some tender leaf, bud or stem, and there employs its 

 life in sucking out the juices of the tree. It is of a pale greenish colour, and somewhat 



less than a tenth of an inch in length. The 

 accompanying illustration (fig. 13) of a greatly 

 magnified winged male and wingless female, 

 shows the structure and shape of the insect ; its 

 beak, which proceeds from the under side of the 

 head, is here hidden from the view in the male 

 but can be seen in the female. Strange as it 

 may appear, all the plant-lice hatched from the 

 eggs are females, and these in less than a fort- 

 night arrive at maturity, and commence giving 

 birth to living young, which are also females. 

 Every day, during its brief life of about a 

 month, it produces two or three young ones, 

 which in their turn arrive at maturity and in- 

 crease the population in the same ratio. As long as the summer lasts no males are pro- 

 duced, the original fecundation of the females in the eggs apparently sufficing for the 

 numerous generations that follow; late in the autumn, however, winged males are born, 

 and these, uniting with the females, become the parents of the eggs for the following 

 year. Their natural history is thus most strange, and contrary to all experience in other 

 orders of insects. 



Almost all the different species of plant-lice secrete a sweetish fluid called honey-dew, 

 which is ejected from the two projecting horns, or nectaries, on each side of the abdomen 

 behind. This fluid, when it falls upon the leaves and branches beneath a colony of these 

 insects, evaporates and forms a sweet glutinous substance upon which many insects are 

 fond of feeding. The ants particularly are fond of this sweetness, and not content with 

 obtaining it as it is discharged by the aphis, they actually perform an operation upon the 

 plant-louse, very much resembling the process of milking a cow, and cause it to discharge 

 its sweetness for their own particular benefit. 



