366 A YEAR OF COSTA RICAN NATURAL HISTORY 



these plants were sufficiently numerous and close together 

 to make passing them a matter of some discomfort. In 

 his recent Observations on the Central American Acacia Ants, 

 Prof. W. M. Wheeler has distinguished between the obliga- 

 tory and the facultative ants, the former being those 

 which live exclusively on the acacias. The facultative ants 

 only occasionally live upon the bull's horn thorn, as they 

 inhabit other plants also. So far as Costa Rica is concerned. 

 Professor Wheeler lists three species of ants of the obligatory 

 class (the two Pseudomyrmas just mentioned and Ps. spin- 

 icola), and three facultative species {Ps. nigropilosa, Ps. sub- 

 tillissima, Camponotus planatus). 



Thomas Belt, from his studies of bull's horn thorns and 

 ants, came to the conclusion, set forth in his charming 

 Naturalist in Nicaragua, that: "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. For these services the ants are not only 

 securely housed by the plant, but are provided with a bounti- 

 ful supply of food, and to secure their attendance at the right 

 time and place, the food is so arranged and distributed as 

 to effect that object with wonderful perfection." He then 

 describes the crater-formed gland, at the base of each pair 

 of leaflets, secreting a honey-like liquid, and the minute 

 yellow fruit-like bodies (since called "Beltian bodies") 

 at the tips of unfolding pinnules, both of which are used as 

 food by the ants which dwell in the hollowed-out thorns. 



Professor Wheeler's observations in Panama and Guate- 

 mala have led him to write that although it must be ad- 

 mitted that some of these dendrophllous ants {Pseudomyrma, 

 Azteca) sting and bite severely, and may therefore defend the 

 plants, this is, of course, merely a coincidence or by-product, 

 as it were, of the true defense which the ants exercise in 



