Brood parasitism

“ How great a number of animal parasites exists may be gathered from the fact that all classes of animals are subject to them. Some of the larger groups, such as Sporozoa, Cestoda, Trematoda and Acanthocephala, consist entirely of parasitic species, and parasitism even occurs among the vertebrates (Myxine) . ”

“ In fact, it becomes difficult to say where the line of parasitism should be drawn. I cannot, however, ignore all notice of the insect tormentors, whether strictly parasitic or not.
At least fifty different species have been regarded as playing the rôle of parasitism in man. ”

“ Owen, "may be conceived when it is stated that almost every known animal has its peculiar species, and generally more than one, sometimes as many as, or even more kinds than, infest the human body." This parasitism begins among the most minute creatures and pervades the entire animal kingdom from the lowest to the highest. Even Protozoa, made visible to us only by the microscope, are infested, as is Paramœcium by broods of Sphærophrya; while in large and complex animals parasites are everywhere present in great variety. ”

“ We may assume, therefore, at least as a working hypothesis, that a lethal parasite is one that is new to its host, and that a harmless parasite is one long established. Since all parasites must have been new to their proper hosts at some period, recent or remote, in the history of the species, it would follow that the first commencement of parasitism would be in almost all cases a life and death struggle, as it were, between the two organisms concerned, and it is quite conceivable that the host might succumb in the struggle and so be exterminated. ”

“ Whilst very many flies, bugs, lice, and fleas persecute animals, not a few of them also attack man. Several of the species are genuine parasites, others are semi-parasitic, and others, again, are altogether outside the border-land of parasitism in the ordinary sense of the term. ”

“ It will be seen from the foregoing discussion that the leading peculiarity of insect parasitism, at least in such groups as the Hymenoptera and Diptera, which are almost the only ones of value in controlling noxious insects, is the restriction of the parasitic habit to the sluggish larva and the specialization of the free adult for the purpose of disseminating the species and of placing the coming generation in intimate contact with the host. ”

“ This is now completely overshadowed by the fact that so many of these parasitic forms are more than simple parasites, they are transmitters of other microscopic parasites which are pathogenic to man. Yet the importance of insects as parasites still remains and must be considered in a discussion of the relation of insects to the health of man. In taking up the subject we shall first consider some general features of the phenomenon of animal parasitism. ”

“ For instance, though the louse, like many decidedly temporary parasites, has lost its wings—a characteristic of insects—in consequence of parasitism, yet nobody would deny its insect nature; such also occurs in other temporary parasites (Cimex, Pulex) . On the other hand, the changes in a number of permanent ectoparasites (such as parasitic Crustacea) are far more considerable, and correspond with those that have occurred in permanent endoparasites. ”

“ Parasitism is a phenomenon common in all the large groups of animals, though the parasites themselves are mostly invertebrates. There are parasitic Protozoa, worms, crustaceans, insects, and molluscs, and a few vertebrates.
Some parasites, like the fleas and lice, live on the surface of the body of the host. These are called external parasites. Others, as the tapeworms, live exclusively inside the body; such are called internal parasites. Again, some, as the bird-lice, which are external parasites feeding on the feathers of birds, spend their whole lifetime on the host ”

“ Paleontology seems to show very clearly the conditions that have favored the enormous development of parasitism among insects especially within comparatively recent times. Some of these conditions are:
1. The diminution in insect stature which occurred in the late Carboniferous and during the Permian and seems to have been originally in great part an adaptation to increased reproduction and dispersal. Other things equal, a small animal will, for very obvious reasons, become a parasite more easily than a large one. ”

“ So with the parasites of fishes. Whenever for any reason or reasons parasitism of any sort becomes so prevalent with any species as to amount to a disease, the remedy will be suggested, and in some cases may be practically applied. If, for example, it were thought desirable to counteract the influences which are at work to cause the parasitism of the trout of Yellowstone Lake, it could be very largely accomplished by breaking up the breeding-places of the pelican on the islands of the lake. ”

“ And not only is practically the whole enormous group of moths and butterflies to be regarded as parasitic, but the same is true also of untold legions of plant-lice, scale-insects and beetles. Hyperparasitism, which may be regarded as a kind of permutation of parasitism, must also be mentioned in this connection, because it gives us a glimpse of the virgin fields which the holometabolic insects, owing to their peculiar method of development, are beginning to invade. ”

“ Although I may have had little difficulty in convincing you that parasitism is a very specialized kind of behavior, you will probably still be of the opinion that there is something inherently and radically wrong with animals that resort to it rather than to predatism, mutualism or some other means of maintaining their vital activities. It must, of course, be admitted that in becoming satellites of their hosts, parasites have renounced the primitive, wasteful and erratic freedom of the predator and are compelled to mould their activities on those of the host. ”

“ It therefore follows that the characteristics of parasites lie, not in their structure, but in the manner of their existence.
Parasitism itself occurs in various ways and degrees. According to R. Leuckart, we should distinguish between OCCASIONAL (temporary) and PERMANENT (stationary) PARASITISM. Occasional parasites, such as the flea (Pulex irritans) , the bed-bug (Cimex lectularius) , the leech (Hirudo medicinalis) , and others, only seek their “host” to obtain nourishment and find shelter while thus occupied. ”

