THE Cows THAT ANTS MILK 15 



them covered all over with a soft sticky substance, 

 sweet to the taste, and spread in a thin layer upon 

 the surface of the foliage. This sweet stuff is honey- 

 dew, and it is manufactured solely by various kinds 

 of aphides, without whose trade-mark none other is 

 genuine. Why do they make it ? Not, you may be 

 sure, out of pure unselfish moral desire to benefit 

 the ants and other beasts that like it. In^he animal 

 world, nothing for nothing is the principle of con- 

 duct. The true secret of the origin of honey-dew 

 appears to be this. Aphides live entirely off a light 

 diet of vegetable juices ; now, these juices are rich 

 in compounds of hydrogen and carbon, especially 

 sugar (or rather, to be strictly scientific, glucose), 

 but are relatively deficient in nitrogenous materials, 

 which last are needed as producers of movement by 

 all animals, however sluggish. In order, therefore, 

 to procure enough nitrogenous matter for its simple 

 needs, your aphis is obliged to eat its way through 

 a quite, superfluous amount of sweets, or of sugar- 

 forming substances. It is almost as though we 

 ourselves had to swallow daily a barrel of treacle 

 so as to reach at the bottom an ounce of beefsteak. 

 To get rid of this surplus of sugar (or rather, un- 

 digested glucose) almost all aphides (for they are 

 a large family, with many separate kinds) have 

 acquired a pair of peculiar organs, known as honey- 

 tubes, on the backs of their bodies. Sometimes, 

 when distended with superfluous food, they simply 

 blow out the honey-dew secreted by these tubes on 

 to the leaves below them. 



