THE INSECT WORLD. 



123 



It is a short step from the Aleyrodidce to the plant-hce, be- 

 longing to the family Aphididce. Plant-lice are well known to 

 agriculturists by the injury they cause, and they are interesting 

 to the naturalist from their life history. Here we have the most 

 striking apparent exception to the general rule that insects are 

 developed from eggs, and yet perhaps the exception is more 

 apparent than real. At all events, parthenogenesis, or repro- 

 duction without the intervention of a male, occurs normally in a 

 large percentage of the species. Of course there are many differ- 

 ences in hfe habits, but a general account, covering most of the 

 cases, is all that can be attempted here. As a rule, plant-lice 



Fig. 92. 



c, female hop-louse, shov/ing eggs through skin ; b, the stem-mother that starts the 

 transformation ; much enlarged. 



winter in the egg stage ; but this is subject to many exceptions, 

 especially in the warmer parts of the country. Early in spring, 

 as soon as there is a trace of reviving vegetation, these eggs 

 hatch. The insect that now appears is wingless, and usually 

 remains so, but grows rapidly by sucking the plant juices, and 

 soon begins to produce living young. It is called a ' ' stem- 

 mother," because it is the source from which numerous genera- 

 tions issue during the season. All the young born by this stem- 

 mother are, like herself, without sex ; that is, they are neither 

 males nor sexually-developed females. The rate at which they 

 are born varies, but as many as eight living young have been 



