The Origin of the Markings of Caterpillars. 235 



green ground ; these slope in the direction of the 

 caudal horn. Owing to the transparency of the 

 skin, a dark green dorsal line appears in the posi- 

 tion of the underlying dorsal vessel, the green 

 contents of the alimentary canal being distinctly 

 visible through the absence of adipose matter in 

 the tissues. The larvae possess also a fine whitish 

 subdorsal line, which extends from the horn to the 

 head. The horn at this stage becomes black with 

 a yellowish red base. 



In the third stage, which occurs after six or seven 

 days, the oblique stripes appear darker, and the 

 subdorsal line disappears. 



Fourth Stage. 



After another period of 4 5 days the third 

 moult takes place, and there now commences 

 a dimorphism which will perhaps be better desig- 

 nated as variability, since the two extremes are 

 connected by transitional forms. The majority of 

 the larvae have, as in the preceding stage, pure 

 white oblique stripes, but many of them possess a 

 blood-red spot on the anterior side of the stripes, 

 this spot showing all gradations in size and depth 

 of colour between maximum development and a 

 mere trace. Special interest attaches to these 

 spots, as they are the first rudiments of the coloured 

 border of the oblique stripes which occurs in so 

 many Sphinx caterpillars. 



In the fifth stage the last of the larval de- 



