Another thing to be remarked is the fact that many butterflies protectively colored, such as those mentioned above, usually alight on some object similar in color to the lower side of their own wings. These butterflies are accustomed to rest upon the trunks of trees, and almost invariably select one with dark-colored bark, avoiding green or light-colored trees such as birches and sycamores. It is not claimed the individual butterfly, after examining the colors of its wings, casts about for a perch to match, but it is quite conceivable that those which were attracted to light-colored bark have been gradually weeded out of the species. OFFENSIVE ODORS AND TASTES Besides the method of protective coloration, some caterpillars are protected against birds by sharp spines or hairs; others by peculiar markings and attitudes said to approximate the appearance of serpents or other dangerous objects. Many butterflies, it has been observed, are protected by still other methods. The common Monarch (_Anosia plexippus_), the Zebra butterfly (_Heliconius charitonius_), and the Red Silverwing (_Dione vanillae_) are very conspicuous butterflies, yet they flutter leisurely about unmolested by birds and other enemies. This protection is due to what Alfred Russell Wallace called “a strong, pungent, semi-aromatic odor, which seems to pervade all the juices of their system.” It has long been known that certain butterflies produced disagreeable odors, but Fritz Müller, working in Brazil as early as 1876, was the first to give the matter any very serious attention. Since Müller’s time extensive studies have been made by Colonel Longstaff and Dr. Dixey, two English entomologists, but very little has been done in North America. It is certainly true, however, that the three butterflies mentioned above (and doubtless many others) are possessed of some taste or odor offensive to their enemies. _Dione vanillae_ is more abundant in South America than in the United States, and was one of the species investigated by Müller, who writes: “The males ... when seized, open wide the anal valvulae, from the inner side of which there appear two glands yielding a strong and nauseous smell. The females, on the contrary, emit a similar smell from a yellow gland extruded on the dorsum between the last and penultimate segments.” Longstaff, who studied this butterfly in Jamaica, describes it as “a beautiful but ill-smelling Fritillary” and says it smells like cow-dung. WARNING COLORATION