Insects and their world 



The dorsal ocelli have something of the structure of an eye, but they 

 do not seem to be good enough to produce any sort of detailed picture. 

 They are believed to record the general light intensity, and to be more 

 sensitive to fluctuations in light intensity than are the compound eyes. 

 It has been suggested that they may help flying insects to keep their 

 balance in the air, but a great many flying insects do quite well without 

 them; moreover they may be present in one genus of insects, and absent 

 in a close relative. The most likely function of dorsal oceUi is that when 

 stimulated by Hght they may 'tone up' the insect's nervous system, and 

 put it on the alert. 



Lateral ocelli (stemmata) occur on the sides of head of larvae of many 

 Orders. They have the same 'toning up' funaion as the dorsal oceUi, 

 and in addition they seem to give some larvae a rough idea of nearby 

 objects. Where there is a group of six oceUi on each side of the head, as 

 in caterpillars of Lepidoptera, they may combine to form a rudimentary 

 eye, but this is much less effective than the compound eye of an adult 

 insect. 



Hearing 

 Hearing is not a commonplace activity among insects, as it is with us, 

 nor is it certain that all insects can hear. On the contrary, it seems Hkely 

 that most insects live in a soundless world, and that those that have 

 developed hearing as a special sense use it mainly to solve the problem 

 of how to find the other sex for mating purposes. 



Even among insects that show any response to sound, only a minority 

 have special organs for it. Most make use of the hairs of the surface 

 of the body, which may be set vibrating by sound-waves, thus exciting 

 the nerves of the hair-socket. This is really only a special development 

 of the sense of touchy, mentioned later in this capter (Fig. 38a). 



Caterpillars can 'hear' in this way with the hairs all over the body, 

 and will show that they hear by stopping movement, and 'freezing'. In 

 ants, and especially in male mosquitoes, it is the hairs of the antennae 

 that are specially developed for this purpose. The hairs of caterpillars 

 are said to respond to notes ranging from three octaves below middle 

 C to two oaaves above this, but as the response to the high notes is not 

 synchronous — i.e. the hair vibrates irregularly, or jarringly, and not in 

 tune with the sound — it does not seem that the insect can Usten to such 

 sounds in any discriminating way. 



Moreover, if these high notes are so badly received, it means that the 

 higher overtones of ordinary sounds will be distorted, and reception 

 will be far from being 'hi-fi'. In all, therefore, it seems that the musical 



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