Naturalists have paid great attention to these latter insects, as much on account of their mode of life as of the difficulties which they have suggested to entomologists in the appreciation of their natural affinities. Are they coleoptera, as was for a long time, and perhaps correctly, supposed, or do they form a distinct order by themselves? However this may be, these are the facts known concerning them, according to the recent observations of Mons. Chapmann, a conscientious naturalist. The females do not lay their eggs in the nests of wasps, but the larvæ, under the form of meloë, penetrate into the cells, by the assistance of the larvæ of the wasps, which carry them hidden between the second and third ring. The larvæ of the Rhipiptera are developed at the expense of the larvæ of the wasp, suck their blood, swell, and their skin remains adhering to the fourth segment.
Fig. 78.—Black Stylops, female, showing the embryos in the abdomen.
When the rhipipterous insect is six millimètres in length, it changes its skin the second time, and this splits on the back, so that the skin remains fixed between the larva of the parasite and that of the wasp. It then sucks the rest of the juices of the young wasp, and becomes a nymph in the prison which it has formed for itself. This evolution lasts from twelve to twenty-four hours.
Many male crustaceans, though they differ materially from their females in form as well as in manner of life, do not remove far from their partners in order to procure the assistance which they need. The insects which now occupy our attention are entirely different in this respect. The male preserves his usual appearance during the whole of his life, as well as the attributes and independence of free insects; while the female seeks for assistance with regard both to food and lodging from the time she leaves the egg; she is still wrapped up in swaddling clothes when she receives the male, as when she came forth from the egg.
The worms of this category are usually fully formed without undergoing metamorphoses; and if the place which they choose at their exit from the egg is not precisely their cradle and their tomb, at least all the phases of their monotonous life occur around it. They may be ranked among the most beautiful and the largest of parasitical worms; and as they are hermaphrodites, we find no greater diversity in the several forms than in their differences of age. All have their reproduction certain, and their eggs are less numerous for this reason. There are some of them that lay only one egg at a time, and this egg sometimes appears but once during a season. This explains why the eggs of some of these worms have not yet been recognized.
We may place at the head of this group the Tristomum, which has only been discovered a few years. We owe to Baster the knowledge of a beautiful and large species, which inhabits the body of the halibut. Naturalists have given it the name of Epibdella. This worm is of the size of the human nail; it resembles in form a box leaf; by the aid of its suckers it clings to the skin of its host like a scale; and is sometimes mistaken for one. It is of an oval form, and of a dull white colour; it can scarcely be distinguished from the skin of the fish. We may have it before our eyes for a long time before we perceive it.
Another Epibdella lives on the skin and on different parts of the body of the European maigre, or the Virgin Mary’s fish; it is covered with pigment spots which cause it still more to resemble the large scales of its host. This fish, which is also called the Sciæna aquila, has its skin covered with similar scales, and they are of the same colour, both on the back and belly.
Another large and fine worm of this group lives on the gills of the sturgeon, and is distinguished by its suckers as well as by its great mobility. The epibdellæ preserve their scale-like form during their greatest contractions, but these worms change with every movement. The Nitschia elegans, for such is the name by which it is distinguished, is not rare on the sturgeon as we see it in our markets. Among the many parasites in this category, there is a very remarkable one which deserves particular mention. It lives abundantly on fresh-water fishes, preferring to attach itself to their gills; it is found most commonly on the bream. For our knowledge of these worms we are indebted to Nordmann.
They bear the name of Diplozoon paradoxum, and are always double, that is to say, always united like Siamese twins, being organically fastened together; they leave the egg, like their congeners, isolated and hermaphrodite, instal themselves separately on their host, and a little time after their choice of a resting-place, they unite so that the tissues, I was about to say the organs, are welded to each other. They cross like two strokes of an x. It is in this position that they live and die, after having produced large and beautiful eggs provided with a very long cable. These eggs are laid separately, and attached to the gills of the fishes which give them shelter. At the end of a fortnight the ciliated embryo comes forth, being provided with two eyes, and seeks to establish itself on a fresh host.
