THE SWALLOWTAILS 73 



found. The species occurs over a very large part of the 

 North American continent, being found from ocean to 

 ocean and from Canada to Florida. In the region north of 

 approximately the fortieth degree of latitude there is but 

 one form of the insect — the familiar yellow-and-black 

 striped butterfly which every one has seen visiting the hlac 

 blossoms in May or June. South of this, however, part 

 of the females take on an entirely different appearance, 

 being almost wholly black with the hind wings touched 

 with Hues of blue and bordered with crescents of yellow 

 and orange. The curious thing about it is that a certain 

 mother butterfly may lay a dozen eggs part of which will 

 develop into the usual yellow form and the rest into the 

 black form, both lots being of the same sex. This black 

 form is so entirely distinct in appearance that the two were 

 originally described as separate species, and they were 

 long considered such, until breeding experiments deter- 

 mined the precise condition. (See plates , pages 17 and 96.) 

 This species is of interest also for another reason. The 

 caterpillars during their later life are remarkable examples 

 of that curious resemblance to the head of a serpent which 

 is thought to have a real protective value in frightening 

 away attacking birds and possibly other enemies. The 

 rings of the body just back of the head are much swollen 

 and on the top of the swollen part there are two large 

 circular marks which bear a striking resemblance to eyes. 

 When the insect is at rest it withdraws its head and holds 

 up the front of the body in such a way as certainly to sug- 

 gest at the first glance that one is looking at the head of a 

 small snake, an impression which is likely to be enhanced 

 when the caterpillar pushes out the curious yellow scent 

 organs from the ring near the top of the head, these 



