True parasitism is not to be expected among the winged insects, but many wingless types, such as lice, are parasites, and some free-living winged insects, like warble-flies, have parasitic larvae. The blood-sucking lice or Pediculidae constitute an order ecto parasitic on mammals. They are marked by absence of wings, small head with simple eyes or none, large abdomen and by the adaptation of the claws to clutch the hairs; but it cannot be said that lice are particularly degenerate. They appear to us to be not far removed from the predatory. This is even more marked in the case of the biting-lice or bird-lice (Mallophaga), which occur on birds and a few mammals, feeding not on blood, but on skin-cells and fragments of feathers and hairs. Some birds shelter numerous species of Mallophaga ; thus the hen has nine; but occasionally a particular species of bird-louse is restricted to a particular species of bird. Related species are often found on related hosts. This specificity is often to be noticed in parasites, yet there are others, such as the liver-fluke, that have many different hosts. But as to the Mallophaga, there does not seem to be much of the parasite about them; they are skin-scavengers, and the same may be said of fleas and of the sheep-tick (Melo phagus ovinus), a wingless fly. It is different. however, with the larvae of bot-flies and warble-flies, which illustrate temporary parasitism. A curiously isolated case of alleged parasitism is Platypsyllus castoris, a beetle found on beavers. In many of the ichneumon-flies and related types, which lay eggs in caterpillars and the like, the predatory larvae are themselves parasitized. This "hyper-parasitism" is sometimes carried tar. Thus the caterpillars of Hernerocampa leucostigma which defoliate many trees in the north-eastern United States have 33 primary parasites, 13 secondary parasites, 2 (perhaps 5) tertiary parasites, and one of these (the chalcid Asecodes albitarsis) may be in some cases not tertiary but quaternary.
Among arachnids various degrees of parasitism are illustrated by mites and ticks, some externally adherent, others burrowing in the skin, a few, like the bee-mite, penetrating deeply. Some times placed in the vicinity of arachnids are the vermiform pentastomids, e.g., Linguatula, found in the nasal cavities and frontal sinuses of carnivores, crocodiles, snakes and some other flesh-eating animals. Apart from a few gastropods, such as Ento concha mirabilis attached to blood-vessels in Synapta, there are no parasitic molluscs. Nor are there any parasitic vertebrates except the pigmy males of some angler-fishes.
A survey of parasitic animals shows the widespread distribu tion of this mode of life. But it is of rare occurrence (a) among types that are sensitive to stagnancy; thus there are no parasitic echinoderms; (b) among types that breathe dry air; thus endo parasitism is relatively uncommon among insects; and (c) among types whose shape of body is markedly unsuitable.
mon house-fly is often seen dying from the ravages of a fungus, Empusa muscae, the spores forming a white powder around the moribund insect ; another species is from man's point of view very useful as a check on the multiplication of green-flies.
An estimate of the number of parasitic animals that infest plants varies according to the conception of parasitism. The phytoptid gall-mites, which cause swellings on many plants, have only two pairs of legs, very simple mouth-parts, and a worm-like body, thus showing some marks of the degeneration so often asso ciated with thoroughgoing parasitism; but it seems hardly justi fiable to rank the numerous leaf-miners and stem-borers as para sites in the strict sense. They have adopted an internal herbivorous mode of life. Similarly it seems doubtful if a species of trypano some that lives in some spurges is really a parasite.
Of plants parasitic on plants there are multitudinous instances, though not approaching the diversity illustrated by animals para sitic on animals. At the one pole are parasitic moulds, mildews and rusts ; at the other pole the dodder on clover or the toothwort on hazel roots.
Very clear among parasitic plants is the contrast between partial and complete parasitism. Thus the mistletoe is a partial parasite, for it is usually believed to take nothing but water and salts from the tree on which it grows. Its own green leaves are capable of normal photosynthesis. Yet there is no hint of recipro city on its part, as has been experimentally corroborated by de foliating the bearer. In contrast to the mistletoe, the leafless and chlorophyll-less dodder is a familiar illustration of complete parasitism, for it depends on its host not only for water and salts, but for organic food in the form of carbohydrates and proteins. Eyebright and yellow rattle are good instances of partial parasites, for while they have green leaves and can absorb soil-water, they get on better if their roots come into organic continuity with the roots of neighbouring plants such as grasses. The related cow wheat, though green, cannot survive without external aid. In the Alpine Tozzia, belonging to the same family, the whole first year is spent as a complete parasite underground; in the second year there rises a flowering shoot with yellow-green leaves. In broom rapes the parasitic dependence has gone still further, for the seeds will not germinate unless they are in contact with a suitable host, such as broom; there is no chlorophyll and apart from the under ground absorbing system there is nothing left but flower-stalk. The extreme simplification of the vegetative system is seen in the Rafflesias, where "the whole vegetative body of the parasite may live inside the host-plant, reduced to a spreading weft of undif ferentiated filaments, roots, stem and leaf alike lost." From this degenerate vegetative body, none the less effectively adapted for absorption, there burst forth strange and brilliant blossoms. One of them, R. arnoldii of Sumatra, is the largest of known flowers, with the immense diameter of a yard.