274 THE COLOURS OF ANIMALS 



whom the birds expect to be fed, it is almost certain 

 to be attacked before it has a chance of assuming its 

 terrifying attitude. In conducting such an experi- 

 ment a healthy vigorous larva should be chosen and 

 carefully introduced, so that it may have the same 

 opportunities of defence which it would possess in 

 a wild state. 



The larva of Puss Moth can farther defend itself by 

 ejecting aD irritant fluid 



All the defensive measures hitherto described are 

 of a passive nature, but if further attacked the cater- 

 pillar can defend itself in a very effective manner. 

 The lower part of the red margin below the real head 

 of the animal is perforated by a slit-like opening (see 

 fig. 60), leading into a gland which secretes a clear 

 fluid. This fluid is stored up in considerable quantity 

 and is ejected with great force when the caterpillar is 

 irritated. The ' face ' being turned towards any point 

 at which the larva is touched, the stream is sent in 

 the direction of the enemy. It has been long known 

 that this fluid causes acute pain if it enters the eye. 



In working out the chemistry of this secretion I 

 have been very kindly helped by many eminent 

 chemists. My thanks are especially due to Professor 

 E. Meldola and Mr. A. G. Vernon Harcourt. The 

 secretion proved to be a mixture of formic acid and 

 water : in a mature larva the proportion of acid is as 



