iv ANTS 139 



entirely on the tree they inhabit, the food being usually sweet 

 stuff secreted by glands of the plant. It is thought that the 

 ants in return are of considerable benefit to the plant by defend- 

 ing it from various small enemies, and this kind of symbiosis has 

 received much attention from naturalists. A very curious con- 

 dition exists in the epiphytic plants of the genera Myrmecodia 

 and Hydnophytum ; these plants form large bulb-like (Fig. 59) 

 excrescences which, when cut into, are found to be divided into 

 chambers quite similar to those frequently made by ants. Though 

 these structures are usually actually inhabited by ants, it appears 

 that they are really produced by the plant independent of the 

 Insects. 



Variability and Polymorphism of Ants. Throughout the 

 Hymenoptera there are scattered cases in which one of the sexes 

 appears in dimorphic form. In the social kinds of bees and 

 wasps the female sex exists in two conditions, a reproductive 

 one called queen, and an infertile one called worker, the limits 

 between the two forms seeming in some cases (honey-bee) to be 

 absolute as regards certain structures. This sharp distinction 

 in structure is rare ; while as regards fertility intermediate con- 

 ditions are numerous, and may indeed be induced by changing 

 the social state of a community. 1 In ants the phenomena of the 

 kind we are alluding to are very much more complex. There are 

 no solitary ants ; associations are the rule (we shall see there are 

 one or two cases in which the association is with individuals of 

 other species). In correlation with great proclivity to socialism 

 we find an extraordinary increase in the variety of the forms of 

 which species are made up. In addition to the male and female 

 individuals of which the species of Insects usually consist, there 

 are in ants workers of various kinds, and soldiers, all of which 

 are modified infertile females. But in addition to the existence 

 of these castes of infertile females, we find also numerous cases 

 of variability or of dimorphism of the sexual individuals ; and 

 this in both sexes, though more usually in the female. Thus 

 there exists in ants an extraordinary variety in the polymorphism 

 of forms, as shown by the table on p. 141, where several very 

 peculiar conditions are recorded. 



The complex nature of these phenomena has only recently 



1 The parthenogenetic young produced by worker females are invariably of the 

 male sex. 



