SENSE OF TEMPERATURE.-ANTENNAS OF INSECTS. 393 
without the other being affected. It is rather of a comparative 
than of a positive kind ; that is, we form our estimate of tem¬ 
perature rather by comparing it with that to which our body 
(or the part of it employed to test the heat or cold) has been 
previously exposed, than by any knowledge which we derive 
through the sensation as to the actual degree of heat or cold 
to which the organ is exposed. Thus, if we plunge one hand 
into a basin of hot water, and the other into cold, and then 
transfer both of, them to a basin of tepid water, this will feel 
cold to the hand which has been previously accustomed to 
the heat, and warm to the other. In the same manner, the 
temperature of Quito, which is situated half-way up a lofty 
mountain, is felt to be chilly by a person who has ascended 
from the burning plains at its base, whilst it seems intensely 
hot to another who has descended from its snow-capped sum¬ 
mit ; the residents in the town at the same dime regarding it 
as moderate,—neither hot nor cold. It is a curious circum¬ 
stance, that a weak impression made on a large surface seems 
more powerful than a stronger impression made on a small 
surface; thus, if the fore-finger of one hand be immersed in 
water at 104°, and the whole of the other hand be plunged in 
water at 102°, the cooler water will be thought the warmer; 
whence the well-known fact, that water in which a finger can 
be held will scald the whole hand. 
498. Where any special organs of Touch exist in Inverte- 
brated Animals, they are for a « 
the most part prolongations 
from the portion of the head 
near the mouth. This is 
the case with the arms of 
the Cuttle-fish, and with the 
tentacula of the lower Mol- 
lusca which are similar in 
position. Among Crustacea 
and Insects, the antennas or 
feelers (fig. 198, a , a) appear 
to be the special organs of 
touch. These are frequently Fig - W8 .-capmcorh-be*tle. 
very long, and present an extraordinary variety in their forms, 
of which some are depicted in fig. 199. They contain, 
for the most part, a large number of joints (in the Mole- 
