As a typical example of the environment in which these butterflies live, we may take the alpine valleys of such mountain regions as Pike's Peak. Prof. M. J. Elrod has described a visit where, at an altitude of 11,500 feet in the month of August, _Parnassius smintheus_ was flying by thousands, and the earlier stages were so abundant that a water ditch had the surface covered as far as one could see with the dead or dying caterpillars. In such situations, where ice forms at night, and snow frequently falls by day, these butterflies develop apparently in greater numbers than almost any of our other species are known to do in warmer regions. THE SWALLOWTAILS FAMILY _Papilionidae_ This is probably the most distinctive family of all our familiar butterflies. Its members are characterized by being on the whole the largest butterflies in our region and by having the hind wings prolonged into curious tail-like projections, suggestive of those of a swallow. In general, the basal color of the wings is blackish though this is commonly marked in various striking ways with yellow, green, or blue, while the margins of the wings are commonly adorned with red or orange spots. These butterflies are also characterized by certain peculiarities in the branching of the wing veins which will be found pictured in more technical works. The caterpillars of these butterflies have the characteristic form pictured on the plate of the Swallowtails opposite page _80_. When full grown they are large, fairly smooth-bodied worms, showing at most on the surface sparse fine hairs or fleshy threadlike projections. Their most characteristic feature is found in the scent organs called _osmateria_ situated in the back just behind the head. These are thrust out, generally, when the caterpillar is disturbed and appear as orange Y- or V-shaped organs from which an offensive odor is commonly given off. They are supposed to serve the purpose of preventing injury by enemies, possibly birds, monkeys, and other vertebrates. Structurally, they are like long tubular pockets that can be turned inside out. When the pocket is in place it is getting a pocketful of odors. When it is inverted it lets these odors free. On this account Professor Comstock has aptly called these caterpillars "the polecats of the insect world." When ready to pupate, these Papilio caterpillars spin a web of silk upon some more or less flattened surface and a loop of silk near by. They entangle their hind legs in the former and keep their heads through the latter so the loop supports the body a little behind the head. Then they change to chrysalids which are held in place by these sets of silken threads. The chrysalids are rather large and angular and generally take on colors approximating their surroundings. They vary so much in different species that one familiar with them can recognize the chrysalis and know the kind of butterfly it will produce. =The Black Swallowtail= _Papilio polyxenes_