INSECTS. 161 



which, after repeated moults, fly away and start a 

 new colony in another plant. Since aphides have 

 many enemies (starlings, sparrows, grasshopper 

 warblers, etc., lady-birds and their larvae, drone fly 

 larvae, lace fly larvae), and are often kUled in large 

 numbers by wind and rain, it only occasionally 

 happens, particularly in dry summers, that they 

 entirely or largely destroy the plants they infest. 

 They injure plants, not only by drawing away their 

 nourishment, but also by the separation of a sugary 

 sticky fluid from the anus. If the minute drops of 

 this fluid fall from the upper parts of an infested 

 plant to the lower (garden and field beans), or from the 

 leaves of an infested tree to the plants growing at its 

 foot, or, as sometimes happens, are carried by the 

 wind to more distant plants, great damage may be 

 caused. The fluid evaporates and leaves behind a 

 shining sticky substance, which closes up the stomata 

 of the leaves, and partially checks exchange of gases 

 (assimilation and respiration). Particles of dust, sand, 

 and smoke carried by the wind, and also the cast 

 skins of the aphides, stick to the surface of the leaves 

 and render exchange of gases still more diflicult. The 

 leaves develop brown dirty patches, and die off". 

 Besides this, the spores of disease- producing fungi, 

 carried by the wind, stick very easily to the places 

 covered by the sweet fluid, and readily germinate in 

 it. Aphides may thus be the indirect cause of 

 several diseases {e.g. smut). These insects are de- 

 structive, therefore, to other plants than those in- 

 fested by them. Remedies: Spraying with any one 

 of the fluids destructive to them — soapy water; a 

 decoction of quassia chips ; tobacco water, not too con- 

 centrated ; Nessler's fluid (1^ ozs. soft soap, 2 J ozs. 

 tobacco mixture, 2 ozs. fusel alcohol, half a pint 

 ordinary alcohol, diluted with rain water up to a 

 quart: when used, mix with one-fifth the quantity 

 of rain water) ; Koch's fluid (2 lbs. soft soap dissolved 



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