APHIDES 51 



" frog-hoppers "; but this name also includes the 

 lassidce, members of which may be distinguished by 

 their triangular scutellum. In Cercopida the prono- 

 tum has an angular notch on the hind-margin. This 

 receives part of the scutellum, which is accordingly 

 rhomboidal in outline. 



The series Dimera, whose members have two- 

 jointed tarsi, comprises three families. The " green- 

 fly," plant-lice, or aphides, 1 are so numerous and so 

 harmful to all kinds of cultivated plants, that they 

 are familiar to everyone ; but certain facts connected 

 with their increase and development are less well 

 known. The life-cycle of a typical aphis may be 

 briefly summarized as follows : Eggs are laid in the 

 autumn on the twigs or bark of plants. From these, 

 in the spring, larvae hatch. They rapidly complete 

 their metamorphosis, and when adult prove to be all 

 females, vested with the remarkable faculty of par- 

 thenogenesis, or virgin reproduction. They are also 

 viviparous i.e., they produce living young. Succes- 

 sive generations of these self-contained females appear 

 throughout the summer, some being wingless, while 

 others are winged, and thus able to fly from one plant 

 to another. The last generation of the year com- 

 prises both males and oviparous females, the latter 

 laying the eggs that are destined to start the next 

 year's attack. These are the usual facts; but the 

 complete life-cycle of some aphides extends over 

 several years, and may include periodic migrations 

 between two distinct kinds of plants. In fine, the 

 life-stories of aphides are more complicated than 

 those of any other insect, and many of them still call 

 for elucidation. Some species, such as the well- 

 1 Aphida. 



