34 GENERAL ZOOLOGY 



(about an eighth of an inch) in length, with a somewhat pear- 

 shaped body, and with or without wings. In most species 

 there is found projecting from the back of the sixth abdom- 

 inal somite a pair of slender tubes, which secrete a sticky, 

 waxen substance, which is probably protective in its nature. 

 Aphids are sought for by ants for the sake of a sweet sub- 

 stance called "honeydew," poured out from the alimentary 

 canal. We shall see later how certain species of ants have 

 taken to protecting aphids in order to secure a constant 

 supply of this food. 



The life-history of some of the aphids is most remarkable. 

 An aphid colony in the summer may consist almost entirely 

 of wingless females, which have the power of producing gen- 

 eration after generation of living young without fertilization. 

 This form of reproduction is known as parthenogenesis. The 

 young so produced are females, and many of them are wing- 

 less, though winged females are produced which start colo- 

 nies in other places, sometimes on a different food-plant. 

 Both winged and wingless females are able to produce young 

 parthenogenetically within from ten to twenty days. This 

 kind of reproduction goes on till the approach of cold weather 

 or the failure of the food- supply, when males are produced. 

 After pairing, the female lays eggs which last through the 

 winter and hatch into females in the spring, which start a 

 new colony as already described. 



One of the greatest enemies of the grape is an aphid 

 known as Phylloxe'ra vasta'trix, which has been the scourge 

 of many vineyards in France. It lives both on the roots and 

 on the leaves. Other species, called woolly aphids, have the 

 power of secreting a white downy substance, which entirely 

 covers their bodies. One of these, which lives in colonies, 

 making white masses on the alder, is known as the alder- 

 blight. This insect secretes honeydew, and is visited by 

 ants. 



