590 MEMIPTERA-HOMOPTERA CHAP. 



of Aphids is such that, drawing as they do their nutriment 

 directly from the plant in its growing state, in the course of 

 two or three years there would he no nutriment available for 

 other animals, except such as might be derived from plants not 

 attacked by Aphids. The numbers of Aphidae would be so great 

 that they could not be expressed by ordinary numerical methods, 

 and their increase would be actually limited only by the relations 

 existing between different kinds of plants, and between plants 

 and Aphids. This result is avoided by the fact that Aphids are 

 themselves the victims of a whole army of Insect -enemies. They 

 have the numerous members of a special group (Braconidae, 

 Aphidiid.es) of minute Hymenoptera to live inside their bodies, and 

 many Aculeate Hymenoptera depend entirely on the Aphidae as 

 the source of food for their own progeny. The Lady-birds 

 Coccinellidae live on Aphids and Coccids, and themselves in- 

 crease to such an extent as to be in many years a conspicuous 

 part of the Insect world. Crowds of the larvae of Hemerobiids 

 and Syrphids are constantly engaged in spearing and sucking 

 the Aphides. Hence the old naturalist Bonnet said that, just 

 as we sow grain for our benefit, Nature has sown Aphids for the 

 benefit of multitudes of different Insects. He might have added 

 that these different Insects are for the benefit of man, it being 

 dear that without them the population of the world must 

 rapidly decrease. 



Ants treat Aphidae more intelligently than most other Insects 

 do, for they do not destroy the helpless creatures, but utilise 

 their products in the way man does those of the cows he keeps. 

 The relations between ants and Aphids is itself an extensive 

 chapter in Natural History ; many facts have been brought to 

 light showing that the ants manage the Aphids in a prudent or 

 intelligent manner, distributing them when too numerous in one 

 place, keeping guard over them, even building shelters for them, 

 and in some cases keeping them in direct association, by retaining 

 the Aphids in their own dwellings. The further investigation of 

 these points goes, the more it tends to raise the actions of the 

 ants to the level we call in ourselves intelligent. It would even 

 ;q>pear that the ants are acquainted with the migrations of the 

 Aphids from one species of plant to another, Webster informing 

 us that as the Aphis-population on an apple tree multiplied the 

 ants in attendance anticipated their migration to wheat and grass 



