374 The Naturalist in La Plata. 



little tocsins made them. And presently I had to 

 return to my muttons ; and afterwards there was no 

 opportunity of revisiting the spot to observe so 

 singular a habit again and collect specimens. It 

 was a very slender grasshopper, about an inch and 

 a half long, of a uniform, tawny, protective colour 

 the colour of an old dead leaf. It also possessed 

 a protective habit common to most grasshoppers, of 

 embracing a slender vertical stem with its four fine 

 front legs, and moving cunningly round so as to 

 keep the stem always in front of it to screen itself 

 from sight. Only other grasshoppers are silent 

 when alarmed, and the silence and masking action 

 are related, and together prevent the insect from 

 being detected. But this particular species, or race, 

 or colony, living on the sides of the isolated sierra, 

 had acquired a contrary habit, resembling a habit 

 of gregarious birds and mammals. For this inform- 

 ing sound (unless it mimicked some warning-sound, 

 as of a rattlesnake, which it didn't) could not pos- 

 sibly be beneficial to individuals living alone, as 

 grasshoppers generally do, but, on the contrary, 

 only detrimental ; and such a habit was therefore 

 purely for the public good, and could only have 

 arisen in a species that always lived in commu- 

 nities. 



On another occasion, in the middle of the hot 

 season, I was travelling alone across-country in a 

 locality which was new to me, a few leagues east of 

 La Plata River, in its widest part. About eleven 

 o'clock in the morning I came to a low-lying level 

 plain where the close-cropped grass was vivid green, 

 although elsewhere all over the country the vegeta- 



