The abdomen of the butterfly contains a number of slender _Malpighian tubules_, in contact with the blood contained in the various cavities. These tubules extract waste matter from the blood, functioning just as kidneys do in the higher animals. The butterfly has no bladder or urethra, however; the Malpighian tubules empty into the small intestine, and the urine passes out of the body with the fecal matter. THE NERVOUS SYSTEM The nervous system consists of the brain, the subesophageal ganglion, and the ventral nerve cord, together with branches of these structures. The _brain_ is a large mass of nerve tissue in the head just above the esophagus. The two _optic nerves_ which supply the large compound eyes make up the principal part of the brain, being much larger and more complicated than the _cerebrum_, which is supposed to be the organ of sensation. The _subesophageal_ ganglion is a sort of second brain lying just below the esophagus; it gives off nerves which supply the mouth parts and control the mechanism of feeding. The _ventral nerve cord_ runs back from the subesophageal ganglion and traverses almost the entire length of the body, being analogous to the spinal cord of the vertebrates. It bears three _ganglia_ in the thoracic region which give off nerves to the legs, wings, and thoracic muscles. Other ganglia, located in the abdomen, have many branching nerves which are distributed to the abdominal muscles and the viscera. THE REPRODUCTIVE ORGANS The _ovaries_ in the female butterfly are sometimes so large as to crowd the other organs in the abdominal cavity. They communicate by means of tubes called _oviducts_ with the _copulatory_ apparatus at the end of the abdomen. The _testes_ of the male butterfly are usually combined into a single organ; they discharge the _seminal fluid_ into the _vas deferens_, whence it is conducted to a sort of pouch near the penultimate segment of the abdomen. In copulation the ends of the male and female abdomens are locked together by certain clasping appendages, and the seminal fluid of the male is forced into the body of the female, where it meets and fertilizes the eggs as they descend from the ovaries. CHAPTER III BUTTERFLY METAMORPHOSIS Some insects, grasshoppers for example, pass through an _incomplete metamorphosis_; that is, the young grasshopper is very much like its parents except as regards size. This is the same sort of development found among birds, reptiles, and other vertebrates. The egg of a butterfly, however, does not hatch into a miniature replica of its parents, but into an altogether different sort of creature, which must pass through a _complete metamorphosis_ before it becomes a butterfly. To put the matter briefly, there are four distinct stages in the life of a butterfly: the egg, the caterpillar, the chrysalis, and the imago, or butterfly proper. THE FIRST STAGE OR EGG