282 Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



stages were by no means mutually exclusive, or even necessarily 

 successive, for each new method was an additional resource, not often 

 to replace the rest, but in nearly all cases to be used with them. 

 Hence the one of the commonest associations is a ground tint partially 

 due to derived pigments in the blood, and also sometimes segregated 

 beneath the cuticle, while special markings are conferred by true pig- 

 ments generally placed in the hypodermis cells, although sometimes 

 cuticular in position. The derived pigments more often confer 

 general resemblances, the true pigments, special resemblances. Again 

 the association is well seen in the use made of fat as a reflecting 

 surface behind the transparent green blood. The fact that the colour 

 due to derived pigments is almost incidental at first is shown by the 

 possession of green blood by certain leaf-mining genera entirely con- 

 cealed from view, as is pointed out by Mr. Meldola. 



We can now summarise the causes of colour in larvae with some 

 attempt at the historic order of their employment :— 



I. The internal tissues and organs f^' ^J% es ^ ve tract. 



with ready made colour \ ' ' ' 



Ic. Dorsal vesseL 



II. The passage of derived pigments ^ m , , 

 i-i ^ , s ., n « X 5 I «• The blood, 



through the walls oi the diges- > , mi _ , ,. , . . 



. & . . . ° | o. The subcuticular tissues, 



tive tract into . . J 



III. The appearance of true pigment f a. The hypodermis. 



in \b. The cuticle. 



The above causes will, I believe, explain all larval and pupal colours, 

 except such cases as the metallic tints of certain pupae which eannot 

 be entirely due to coloured substances. 



9. Spectroscopic Examination of the Blood of Lepidopterous Larvce 



and Pupm. 



In the following investigation Zeiss's microspectroscope was always 

 employed, and proved to be extremely delicate and convenient on all 

 occasions. The instrument was very kindly lent to me by Professor 

 Burdon Sanderson. A paraffin lamp was at first used as the means of 

 illumination, and it acted very well for the less refrangible half of 

 the spectrum, but in all later work bright sunlight was alone employed 

 because of its immense superiority at the violet end. 



The blood was always obtained in ^such a way as to prevent admix- 

 ture with the contents of the digestive tract or any secretions. 

 Success in this attempt is aided by the fact that the blood exists 

 under considerable pressure, so that after a minute prick, only just 

 penetrating the hypodermis, a considerable quantity at once issues. In 

 larvae the distal pa.rts of the claspers were generally pricked, the spot 

 being chosen because of its .situation at some distance from the diges- 



