CHAPTER V.
PARASITES.
“En plongeant si bas dans la vie, je croyais y rencontrer les fatalités physiques, et j’y trouve la justice, l’immortalité, l’espérance.”—Michelet, l’Insecte.
The parasite is he whose profession it is to live at the expense of his neighbour, and whose only employment consists in taking advantage of him, but prudently, so as not to endanger his life. He is a pauper who needs help, lest he should die on the public highway, but who practises the precept—not to kill the fowl in order to get the eggs. It is at once seen that he is essentially different from the messmate who is simply a companion at table. The beast of prey kills its victim in order to feed upon his flesh, the parasite does not kill; on the contrary he profits by all the advantages enjoyed by the host on whom he thrusts his presence.
The limits which separate the animals of prey from the parasite are usually very clearly marked; yet the larva of the ichneumon, which eats its nurse, piece after piece, resembles a carnivorous animal as much as a parasite. There are indeed certain animals which take advantage of the good condition of their Amphitryon,
but which render to him in return precious services. Thus those which live on the produce of the secretions, or which clear the system of useless materials in exchange for the hospitality which they receive, are not true parasites. These services are of a very different character, and the duties which they sometimes perform for each other are in some respects analogous to medical care.
Every animal has its own parasites, which always come from without. With some few exceptions, they are introduced by means of food or drink. In order to ascertain their origin, the naturalist must beforehand study the food, that is to say, the prey or the plant which furnishes the habitual nourishment of the host which gives them shelter.
A carnivorous animal, however, does not in general content himself with a single kind of prey—one voracious animal of this class devours all that comes in its way; another, more of an epicure than a glutton, chooses with more discernment. But in the midst of this varied kind of food there is always some species which forms the staple of the usual bill of fare, and it is necessary to find out what this is if we wish to ascertain the parentage and the metamorphoses of the parasite, since it is that which conducts the parasite to its new destination. The mouse is destined to the cat, and the rabbit to the dog; in the same manner, each one of the herbivora is intended to be the prey of a carnivorous animal, if not larger and stronger than itself, at least more cunning. It is of great importance to discover the animal which conducts the new-comer into his habitation. When we know it, we have only to introduce into it the stranger
guest, that sooner or later he may pass into the body of his accustomed Amphitryon. In order thoroughly to know these sedentary or vagabond populations, we must not only study them at the different periods of the year, and under all the conditions of their irregular life, but it is necessary to follow them from the moment that they quit the egg till their complete evolution, closely noticing all that relates to their reproduction.
In the dung of the cow, by the side of the elegant Pilobolus, live masses of small eels, born in the stomach of the animal, which wind and twist like microscopical serpents, and do not seek the slightest help from the organ which shelters them. They are hatched in the interior of the stomach, as if it took place in the meadow. These little eels have evidently only the appearance of parasites, and it may be that they render some service in some of the organs through which they pass. This may also be the case with those which live on the feces of others, or which, lodged in the rectum, watch for the prey which is attracted by the odour. These, especially the latter, are rather messmates than parasites. True parasites are animals entirely dependent on their neighbours, unable to provide for themselves, fed entirely at the expense of others. It is generally supposed that parasites are exceptional beings, requiring a place by themselves in the animal hierarchy, and knowing nothing of the world except the organ which shelters them. This is an error. There are few animals, however sedentary they may be, which are not wanderers at some period of their lives, and it is not even uncommon to find some which live alternately as noblemen or as beggars. Many of them only deserve to be placed