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Show 206 DARWINISM CIIAP. those fccdincr on particubr species of phtnts would rapidly acquire the b peculiar tints and markings ,be.~ adap~~d. to conceal them upon those plants. Then, every httle vauat~on that, once in a hundred years perhaps, led to the preservatiOn of some larva which was thereby rather bct~er concealed than its fellows, would form the starting- pomt. of a. f~ut~er development, leading ultimately to that perfectiOn of ImitatiOn in details which now astonishes us. The researches of Dr. Weissmann illustrate this progressive adaptation .. The. very young larvm of several sp~cios arc green or yellow1s~ wit~o:lt any markings; they then, m subsequent mo.ults, obtam ce1 tam markincrs some of which are often lost agam before the larva is full; ~rown. The early stages of those species whi?h, like elephant hawk-moths (Ch~roc:u~pa), have the. antonor soaments elonaated and retractile, w1th largo oye-hke spots to bimitate the head of a vertebrate, are at first like those of non-retractile species, the anterior segments being as largo as the rest. After tho first moult they become smaller, comparatively ; but it is only after tho second moult t~at the ocelli begin to appear, and those are not fully dcfinedftlllhaf~er the third moult. This progressive development o t o Individual- the ontogeny-gives us a clue to tho ancestral development of the whole race-tho phylogeny; and we arc enabled to picture to ourselves the very slow. and gradual steps by which the existing perfect adaptatiOn has been brouaht about. In many larvre groat variability still exists, and in some there are two or more distinctly-coloured forms - usually a dark and a light or a brown and a green form. The larva of the humming-bird hawk-moth (Macroglossa stellatarum) varies in this manner, and Dr. Weissmann raised five varieties from a batch of eggs from one moth. It feeds on species of bedstraw (Galium verum and G. mollugo ~· arHl as the areen forms are less abundant than the brown, It h:ts b probably undergone some recent change of .food -plant or of habits which renders brown the more protective colour. Special Protective Colouring of Butterflies. We will now consider a few cases of special protective colouring in the perfect butterfly or moth. Mr. Mansel W eale states that in South Africa there is a great prevalence VIII ORIGIN AND USES OF COLOUR IN ANIMALS 207 of white and silvery foliage or bark, sometimes of d:tzzling brilliancy, and that many insects and their larvrc have brilliant silvery tints which arc protective, among them being three species of butterflies whose undersides arc silvery, and which arc thus effectually protected when at rost.1 A common African buttorAy (Atericn, mcloagris) always settles on tho ground with closed wings, which so clo ely resemble the soil of the district that it can with difficulty be seen, and the colour varies with tho . oil in different localities. Thus specimens from Sonegambia wore dull brown, tho soil being reddish sand and iron -clay; tho. c from Calah:w and Cameroons were light brown with numerous small white spots, the soil of those countries being light brown clay with small quartz pebbles; while in other localities where the colours of the soil were more varied tho colonrs of the butterfly varied also. Here we have variation in a single species which has become specialised in certain areas to harmonise with the colour of the soil. 2 Many butterflies, in all parts of the world, resemble dead leaves on their under side, but those in which this form of protection is carried to the greatest perfection are the species of the Eastern genus Kallima. In India K. inachis, and in the larger Malay islands K. paralekta, are very common. They are rather large and showy butterflies, orange and bluish on the upper side, with a very rapid flight, and frequenting dry forests. Their habit is to settle always where there is some dead or decaying foliage, and the shape and colour of the wings (on the under surface), together with the attitude of tho insect, is such as to produce an absolutely perfect imitation of a dead leaf. This is effected by the butterfly always settling on a twig, with the short tail of the hind wings just touching it and forming the leaf -stalk ' From this a dark curved line runs across to the elongated tip of the upper wings, imitating the midrib, on both sides of which are oblique lines, formed partly by the nervures and partly by markings, which give the effect of the usual veining of a leaf. The head and antonnre fit exactly between the closed upper wings so as not to interfere with the outline, 1 Trans. Ent. Soc. Land., 1878, p. 185. 2 Ibid. ( P'I'Oceedings, p. xlii.) |