A LA J U EL A AND THE VOLCANO POA'S 333 



amused ourselves by throwing stones toward them until 

 they were induced to move into shelter, skimming over the 

 surface of the water and using both legs and tail as locomo- 

 tive organs. Many lizards can travel a surprising distance 

 in this way. They move so quickly that it is difficult to see 

 just how they do it, but they do not appear to be at all sub- 

 merged. 



Still farther south, after crossing several potreros, on the 

 banks of a little stream, were numerous bull's horn thorns 

 growing mingled with urucas and jocotes. Although they 

 did not seem to be growing actively we found some young 

 shoots with the yellow so-called food-bodies on the tips of 

 the tender green leaflets and the three conical glands in a 

 row on the petiole still exuding their sweet secretion. This 

 bull's horn thorn was Acacia costaricensis and derives its pop- 

 ular name from the strong, sharp, paired thorns, which 

 quite closely resemble the horns and forehead of a bull. In 

 many species these thorns are commonly hollowed out and 

 tenanted by small stinging and biting ants {Pseudomyrma 

 sp.) which feed upon the nectar and "food-bodies" and 

 have been supposed to be of great protective value to the 

 plant. The ants on the thorns in this potrero were both nu- 

 merous and aggressive. The largest bush had also a big 

 wasps' nest hanging from its branches. We wanted to pho- 

 tograph a small spray of this plant, and our efforts to do 

 so, although finally successful, were attended with various 

 amusing difficulties. The plant is extremely sensitive to 

 shaking and no matter how gently the spray was removed 

 and laid on the background — the camera on its tilting top 

 being focused and ready — the leaflets would close a little. 

 The ants occupying the thorns also objected to the shaking, 

 and it proved a hard task to handle the spray "gently" 

 when a stream of savagely-biting little furies poured down 

 the stem ready to attack the intruder with jaws and stings! 



