26 HOUSE, GARDEN, AND FIELD 



fluids, elaborated in the leaf, and meant to supply the 

 growing tissues of the sycamore, are tapped, and drawn 

 into the mouth of the aphid by capillarity, the closely 

 applied bristles acting like the threads of a lamp-wick. 

 The only insects which can avail themselves of this source 

 of sugary food are Homopterous, such as aphids, scale- 

 insects and cicadas. The cicadas of tropical South America 

 emit showers of liquid from certain trees which they 

 regularly haunt, and which are called " rain- trees " be- 

 cause of the fine drops which seem to distil from them. 



The spreading of the drops of honey- dew into the films 

 which glaze the leaves of the sycamore is caused by dew 

 or fine rain, which moistens the drops and afterwards 

 evaporates. Sycamore is not the only tree from which 

 honey-dew can be extracted by aphids ; lime, plum, cherry 

 and other common trees can be tapped in the same way. 

 There is no doubt that the trees infested by aphids suffer ; 

 they lose food-substances which are required for new 

 growths, and their leaves are overspread by a sticky sub- 

 stance which checks the exchange of gases, and favours 

 the growth of moulds. It has been ingeniously suggested, 

 however, that the sprinkling of the leaves with honey- dew 

 may be in some measure beneficial to the tree. There are 

 plants which possess honey-glands on their leaves or 

 stipules, " extra-floral nectaries " they are called. These 

 attract ants, which are thought to protect the plant from 

 such dangerous enemies as leaf-eating caterpillars. In 

 certain cases the utility of the ants thus attracted to the 

 plant is unquestionable. Most observers of the sycamore 

 as it grows in English fields and gardens would, I think, 

 come to the conclusion that while the mischief caused by 

 the aphids is real, the protection afforded by the ants is 

 very doubtful. This is not quite decisive of the question, 

 for the sycamore is not an undoubted native with us, and 

 there may be countries where caterpillars are more in- 

 jurious enemies to the sycamore, and ants more valuable 

 allies than our local experience would show. A multitude 



