374 The Naturalist in La Plata. 
little tocsins made them. And presently [ had to 
return tomy 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 soas 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 birdsand 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 oniy have 
arisen in a species that always lived in commu- 
nities. 
On another occasion, in the middle of the hot 
season, I was traveiling 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- 
