238 INDEX 



Sight, sense of: subordinate to 

 smell and hearing in most 

 animals, 20 ; perception of light 

 among lower forms of life, 2 1 ; 

 positive and negative reactions 

 to light, 22-3 ; in spiders, 23, 

 161 ; in insects, and mosaic 

 vision, 23; in amphibians, 24; 

 in reptiles, 24-5 ; in fishes, 25-6 ; 

 in birds, 26-7 ; in mammals, 27- 

 28 ; position of eyes and size of 

 eyeballs indicate habit and be- 

 haviour, 28; marine mammals, 

 29; field observations and experi- 

 ments: use of microscope, 30-1 ; 

 studying animals in captivity, 

 31 ; worms, 32-3; insects, 33-4; 

 spiders, 34-5 ; amphibians, 35- 

 46; reptiles, 46-55; birds, 55- 

 65; mammals, 65-8; — the 

 fundamental principle dictated 

 by physical properties of light, 

 137; structure of the eye, i 38; 

 eyelids and lack of eyelids, 139; 

 most land vertebrates possess 

 third eyelid, 139; eyelashes, 

 139-40; the incomplete eye 

 socket of amphibians, 140; 

 optical system of the eye, 1 40-5 ; 

 a blind spot in all vertebrates, 

 145; wide field of view in most 

 animals, 1 46 ; stereoscopic sight, 

 146; optical mechanism and 

 internal structure of eye in birds, 

 147-9, 1 50- 1 ; use of eyes learnt 

 by experience, 149; fallibility, 

 149-50; and the image in the 

 brain, 150; of fishes in deep and 

 shallow waters, 151-3 ; seals and 

 whales, 153-4; theories of colour 

 vision, 1 54-5 ; evolution from 

 eye-spots to eyes, 1 56-7 ; eyes of 

 molluscs, 157-60; simple eyes 

 (ocelli) and compound eyes, 

 1 60-7 ; diffraction and refrac- 

 tion, 167-8 



Skates, 193 



Sleep, 70 



Slow Worm : acuity of sight, 49 ; 

 response to movement of prey, 

 52-3 ; experiments on its sight, 

 53 ; hearing, 73 ; experiments 

 on its sense of smell, 102 



Slugs : sense of taste, 1 1 1 ; respon- 

 sive to touch, 119; their recep- 

 tors for smell, 201 



Smell, sense of: closely linked with 

 sense of taste, 16, 90, 109, 113, 

 195, 203; a chemical sense, 89; 

 factors affecting scents given off 

 and perceived, 90; and micro- 

 climates, 94; limitations of 

 laboratory experiments, 95-6 ; 

 field observations and experiments: 

 insects, 97-9; fishes, 99-100; 

 reptiles, 1 00-2 ; amphibians, 

 102-3; birds, 103; mammals, 

 103-8; — mammalian use of 

 scent for marking out territory, 

 107; scents given off by female 

 mammals in season, 107-8; 

 smells cannot be classified ob- 

 jectively, 195 ; olfactory mechan- 

 ism of vertebrates, 195-7; ™" 

 portant to most mammals for 

 varied purposes, 197-8; birds, 

 198-9; reptiles, 199-200; am- 

 phibians, 200; insects, 201-3 



Smith, Malcolm, 40 



Smooth Newt, 45 



Smooth Snake, 102 



Snails : sense of taste, in; respon- 

 sive to touch, 119; sight, 157; 

 their receptors for smell, 201 



Snakes : sight : problemis, 24-5 ; 

 devoid of moveable eyelids, 25, 

 139; their elliptical pupils, 28; 

 faulty observation of, 53 ; snake- 

 charmers and the cobra, 53 ; 

 short-sighted, 53, 54; trail prey 

 by scent, 53-4; part played by 

 eyes in capturing prey, 54-5 ; 

 HEARING : lack ears, but sensi- 

 tive to vibrations, 18, 72, 78, 

 177, 178; smell: dependence 

 upon, 1 8, 1 00-2 ; trail prey by 

 scent, 53-4; the tongue a de- 

 tector of smells, 91, 200; field 

 observations and experiments, 

 1 00-3 ; olfactory mechanism, 

 199-200; touch: among pit 

 vipers, 115; keen response to, 

 119; detection of infra-red radia- 

 tion by pit vipers, 215-16 



Snipe, their sense of touch, 92, 1 14 



Sonar. See Echo-location 



