Auditory organs of a simple type are present in most insects. These
consist of fine rods suspended between two points of the cuticle, and
connected with nerve-fibres; they are known as chordotonal organs. In
many cases a more complex ear is developed, which may be situated in
strangely diverse regions of the insect's body. In locusts
(_Acridiidae_) a large ovate, tympanic membrane (fig. 9, G) is
conspicuous on either side of the first abdominal segment; on the
inner surface of this membrane are two horn-like processes in contact
with a delicate sac containing fluid, connected with which are the
actual nerve-endings. In the nearly-related crickets and long-horned
grasshoppers (_Locustidae_) the ears are situated in the shins of the
fore-legs (see fig. 9, F). Just below the knee-joint there is a
swelling, along which two narrow slits run lengthwise. They lead into
chambers, formed by inpushing of the cuticle, whose delicate inner
walls are in contact with air-tubes; on the outer surface of these
latter are ridges, along which the special nerve-endings are arranged.
An ear of another type is found in the swollen second segment of the
feeler in many male gnats and midges, the cuticle between this segment
and the third forming an annular drum which is connected with numerous
nerve-endings, while the fine bristles on the more distal segments
vibrate in response to the note produced by the humming of the female.
[Illustration: From Ridley, _Insect Life_, vol. 7 (U.S. Dept. Agr.).
FIG. 9.--Single Ommatidium of Cockroach's Eye (after Grenacher). B,
Section through compound eye (after Miall and Denny); C, organs of
smell in cockchafer (after Kraepelin); D, a, b, sensory pits on
cercopods of golden-eye fly; c, sensory pit on palp of stone-fly
(after Packard); E, sensory hair (after Miall and Denny); F, ear of
long-horned grasshopper; a, Front shin showing outer opening and
air-tube; b, section (after Graber); G, ear of locust from within
(after Graber). All highly magnified.]
Many of the numerous hairs (fig. 9, E) that cover the body of an
insect have a tactile function. The sense of smell resides chiefly in
the feelers, on whose segments occur tiny pits, often guarded by
peg-like or tooth-like structures and containing rod-like cells (fig.
9, C) in connexion with large nerve-cells. It is said that 13,000 such
olfactory organs are present on the feeler of a wasp, and 40,000 on
the complex antennae of a male cockchafer. Organs of similar type on
the maxillae and epipharynx appear to exercise the function of taste.
[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.