514 PRODUCTION' OF SOUNDS BY INSECTS. 
676. Many Insects have the power of producing a conti¬ 
nuous sound, which probably serves the purpose of intimating 
to each other the neighbourhood of their own kind; and 
even, in some instances, of expressing them feelings : some 
of these sounds are produced only during flight. Of this 
kind is the sharp hum of the Gnat, Mosquito, Gad-fly, &c., 
which, though often a source of extreme annoyance to man 
and beast, serves to give warning of the proximity of these 
blood-thirsty Insects, and is therefore of real service to the 
animals they attack. From recent experiments, however, it 
appears that in Bees and Flies, at least, the sound is not 
produced so much by the vibrations of the wings (to which it 
is commonly attributed), as 
by those of a little mem¬ 
branous plate, situated in 
one of the spiracles or stig¬ 
mata (§ 321) of the thorax; 
for if the apertures of these 
be stopped, no sound is heard, 
though the wings remain in 
movement. But in Cock¬ 
chafers, and other noisy 
Beetles, Butterflies, &c., no 
such apparatus can be dis¬ 
covered. Other sounds are 
produced while the insect is feeding; that occasioned by the 
armies of Locusts, when incalculable millions of powerful jaws 
are in action at the same time, has been compared to the crack¬ 
ling of a flame of fire driven by the wind. Certain two-winged 
Flies, distinguished by a long proboscis (fig. 256), make a 
humming sound whilst sucking honey from flowers ; and the 
same is the case with some of the Hawk-moths. 
677. Some Insects are remarkable for a peculiar mode of 
calling , commanding , or giving an alarm . The neuters or 
soldiers among the White Ants make a vibrating sound, 
rather shriller and quicker than the ticking of a watch, by 
striking against hard substances with their mandibles; this 
seems intended to keep the labourers, who answer it by a 
hiss, upon the alert and at their work. The well-known 
sound termed the “ death-watch” is produced by a small beetle 
termed Anobium (fig. 257), that burrows in old timber; and 
