ON SOME LEPIDOPTEEOUS LARV_^. 89 



had a ridiculous resemblance to little black cats. As they 

 grew older, and after a skin change or two, their sides 

 become green, whilst the back remains dark. When full 

 grown the larva is mostly green, the patch on the back 

 being of a light purple, sometimes much toned down with 

 white. If disturbed, the larva throws its head back in a 

 kind of sphinx-like attitude, drawing back till the fore part 

 is compressed and swollen, giving great prominence to two 

 black marks resembling eyes, and having the true head 

 surrounded by a livid red margin ; thus giving, as Poulton 

 describes it, an intensely exaggerated caricature of a verte- 

 Jbrate face. At the same time the forked tail is erect, and 

 if the creature's body is touched, suddenly and swiftly two 

 pink whips or filaments are protruded from the fork, the 

 tail is still more curved forward, and the whips are shaken 

 over the head. These whips are probably a protection 

 against the attacks of a species of ichneumon fly, to which 

 the caterpillar is very subject. It has still another defence ; 

 for if the irritation is continued, the larva may eject a 

 strongly acrid liquid from a gland beneath the head. Prof. 

 Poulton, who has studied this larva most exhaustively, 

 and has published his results both in the Trans. Ent. 

 Society^ and in his book The Colours of Animals^ has 

 analysed this liquid, and finds that it is a mixture of formic 

 acid and water : in a mature larva the proportion of acid 

 being as high as 40 per cent., and the twentieth of a 

 gramme can be ejected if the caterpillar has not been 

 irritated for some days. " So far as we know at present," 

 he says, " no other animal secretes a fluid containing any- 

 thing which approaches this percentage of strong acid." On 

 one occasion a larva, which I found on a tree in Tyndall's 

 Park, ejected its fluid whilst I was trying to box it ; for- 

 tunately it did not enter my eye, but the liquid is expelled 



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