312 ANTS. 



177 and 178) and a species allied to Pseudomyrma bicolor is given by 

 llelt in " The Naturalist in Nicaragua " ( 1874 ). He describes the large 

 paired thorns tenanted by the ants, the extranuptial nectaries on the leaf- 

 petioles and the yellow food-bodies at the tips of the leaflets, and puts 

 these various structures down as so many symbiotic adaptations. I le 

 says that " hundreds of ants are to be seen running about, especially 

 over the \oung leaves. If one of these be touched, or a branch shaken, 

 the little ants (Pseudomyrma bicolor Guer.) swarm out from the hollow 

 thorns and attack the aggressor with jaws and sting. . . . These ants 

 form a most efficient standing army for the plant, which prevents not 

 only the mammalia from browsing on the leaves, but delivers it from 

 the attacks of a much more dangerous enemy, the leaf-cutting ants." 

 Belt sowed the seeds of Acacia in his garden and reared some of the 

 young plants. " Ants of many kinds were numerous ; but none of 

 them took to the thorns for shelter, nor the glands and fruit-like bodies 

 for foo.d. . . . The leaf-cutting ants attacked the young plants and 

 defoliated them, but I have never seen any of the trees out on the 

 savannahs that are guarded by the Pseudomyrma touched by them, and 

 have no doubt the Acacia is protected from them by its little warriors." 

 There are several other thorn-inhabiting Pseudomyrma ( belli, fnli'cs- 

 ccns, spinicola) that nest in this and other species of Acacia in Central 

 America and Mexico, and a Cremastogastcr is also mentioned by llelt 

 in this connection. Concerning the development of the thorns he says : 

 ' The thorns, when they are first developed, are soft, and filled with a 

 sweetish pulpy substance ; so that the ant, when it makes an entrance 

 into them, finds its new house -full of food. It hollows this out, leaving 

 only the hardened shell of the thorn. Strange to say this treatment 

 seems to favor the development of the thorn, as it increases its size, 

 bulging out towards the base ; whilst in my plants that were not touched 

 by the ants, the thorns turned yellow and dried up into dead but per- 

 sistent prickles. I am not sure, however, that this may not have been 

 due to the habitat of the plant not suiting it." According to Rettig 

 ( 1904) this latter statement is based on insufficient observation, for the 

 enlargement of the thorns is not produced by the ants, although it does 

 not make its appearance till the plant has grown considerably. 



It has been known for some time that the Old World also has its 

 acacias with enlarged thorns tenanted by ants. Gerstaecker (1871), 

 Schweinfurth (i867~'68) and more recently Keller (1892) have called 

 attention to the East African " uwadi " acacia ( A. fistulosa), the greatly 

 inflated thorns of which are white at first but when old become brown 

 or black. According to Keller these thorns are nearly always inhabited 

 by species of Cremastogaster (chiarinii, ruspolii or acacia:). He main- 



