6 THE INSECT WORLD. 
During their different movements, insects move their antennz 
more or less, sometimes slowly and with regularity, at other times 
in all directions. Some insects impart to their antennee a perpetual 
vibration. During flight they are directed in front, Goyette 
to the axis of the bein or repose on the back. 
What is the use of the antennz, resembling, as they do, feathers, 
saws, clubs, &c.? Hverything indicates that these organs play a 
very important part in the life of insects, but their functions are 
imperfectly understood. Experience has shown that they only 
play a subordinate part as feelers, and have nothing to do with 
the senses of taste or smell. There is no other function for them 
to fulfil except that of hearing. 
On this hypothesis the antennz will be the principal instru- 
ments for the transmission of sound-waves. ‘The membrane at 
their base represents a trace of the tympanum which exists among 
the higher animals. This membrane then will be an auditory 
nerve. 
Situated intermediately between the inferior animals, whose 
functions more or less resemble those of plants, and the vertebrates, 
whose functions are localised in a very high degree, insects have 
received, like these latter, special organs for nutrition. The mouth 
is the most exterior of these apparatuses. 
The mouth of insects is formed after two general types, which 
correspond to two kinds of requirements. It is suited in the one 
case to break solid substances, in the 
other to imbibe liquids. 
At first sight there seems no simi- 
larity between the mouth of a grind- 
ing insect and of one living by 
suction. But on examination it is 
found that the parts of the mouth 
in the one animal are exactly ana- 
logous to the same parts in the other, 
Ee ane and that they have only undergone 
modifications suiting them to the 
different purposes which they have to fulfil. 
The mouth of a biting insect is composed of an upper lip, a 
pair of mandibles, a pair of jaws, and a lower lip (Fig. 5). 

