1885.] Colouring of Phytophagous Larvae. 281 



generally of a very yellowish-green. In the case of E. Angular ia the 

 eggs present (on a microscopic examination) a yellowish ground, colour, 

 with lines and patches of green. The pigments, at any rate partially, 

 are dissolved in oil globules. The ova are opaque, and are coloured 

 by reflected light, although no doubt a certain depth of substance is 

 penetrated. In the same manner the newly hatched larva is always 

 yellower than one which has taken a meal, apparently due to the pre- 

 dominance of xanthophyll in the former. In some cases there seems 

 to be a segregation of the small amount of chlorophyll to a special 

 part. Thus the head of the newly hatched 8. Ligustri is green, while 

 the rest of the body is yellow. These facts of the predominance of 

 xanthophyll in the ovum and young larva are no doubt to be explained 

 by the great stability of this derived pigment.. Due to the same cause, 

 this substance can be extracted from dried and blown larvae that have 

 been kept for many years, while the chlorophyll has entirely dis- 

 appeared. It is probable that the green or yellow colour of the ovum 

 is protective, especially considering that large and therefore easily 

 seen ova are most generally tinted in this way, and because they seem 

 to be deposited on the side of the leaf against which they would be 

 least conspicuous (as I have noticed in the case of 8. Ligustri). But 

 the chief use of the pigments must be for the young larva, which 

 often rests for some time before feeding, which is active, and there- 

 fore more easily recognised, and which does not possess a firm resistent 

 covering like that of the ovum. 



8. The Prohahle History: of Larval Colouration. 



It seems probable that food in the digestive tract was the first cause 

 of larval colour. In root-feeding and other colourless larvae the dark 

 contents of the digestive tract are distinctly seen through the trans- 

 parent varieties, and in many transparent coloured larvae the same 

 cause certainly helps to confer depth of colour. It is also probable 

 that a dark dorsal line, due to the fluid contents of the dorsal vessel, 

 was a very early marking, as also must have been the external effect of 

 superficially placed fat. The next step would probably be the passage 

 of the plant pigments through the digestive tract into the blood, and 

 next from the blood into' the subcuticular tissues, finally into so 

 remarkable a tissue as that shown in P. Machaon. The employment 

 of true pigment seems to be on the whole posterior in date to the use 

 of derived pigment, and the former at first appears on the highly 

 protected surfaces, the back and upper parts of the sides, conferring a 

 distinct advantage in its greater number of available tints. In opaqr.e 

 larvae, such as many brown Geometers, the green tints are still seen 

 on the inner surfaces of the large posterior claspers where it was 

 unnecessary to replace the older method of colouration. Bat these 



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