84 



THE EYE IN EVOLUTION 



Latrodectus 



Betta 



Ch laniydosa ur'us 



Boinbinaior 



in warning 



attitude 



animal strives to make itself as conspicuous as possible, may be 

 directed towards several ends. In the first place they may serve the 

 essential biological purpose of reproduction whereby, simulating the 

 sexual riot of the flowers, colour displays, sometimes of extraordinary 

 vividness, are associated with courtship and mating behaviour, a 

 phenomenon seen in marked degree in certain cephalopods and fishes 

 (Hadley, 1929 ; Parker and Brower, 1935) ; it is a function analogous 

 to the conspicuousness of many male birds adopted possibly for display 

 and distraction in contrast to the cryptic inconspicuousness of the 

 brooding female. It is interesting that such sexual dimorphism is 

 rarely seen in birds which feel secure, either because of their fighting 

 ability or in their colonial habits, the latter finding safety in a flock 

 (Mottram, 1915). In the second place they may be designed for 

 aggression, wherein, as if in defiance of all creation, the animal when 

 sufficiently moved to excitement assumes the most blatant hues 

 possible (as in squids, cuttle-fish, teleostean fishes, spiders and lizards : 

 Kleinholz, 1938 ; and others). Less commonly they may have a more 

 social purpose, serving as signals of warning or recognition between 

 members of the same species or as feeding-releasers between parent 

 and offspring (see Marshall, 1936 ; Huxley, 1914-38 ; Cott, 1940-54 ; 

 Armstrong, 1947 ; and others). 



Thus when facing an enemy the venomous Australasian spider, Latrodectus, 

 turns a fiery red, and the cornered green chameleon an inky black, opening 

 widely at the same time its brightly coloured movith. Nowhere, however, in 

 the whole animal kingdom are displays so lavish and theatrical provided as 

 among teleostean fishes in their wild ecstasies of love or fighting ; none so 

 exquisite as the elaborately graceful love-dance of the male European stickleback, 

 Gasterosteus aculeatus, when his incandescent blue-green back and transparent 

 red sides glow like neon lighting ; none so awesome as the life-and death war- 

 dance of the ordinarily brownish-grey male Siamese fighting-fish, Betta pugnax, 

 as his widespread fins light up in a luminous multi-coloured glory of burning 

 passion which for centuries has whetted the gambling instincts of the Siamese as 

 did cock-fights the English. In these cases the stimulus is purely visual for the 

 stickleback will fight its own image in a mirror with the utmost savagery 

 (Tinbergen, 1951). 



In other cases a colour-demonstration is made which, strictly speaking, 

 does not involve a true colour change. The Australian frilled lizard, Chlamydo- 

 saurus, for example, carries arovind its neck a large frill -like fold supported by 

 cartilaginous rods which can be opened like a huge circular umbrella around 

 the head. When scared the frill is closed and the lizard dashes for safety ; when 

 it turns to face its enemy the great greenish -yellow frill splashed with red forms 

 a striking and terrifying picture in contrast to the saffron yellow of its open mouth, 

 before which the eneiuy visually retires discomfited. Again, the small European 

 fire-bellied toad, Bombinator igneus, has its dark under-surface spotted vividly 

 with yellow or red, associated with a poisonous exudate from the skin ; when 

 danger tljreatens the animal throws itself on its back or arches its body to pro- 

 claim its unsuitability as food. 



