The Cows that Ants Milk 15 



tlicm covered all (ncr with a soft sticky substance, 

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

 the surface of the foliaj^e. This sweet stuH" 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, ( of pure unseliisli moral desire to benefit 

 the ant. nd other beasts that like it. In the animal 

 world, nothinj^ for nothing is the principle of con- 

 duct. The true secret of the orij^in of honey-dew 

 appears to be this. Aphides live entirely off a lij^ht 

 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 orgcUis, known as honey- 

 tubes, on the backs of their bodies. Scjmetimes, 

 when distended with superfluous food, they simply 

 blow out the honey-dew secreted by these tubes on 

 to the leaves below them. 



