30 The Structure and Special Physiology of Insects 
besides the midges and mosquitoes possess this type of auditory organ; 
in fact such an organ, more or less well developed, has been found in almost 
every order except the Orthoptera (the order of locusts, crickets, katydids, 
etc.) in which the tympanic auditory organs occur. 
Special isolated hairs scattered sparsely over the 
body, connected with a special peripheral nervous 
arrangement, are believed by some entomologists 
to be a third kind of auditory structure, and are 
called chordotonal organs. Experimentally the 
sense of hearing has been surely determined for 
certain insects. A single striking example of this 
experimentation must here suffice. Mayer fastened 
a live male mosquito to a glass slide, put it under 
a microscope, and had a series of tuning-forks of 
different pitch sounded. When the Ut 4 fork of 
512 vibrations per second was sounded many of 
the antennal hairs were set, sympathetically, into 
strong vibration. Tuning-forks of pitch an octave 
lower and an octave higher also caused more 
vibration than any intermediate notes. The male 
mosquito’s auditory hairs, then, are specially fitted to respond to, i.e., be 
stimulated by, notes of a pitch produced by 512 vibrations. Other, but 
fewer, hairs of different length vibrated in response to other tones. Those 
auditory hairs are most affected which are at right angles to the direction 
from which the sound comes. From this it is obvious that, from the position 
of the antennae and the hairs, a sound will be loudest or most intense if it is 
directly in front of the head. If the mosquito is attracted by sound, it will 
thus be brought straight head end on toward the source of the sound. As a 
Fig. 58.—Longitudinal sec¬ 
tion through ocellus of the 
honey-bee, Apis mellifica. 
/., cuticular lens; i.c., cell¬ 
ular layer of skin; c.b., 
crystalline layer; r.c., ret¬ 
inal cells; o.n., optic 
nerve. (After Redikor- 
zew; greatly magnified.) 
Fig. 59.—Ocellar lens of larva of a saw-fly, Cimbex sp., showing its continuity with the 
chitinized cuticle. (After Redikorzew; greatly magnified.) 
matter of fact, Mayer found the female mosquito’s song to correspond nearly 
to Ut 4 , and that her song set the male’s auditory hairs into vibration. With 
little doubt, the male mosquitoes find the females by their sense of hearing. 
Insects have two kinds of eyes, simple and compound. On most 
species both kinds are found, on some either kind alone, and in a few no 
eyes at all. Blind insects have lost the eyes by degeneration. The most 
