WARNING COLOURS 171 



be true of gregarious species, such as the dark larvae of 

 the Peacock or Small Tortoiseshell butterflies, which 

 feed freely exposed on the tops of nettles, and which are 

 known to be refused by some insect-eating animals. 1 



The various unpleasant qualities possessed by 

 caterpillars with Warning Colours 



Other unpleasant attributes, as well as that of 

 a nauseous taste, may be associated with Warning 

 Colours. A strongly smelling or irritant fluid may be 

 discharged from special glands on the approach of an 

 enemy. Glands of this kind occur on the back of 

 many common caterpillars, such as the brilliantly 

 coloured ' Palmer worm ' (larva of Porthesia auriflua), 

 or the onspicuous ' Hop-dog ' (larva of Orgyia 

 pudilunda). The larvae of some common gregarious 

 saw-flies (Hymenoptera), such as Croesus septentrionalis, 

 which completely denudes the branches of birch trees, 

 have a number of odoriferous glands along the middle 

 of the ventral surface. When disturbed, the body is 

 turned forward over the head, and the glands are 

 everted so that their secretion escapes into the 

 air. The meaning of the gregarious habit is very 



1 The gregarious habit may render an insect so conspicuous that 

 it is unnecessary for it to acquire bright colours. The ' warning ' 

 significance of the gregarious habit was first suggested by Fritz 

 Miiller (Kosmos, Dec. 1877). An abstract of this paper has been 

 published by Professor Meldola (Proc. Ent. Soc. Lond. 1878, pp. vi. 

 and vii.) 



