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 

 lo 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.e., 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 



