During their different movements, insects move their antennæ more or less, sometimes slowly and with regularity, at other times in all directions. Some insects impart to their antennæ a perpetual vibration. During flight they are directed in front, perpendicular to the axis of the body, or else they repose on the back.
What is the use of the antennæ, resembling as they do, feathers, saws, clubs, &c.? Everything 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 antennæ will be the principal instruments 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 have some connection with an auditory nerve.
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 similarity between the mouth of a biting insect and of one living by suction. But on examination it is found that the parts of the mouth in the one are exactly analogous to the same parts in the other, and that they have only 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]).
| Fig. 5.—Mouth of a masticating insect. | Fig. 6.—Thorax of Acrocinus longimanus (a beetle). |
The lower lip and the jaws carry on the outside certain appendages or filaments which have received the name of palpi.
When speaking of sucking insects, and in general of the various orders of insects, we shall speak more in detail of the various parts of the mouth.