“ The young forms which have invaded the final host become mature in the latter, or after a longer or shorter period of parasitism again wander forth (as the Œstridæ, Ichneumonidæ, etc.) , and reach the adult stage in the outer world. The young stages, during which the parasites undergo metamorphoses or are even capable of producing one or several intermediate generations, are passed in the intermediate hosts until, as a rule, they are passively carried into the final host and there complete their cycle of development by the formation of the organs of generation. ”

“ In order to avoid errors, the term “incidental parasites” should be confined to true parasites which, besides living in their normal host, may also live in other hosts. Leuckart speaks of FACULTATIVE PARASITISM in such forms as Leptodera. L. Oerley3 succeeded in artificially causing Leptodera (Rhabditis) pellio to assume facultative parasitism by introducing these worms into the vagina of mice, where the parasites remained alive and multiplied. Leptodera pellio dies in the intestines of mammals and man; it remains alive in frogs, but always escapes into the open with the fæces. ”

“ Other Parasitic Animals cause Disease.—Besides parasitic protozoans other forms of animals have been found that cause disease. Chief among these are certain round and flat worms, which have come to live as parasites on man and other animals. A one-sided relationship has thus come into existence where the worm receives its living from the host, as the animal is called on which the parasite lives. Consequently the parasite frequently becomes fastened to its host during adult life and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed. ”

“ The late Dr Rowe, an acute observer and successful stock-breeder, who wrote chiefly in connection with the sanitary bearings of parasitism, remarked that “the native animals of Australia are much infested with internal parasites. Some of those now found in the kangaroo and the smaller marsupials may have been derived from our domestic animals; but tapeworms and other internal parasites have been met with in animals occupying regions wholly unsettled.” ”

“ We have the parasitism in which one creature habitually accompanies another to share its prey, like the annelid which takes up its abode in a hermit-crab's shell, and snatches from the hermit-crab the morsels of food it is eating. We have again the commoner parasitism of the Epizoa—animals which attach themselves to the surfaces of other animals, and feed on their juices or on their secretions. And once more, we have the {398}equally common parasitism of the Entozoa—creatures which live within other creatures. ”

“ The habits of the Diptera easily pass over into parasitism, and a very large number are temporary or permanent parasites in the adult or larval stages. Most of the larvae of the Cecidomyiidae live in plants and form galls or other deformities. The bloodsucking habits of mosquitoes and gnats and sand-flies have not led to any special development in the direction of parasitism. ”

“ Internal parasites and "saprophytes," that is, beings which live on dead and decayed organic matter, on the other hand, live surrounded by a supply of dissolved food; and rarely do we find larger cells, richer in reserves, than in the parasitic Sporozoa, which owe their name to the importance of brood-formation in their life-history. In Radiolaria (p. 75 f.) a central capsule separates off an inner layer of protoplasm; the outer layer is the one in which digestion is performed, while the inner layer stores up reserves; and here brood-formation appears to be the rule. ”

“ More repulsive than the parasite female herself, because a yet further product of decay, it is yet only the scent of his mother’s boudoir that we smell in his hair. He is like to the bald patches and rotten wool on the back of a scabby sheep; which indeed indicate that, deep beneath the surface, a parasite insect is eating its way into the flesh, but which are not so much the cause of disease, as its final manifestation. ”

“ A number of diseases in man and other animals are due to the presence of these Sporozoans, for they are all parasitic. Few if any animals are exempt from their attacks. They even attack other minute Protozoa. One hundred and fifty-seven species have been recorded as attacking insects, one hundred species attack birds, fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through the list. Ten have been recorded as attacking man. In some instances the parasite is always present in the host and some hosts may harbor several different species of Sporozoa. ”

“ Then, too, the parasite often gives up all its independence and becomes wholly dependent on its host or hosts not only for its food but for its dissemination from one animal to another, in order that the species may not perish with the host. But in return for all this it has gained a life of ease, free from most of the dangers that beset the more independent animals, and is thus able to devote its whole time and energy to development and the propagation of the species. ”

“ Now, since the parasites never attack more than one of the developmental stages of the insect—egg or larva, according to parasitic species—it results, from what precedes, that there will always be, at the period of egg-laying, existing individuals of the species which will escape them because they are in a developmental condition in which they are not pierced; and when the generation of parasites has passed, these individuals will remain unharmed and constitute the indispensable reserve for the perpetuation of the species. ”

“ He discovered and described developmental stages of the parasite within the intestine of the insect. In other words, the tsetse-fly (in nature, Glossina morsitans) , serves as an essential host, within which an important part of the life cycle of the parasite is undergone. These conclusions were quickly verified by Bruce and numerous other workers and are no longer open to question. Klein and Taute are even inclined to think that mechanical transmission plays practically no rôle in nature, unless the fly is interrupted while feeding and passes immediately to a new animal. ”

“ The phytophagous insect which carries in its interior the larva of the parasite, continues to grow and to feed upon vegetation, and is killed by the parasitic insect only when the latter has reached its full development, and when the host has done all the damage it is capable of doing during its existence. The benefit accomplished by the parasite is manifest only in the following generation, and consists in the suppression of the descendants that would have been mothered by the phytophagous larvæ, if they had been able to develop until their transformation into adult insects. ”

“ A cuticular or intestinal parasite is, as a rule, less harmful than a blood parasite. A helminth, such as an Ascaris lumbricoides or a tapeworm, that feeds on the residues of foodstuffs within the intestine, will hardly affect its host by depriving it of this material. The case is different when the parasites are very numerous, especially when the heavily infested host happens to be a young individual needing all it ingests for its own requirements, and therefore unable to sustain the drain of numerous intruders in the intestine. ”