126 --/^V ECONOMIC ENTOMOLOGY. 



\wg the suiniiKT upon a plant which dies down to the ground, 

 leaving nothing through the winter. In such cases there is an 

 alternate food-plant, upon wliich the winter and early spring are 

 passed. From this the insects migrate in early summer and to 

 it they return when cold weather sets in. Such a case we have 

 in the hop-louse, which spends the summer upon the hop, in- 

 creasing greatly in number in favorable seasons and often causing 

 much injury. When the vines mature and die, males develop, 

 and all the lice fly to plum-trees. Here the female is born, the 

 sexes mate, and eggs are laid. In the spring two or more gen- 

 erations mature upon the plum, and, when the vines are again 

 well started, winged forms develop and migrate to their summer 

 food-plant. This sort of migration is not unusual, although it 

 has not been traced out in many cases. 



Another example we find in the "melon-louse," which has a 

 considerable range of food-plants, including cotton, orange, 

 strawberry, and nearly all the common weeds of our fields. If 

 circumstances favor their increase in spring, winged forms are 

 produced which migrate and settle upon melon fields, providing 

 for colonies during the summer. 



The scientific problems connected with this method of repro- 

 duction and spread are of great interest, but cannot be entered 

 upon here ; the mere statement of the case being sufficient for 

 practical purposes. Plant-lice are so commonly known that a 

 detailed description of their appearance is unnecessary ; but it is 

 well to call attention to the presence of a pair of little tubes or 

 cornicles near the end of the abdomen, projecting from the upper 

 surface. These are called honey-tubes, and from them is excreted 

 a sweetish liquid known as honey-dew. Sometimes, when food 

 is abundant and the insects are active, the amount of sap they 

 pump out of the plants is so great that, in order to ease them- 

 selves, they void it in little streams through the anus, as well as 

 in drops through the honey-tubes. Thus the leaves of infested 

 plants become sticky or glazed with a sweetish liquid, on which 

 a black fungus rapidly develops, the leaf being frequently killed 

 by simply choking to death. Sometimes the vegetation beneath 

 a tree becomes thoroughly coated in the same way, or, when 

 shade-trees in cities are infested, the pavement becomes wet and 

 slippery with the viscid liquid. This honey-dew is often attractive 



