The life-histories of the parasites that attack caterpillars vary greatly. The simplest are those of the large Ichneumon flies: The mother fly lays an egg beneath the skin of the caterpillar. The egg hatches into a larva that absorbs the fatty parts of the body of the caterpillar, gradually growing larger and larger until at last it reaches a length of possibly an inch. By this time it is likely to have absorbed so large a part of the inside of the caterpillar that the latter dies. The parasite larva now changes to a pupa, either inside or outside the skin of the caterpillar, and a little later changes again to an adult Ichneumon fly. [Illustration: Tachinid Parasite: _a_, fly; _b_, puparium, magnified.] In the case just given, one egg only was deposited within the skin of the caterpillar. In many others, however, a large number of eggs may be so deposited by a single fly. A special group of Ichneumon flies, called the Microgasters, contains many parasites that have this peculiarity. The Microgaster larvae on coming forth from the caterpillar have the habit of spinning tiny cocoons within which they change to pupae. By collecting some cabbage worms which are nearly full grown, and keeping them in a glass jar one can generally get a considerable number of these Microgaster cocoons and rear the flies from them. Another group of caterpillar parasites is still more minute. They are called the Chalcid flies. Their life-histories are full of interest, and might easily furnish opportunity for a long lifetime of study and experiment. One is likely to get hundreds of these Chalcid flies from a single caterpillar. Another interesting group of parasites is that of the two-winged Tachina flies (_see cut on this page_). The life-story of some of these is comparatively simple: a buzzing fly, looking much like a large housefly, lays a small whitish egg upon the skin of a caterpillar. This egg is glued tightly and is large enough to be readily seen by the unaided eye. It hatches into a tiny larva that eats its way through the part of the shell glued to the caterpillar's skin, and through the latter at the same time. So the newly hatched Tachina larva finds itself in the body of its caterpillar host. It lives there, absorbing the fatty juices around it until at last it either kills or stupefies its unfortunate victim. It has then become full grown as a larva, and its last larval skin hardens into a brown pupa-case within which the little creature changes into a pupa. It may or may not have burrowed through the skin of the caterpillar before this happened. A little later the pupa changes to a Tachina fly which breaks apart the pupa-case and flies out into the world. It has lately been found, however, that many Tachinids have much more complicated life-histories than this. I have already discussed some of the more important of these in my book entitled, "Seeing Nature First" (_pages 150-158_).