134 THE FACTORS [Part I 



as much as possible unchanged, will without doubt in numerous cases 

 prove the connexion between the occurrence of certain definite forms of 

 insects and the flowers specially adapted to them. Interesting in this 

 respect is the fact, established by Ridley, that species of Bulbophyllum 

 near Singapore are adapted for pollination by a certain fly with a very 

 specialized taste, and that, of the orchids that are not indigenous, only 

 Dendrobium superbum is visited by this fly. 



2. PLANTS AND ANTS. 



Ants in the temperate zones play an unimportant part in the economy 

 of nature, but in the tropics a leading part. They are the most numerous 

 and the most industrious representatives of the tropical insect-world. 

 They abound everywhere. Untiring in their search for food, and usually 

 quite fearless, they are ever ready for attack, in which they employ 

 either their sharp jaws or poisonous stings. In the eastern part of 

 the tropics they do but little harm to vegetation, for, as in temperate 

 countries, they are for the most part satisfied with the dead parts of 

 plants or with sweet exudations from the nectaries on leaves, to which 

 subject further reference will be made. In tropical America, however, 

 the so-called leaf-cutters, or parasol-ants, of the genus Atta may be 

 reckoned as the most dangerous foes to vegetation. 



i. ANTS AS CULTIVATORS OF FUNGI. 



The foraging expeditions of parasol-ants in tropical America are 

 well known to every traveller, and have frequently been described. 

 Straight across the forest path moves a green stream — travelling pieces 

 of leaf, each as big as a farthing and borne upright on the head of an 

 ant. In some species, large-headed soldiers without any load accompany 

 the procession. The pieces of leaf come from a plant on which the 

 bold little creatures may be easily observed at work. A piece is cut 

 from the margin of a leaf, in a few minutes, by their shear-like mandibles 

 and placed on their heads by a kind of jerking movement. Thus laden, 

 the ant follows the homebound troops (Figs. 70-72). 



The attacked plant is frequently, though not always, abandoned only 

 after all the foliage, except the hard ribs and petioles, has been carried 

 away. It is remarkable that parasol-ants so frequently bring their booty 

 from a great distance, although suitable plants are near at hand ; Belt 

 frequently found them engaged half a mile from their nest. This was 

 probably due to the fact that, as Alf. Moller proved, they reject and 

 select the same species of plant in turns ; a fact that seems explicable 

 only upon the assumption that it relates to the preparation of a certain 

 mixture or to the replacing of components of a mixture that have become 



