384 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



are tenanted by ants that make a small hole for their entrance and exit near 

 one end of the thorn, and also burrow through the partition that separates 

 the two horns; so that the one entrance serves for both. Here they rear their 

 young, and in the wet season every one of the thorns is tenanted ; and 

 hundreds of ants are to be seen running about, especially over the young 

 leaves. If one of these be touched, or a branch shaken, the little ant* 

 (Pseudomyrma bicolar, Guer. ewarm out from the hollow thorns, and attack 



Fig. 2. — Acacia spliacrocephala Schl. & Cham. A, Branch of the type-specimen, show- 

 ing ovoid flower heads and perforations, x, near the tip of the thorns made by ants ; 

 B, a pinnately compound leaf, showing ncctar-gland, y, on the petiole ; C, a single 

 leaflet, showing terminal body rich in nutriment, fed by the ants to their young, cradled 

 in the hollowed thorns. 



the aggressor with jaws and sting. They sting severely, raising a little white 

 lump that does not disappear in less than 24 hours, 



These ants form a most efficient standing army for the plant, which pre- 

 vents 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 bountiful supply of food ; and to secure their attendance at 

 the right time and place, this food is so arranged and distributed as to effect 

 that object with wonderful perfection. The leaves are bipinnate. At the base 

 of each pair of leaflets, on the mid-rib, is a crater-formed gland, which, when 



