6 USES OF INSECTS TO PLANTS. [CHAP. 



insects, will, ccsteris paribns, have an advantage in the 

 struggle for existence, and be most likely to per- 

 petuate their race. 



Every garden indeed is a sufficient proof that in 

 size and colour, flowers are susceptible of great 

 modifications ; and insects unconsciously produce 

 changes similar to those which man effects by 

 design. 



Insects are useful to plants in various ways. Thus, 

 a species of acacia mentioned by Mr. Belt, 1 if unpro- 

 tected, is apt to be stripped of its leaves by a leaf- 

 cutting ant, which uses the leaves, not directly for 

 food, but, according to Mr. Belt, to grow mushrooms 

 on. The acacia, however, bears hollow thorns, and 

 each leaflet produces honey in a crater-formed gland 

 at the base, and a small, sweet, pear-shaped body at 

 the tip. In consequence, it is inhabited by myriads 

 of a small ant, Psciidomyrma bicolor, which nests 

 in the hollow thorns, and thus finds meat, drink, and 

 lodging all provided for it. These ants are con- 

 tinually roaming over the plant, and constitute a 

 most efficient body-guard, not only driving off the 

 leaf-cutting ants, but even in Mr. Belt's opinion, ren- 

 dering the leaves less liable to be eaten by herbivo- 

 rous mammalia. 



The principal service, however, which insects per- 

 form for plants is that of transferring the pollen from 

 one flower to another. 



I will not now enter on the large question why this 

 cross-fertilisation should be an advantage ; but that 



1 F. Muller has observed similar facts in Sta. Catharina. (Nature, 

 vol. x. p. 1 02. ) 



