The preceding cases, in which the adapted parts are used for the ordinary purposes of life of the individual, are not essentially different from the cases in which the organ is used to protect the animal from its enemies. The bad taste of certain insects is supposed to protect them from being eaten by birds. Cases like this of passive protection grade off in turn into those in which, by some reflex or voluntary act, the animal protects itself. The bad-smelling horns of the caterpillar of the black swallow-tailed butterfly (_Papilio polyxenes_) are thrust out when the animal is touched, and it is believed that they serve to protect the caterpillar from attack. The fœtid secretion of the glands of the skunk is believed to serve as a protection to the animal, although the presence of the nauseous odor may lead finally to the extermination of the skunk by man. The sting of bees and of wasps serves to protect the individual from attack. The sting was originally an ovipositor, and used in laying the eggs. It has, secondarily, been changed into an organ of offence. The special instincts and reflex acts furnish a striking group of adaptations. The building of the spider’s web is one of the most remarkable cases of this kind. The construction of the web cannot be the result of imitation, since, in many instances, the young are born in the spring of the year following the death of the parents. Each species of spider has its own type of web, and each web has as characteristic a form as has the spider itself. It is also important to find that a certain type of web may be characteristic of an entire family of spiders. Since, in many cases, the web is the means of securing the insects used for food, it fulfils a purpose necessary for the welfare of the spider.