APHIDES, OR PLANT-LICE. COCCID.E. 189 



of budding in their interior. In the autumn males and females 

 are produced in the same way, and the latter lay eggs, which 

 survive the winter and give birth to viviparous individuals in 

 the ensuing spring. This is an example of what is called the " Al- 

 ternation of Generations," the only one met with amongst Insects, 

 although it frequently occurs amongst the lower classes of the 

 animal kingdom. The rapidity of production of the viviparous 

 individuals is enormous ; nine generations having been produced 

 within three months, and each generation averaging 100 indivi- 

 duals. Hence it may be calculated that, from a single Aphis, 

 10,000 million millions may be generated in that short period. 

 It is not surprising, then, that an immense amount of damage 

 should be done by them, notwithstanding their very small size. 

 Many of the blights so injurious to the gardener and the agricul- 

 turist, consist really of Aphides ; although, from the minuteness 

 of the insects themselves, they frequently escape observation. 

 The Aphis Rosce, or Rose Louse, is one of those best known to 

 the gardener ; whilst the one most destruc- 

 tive to the property of the cultivator on a 

 larger scale is the Aphis Humuli, or Hop 

 Fly. Of the extent of its influence on the 

 FIG. 516,-ApHis BOSJE. production of that vegetable, some idea 

 may be formed from the fact, that the duty paid to the English 

 government on its growth, has varied from 468,000/. to 15,400/. 

 in different years, almost entirely from the absence of this insect 

 in the former case, and its presence in the latter ; and the differ- 

 ence in the actual value of the crop is, of course, far greater. 



786. Section III. MONOMERA. The third section contains 

 but one family, that of Coccus, sometimes called Scale Insects. 

 These, although ordinarily of very small size, are amongst the 

 most injurious to vegetation of the whole tribe. Like the last, 

 they are remarkable for their powers of propagation ; and when 

 they once gain possession of a plant or young tree, its death is 

 almost certain, the minute size of the larvae rendering it almost 

 impossible to exterminate them. They furnish, however, some 

 very important products. The bodies of many species are deeply 

 coloured through their whole substance, and yield dyes of great 

 value ; the richness of which seems to depend upon the nature 



VOL. II. P 



