WARNING COLORS, WITH EXAMPLES 151 



upon the surface of some low prickly-ash leaves, in August. 

 These caterpillars resembled in every detail the droppings of 

 birds. When first observed an accompanying friend was skepti- 

 cal in believing them to be real larvae. When the foHage was 

 rustled the head and forward part of the body of this cater- 

 pillar was thrown upwards, forming a curve over the back. 

 At the same time occurred a remarkable display of the fork- 

 like scarlet-colored osmeterium. This scent organ protruded 

 from a slit just behind the head, but appeared like a snake 

 thrusting out its tongue. From this fleshy process a peculiar 

 scent not unlike valerian, diffused through the air. Just 

 before this performance the slit partially opened, showing a 

 pinkish tinge on each of the edges Instantaneously the forked 

 tubular mechanism was then extended over a quarter of an 

 inch, slightly leaning backwards when fully erected. After 

 the fright is over the osmeterium is as suddenly withdrawn 

 within the lips of the slit, which close in and completely hide 

 all trace of its former existence. All the caterpillars of the 

 swallow-tail butterflies have osmeteria or scent organs that 

 are thrust out during excitement. The volatile substance 

 which is given off during excitation is acid in reaction, and 

 secreted in some oval glands at the base of the fleshy forked 

 structures. If an unwary bird should by chance have the 

 curiosity to try its bill upon the shiny, delicate skin, a hideous 

 surprise that few of our feathered friends would wilfully invite 

 awaits the aggressor. The innocent looking object suddenly 

 turns into the most startling snake-like animal. The peculiar 

 swollen shape of the forward segments, and shining eye-like 

 tubercles give greater accent to this sudden transformation. 



The larger the larva, the more pronounced is the scent given 

 off when they are disturbed. In September, some of the full 

 grown larvae were found about ready to pupate. They were 

 nearly two inches long. The young larvae are dark brown, 

 having the sixth and eleventh segments of a light straw color. 

 This form of coloration exists until the third molt, when the 

 skin becomes shiny, and some of the glazed tubercles which 

 cover the body disappear. The photograph of the mature 

 larva here reproduced shows it resting on a twig of its favorite 

 food plant, the three-leaved hop tree. The markings are so 



