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Among the insects there are many kinds that live parasitically for part of their life, and not a few that live as
parasites for their whole life. The true sucking lice and the bird lice live for their whole lives as external parasites on the bodies of their host, but they are not fixed—that is, they retain their legs and power of locomotion, although they have lost their wings through degeneration. The eggs of the lice are deposited on the hair of the mammal or bird that serves as host; the young hatch and immediately begin to live as parasites, either sucking the blood or feeding on the hair or feathers of the host. In the order Hymenoptera there are several families, all of whose mem
bers live during their larval stage as parasites. We may call all these hymenopterous parasites ichneumon flies. The ichneumon flies are parasites of other insects, especially of the
larvae of beetles and moths and butterflies. In fact, the ichneumon flies do more to keep in check the increase of injurious and destructive caterpillars than do all our artificial remedies for these insect pests. The adult ichneumon fly is four-winged and lives an active, independent life. It lays its eggs either in or on or near some caterpillar or beetle grub, and the young ichneumon, when hatched, burrows into the body of its host, feeding on its tissues, but not attacking such organs as the heart or nervous ganglia, whose injury might mean immediate death to the host. The caterpillar lives with the ichneumon grub within it, usually until nearly time for its pupation. In many instances, indeed, it pupates with the
parasite still feeding within its body, but it never comes to maturity. The larval ichneumon fly pupates either within the body of its host (Fig. 218) or in a tiny silken cocoon outside of its body. From the cocoons the adult winged ichneumon flies emerge, and after mating find another host on whose body to lay their eggs. One of the most remarkable ichneumon flies is Thalesso (Fig. 219), which has a very long, slender, flexible ovipositor, or egg-laying organ. An insect known as the pigeon horntail (Tremer columba) (Fig. 220) deposits its eggs, by means of a strong, piercing ovipositor, half an inch deep in the trunk wood of growing trees. The young or larval Tremer is a soft-bodied white grub, which bores deeply into the trunk of the tree, filling up the burrow behind it with small chips. The Thalessa is a parasite of the Tremer, and “when a female Thalessa finds a tree infested by Tremer, she selects a place
which she judges is opposite a Tremer burrow, and, elevating her long ovipositor in a loop over her back, with its tip on the bark of the tree (Fig. 221), she makes a derrick out of her body and proceeds with great skill and precision to drill a hole into the tree. When the Tremer burrow is reached she deposits an egg in it. The larva that hatches from this egg creens along this burrow until it reaches its victim, and then fastens itself to the horntail larva, which it destroys by sucking its blood. The larva of Thalessa, when full grown, changes to a pupa within the burrow of its host, and the adult gnaws a hole out through the bark if it does not find the hole already made by the Tremer.” The beetles of the family Stylopidae present an interesting case of parasitism. The adult males are winged, but the adult females are wingless and grublike. The larval stylopid attaches itself to a wasp or a bee, and bores into its abdomen. It pupates within the abdomen of the wasp or bee, and lies there with its head projecting slightly from a suture between two of the body rings of its host.
Almost all of the mites and ticks, animals allied to the spiders, live parasitically. Most of them live as external parasites, sucking the blood of their host, but some live underneath the skin like the itch mites (Fig. 222), which cause, in man, the disease known as the itch. Among the vertebrate animals there are not many examples of true parasitism. The hagfishes or borers (Myrine, etc.) have been already mentioned. These are long and cylindrical, eellike creatures, very slimy and very low in structure. The mouth is without jaws, but forms a sucking disk, by which the hagfish attaches itself to the body of some other fish. By means of the rasping teeth on its tongue, it makes a round hole through the skin, usually at the throat. It then devours all the muscular substance of the fish, leaving the viscera untouched. When the fish finally dies it is a mere hulk of skin, scales, bones, and viscera, nearly all the muscle being gone. Then the hagfish slips out and at- Fig. 219.-The large ichneumon fly, tacks another individual. Thalasa, with long ovipositor. The lamprey, another low fish, in similar fashion feeds leechlike on the blood of other fishes, which it obtains by lacerating the flesh with its rasplike teeth, remaining attached by the round sucking disk of its mouth. Certain birds, as the cowbird and the European cuckoo, have a parasitic habit, laying their eggs in the nests of other
birds, leaving their young to be hatched and reared by their unwilling hosts. This is, however, not bodily parasitism, such
as is seen among lower forms.
We may also note that parasitism and consequent structural degeneration are not at all confined to animals. Many plants are parasites and show marked degenerative characteristics. The dodder is a familiar example, clinging to living
green plants and thrusting Fig. 220.—The pigeon horn-tail, Tremer its haustoria or rootlike columba, with strong bearing ovipositor. suckers into their tissue to
draw from them already elaborated nutritive sap. Many fungi like the rusts of cereals, the mildew of roses, etc., are parasitic. Numerous plants, too, are parasites, not on other plants, but on animals. Among these are the hosts of bacteria (simplest of the onecelled plants) that swarm in the tissues of all animals, some of which are causal agents of some of the worst of human and animal diseases (as typhoid fever, diphtheria, and cholera in man, anthrax in cattle). There are also many more Fig. 221.-Thalessa lunator boring. (After Comstock.) highly organized fungi like the whole family of Entomophthora, and the genus Sporotrichum that live in and on the bodies of insects, often killing them by myriads. One of the great checks to the ravages of the corn and wheat-infesting chinch bug (Blissus
in man, anthrax in
leucopterus) of the Mississippi Valley is a parasitic fungus (Sporotrichum globuliferum). In the autumn, house flies may often be seen dead against a windowpane surrounded by a delicate ring or halo of white. This ring is composed of spores of the fungus, Empusa aphidis, which has grown through all the tissues of the fly while alive, finally resulting in its death. The spores are thrown off from tiny fruiting hyphae of the fungus which have grown out through the body wall of the insect. And they serve to inoculate other flies that may come near. Just as in animals, so in plants; parasitic kinds, especially among the ro-T- higher groups as the flowering plants, "" :...” often show marked degeneration. Leaves may be reduced to mere scales, roots are lost, and the waterconducting tissues greatly reduced. This degeneration in plants naturally affects primarily those parts which in the normal plant are devoted to the gathering and elaboration of inorganic food materials, namely, the leaves and stems and roots.
The flowers or reproductive organs usually retain, in parasites, all of their high development. While parasitism is the principal cause of degeneration of animals, other causes may be also concerned. Fixed animals or animals leading inactive or sedentary lives, also become degenerate, even when no parasitism is concerned. The tunicata or sea squirts (Fig. 224) are animals whose simplicity of structure is due to degeneration from the acquisition of a sedentary habit of life. The young or larval tunicate is a free-swimming active tadpolelike creature with organs much like those of the adult of