Caterpillars vary in their social habits. Some species are gregarious, and are found in colonies. These frequently build for themselves defenses, weaving webs of silk among the branches, in which they are in part protected from their enemies and also from the inclemencies of the weather. Most caterpillars are, however, solitary, and no community life is maintained by the vast majority of species. Many species have the habit of drawing together the edges of a leaf, in which way they form a covering for themselves. The caterpillars of some butterflies are wood-boring, and construct tunnels in the pith, or in the soft layers of growing plants. In these cases, being protected and concealed from view, the caterpillars are generally white in their coloration, resembling in this respect the larvæ of wood-boring beetles. A most curious phenomenon has within comparatively recent years been discovered in connection with the larval stage of certain small butterflies belonging to the family _Lycoenidoe_. The caterpillars are carnivorous, or rather aphidivorous; they live upon aphids, or plant-lice, and scale-insects, and cover themselves with the white exudations or mealy secretions of the latter. This trait is characteristic of only one of our North American species, the Harvester (_Feniseca tarquinius_). [Illustration FIG. 21.--Early stages of the goatweed butterfly: _a_, caterpillar; _b_, chrysalis; _c_, leaf drawn together at edges to form a nest. (Natural size.) (Riley.)] In addition to being protected from enemies by having colors which enable them to elude observation, as has been already stated, some caterpillars are provided with other means of defense. The caterpillars of the swallowtail butterflies are provided with a bifurcate or forked organ, generally yellow in color, which is protruded from an opening in the skin back of the head, and which emits a powerful odor (Fig. 22). This protrusive organ evidently exists only for purposes of defense, and the secretion of the odor is analogous to the secretion of evil odors by some of the vertebrate animals, as the skunk. The majority of caterpillars, when attacked by insect or other enemies, defend themselves by quickly hurling the anterior part of the body from side to side. [Illustration FIG. 22.--Head of caterpillar of _Papilio troilus_, with scent-organs, or _osmateria_, protruded.]