Under the form of Diporpa it has a ventral sucker, and a small papilla on its back, and the two individuals are attached to each other cross-wise by the sucker and the papilla. Notwithstanding what Humboldt says in his “Cosmos,” the Diplozoon is not an animal with two heads and two caudal extremities, but is a double animal, two hermaphrodite individuals united, which at first have lived separately, and have become soldered to each other at the period of maturity.
We find a nematode, and consequently an animal with the sexes separate, which presents the same phenomena. The male and female are soldered together, but the female alone undergoes development. It is the Syngamus trachealis of Siebold. It inhabits the tracheal artery of some gallinaceous fowls, and according to recent experiments, it develops itself directly in the tracheal artery of birds.
Another beautiful trematode, the Octocotyle lanceolata, lives abundantly on the gills of the alosa, and another, the Octobothrium merlangus, on those of the whiting. The gills of the Mustelus vulgaris regularly bear another species resembling a leech, but instead of a single sucker there are six; this is the Onchocotyle appendiculata.
The bladder of frogs lodges a very beautiful and large trematode which has lately been studied by many naturalists, the Polystomum integerrimum. Many observations remain to be made on the different phases of the existence of this parasite. Its organization is known, and it has been seen to lay large and beautiful eggs, but its movements have not been observed before its entrance into the bladder.
This Polystomum of the frog—and it is no doubt the same with the species Polystomum ocellatum which inhabits the mouth of the European tortoise (Emys Europæa)—lays eggs only in winter, and the eggs of the young ones do not seem to produce more precocious embryos than those of the adult. The embryos are ciliated, unlike those of many of the ectoparasite worms. They much resemble the gyrodactyles, especially by their bristles; and like these, they inhabit the cavity of the mouth before they migrate into another organ. We may even ask if these singular gyrodactyles, so peculiar in many respects, are not the larval forms of trematodes allied to the polystomum.
Several important works have lately appeared on the Polystomum integerrimum, by Mons. Stiéda in 1870, by Mons. E. Zeller and Mons. Willemoes-Suhm in 1872.
The gyrodactyles, which we have just mentioned, are among the most curious worms that have been discovered during late years. They are of small size, and live in the gills of fishes, often in great numbers, and move with considerable agility. They are armed with very variable hooks, which serve to anchor them; and sometimes a digestive canal and organs of sensation are found in them.
The Gyrodactylus elegans bears within it a young one which already has hooks, and in this young one, which is not yet born, we see another generation with the same organs, so that three generations are thus enclosed. The daughter is ready at the moment of her birth to give birth to another daughter. According to another mode of interpretation, the mother and daughter are sisters; the elder is found at the periphery, the younger at the centre. These worms are found abundantly in the gills of the cyprinidæ, or white fishes. We have only to scrape gently the surface of the gills with a scalpel, and thus remove a small quantity of a mucous substance, place it on a slide of a microscope, cover it with thin glass, and examine it immediately with the compound microscope. We cannot repeat this three times without finding gyrodactyles.
There are also many insects which live as parasites on plants, and demand from them both a resting-place and their food. Almost all the Hemiptera are among these; we have already mentioned them. The hemiptera, which live on the sap of vegetables, are parasites in the same manner as those which live at the expense of animals. We ought not to make a difference between the manner of life of the bugs of plants and those of animals. It may be said that Providence has placed these beings as riders on both the vegetable and animal kingdoms to restrain them with a bridle. What the gardener does to plants, the aphis has often done before in order to arrest a too vigorous and rapid growth.
Fig. 80.—Cochineal insect, male (Coccus cacti), natural size and magnified.
The cochineal insect (Coccus cacti) Figs. 80 and 81, originally from Mexico, lives on the cactus nopal as a true parasite, and furnishes a precious colouring matter, carmine. This insect has been introduced into the Antilles, Spain, the Canary Isles, Algeria, and Java.
Lake is produced by a species of the same genus, originally a native of India (Coccus lacca).
Aphides (Fig. 82) feed on the sap of plants; they multiply rapidly without the male insect. Rose-trees, and more especially their buds, are attacked by a species of a green colour, of which we give a representation (Fig. 83).
An aphis, the Phylloxera vastatrix, has, a short time since, invaded the vineyards, and small as it is, it is dreaded as a plague which scatters ruin in its path. According to recent observations this insect has a double series of generations which precede each other: the mother type and the tubercular type. But this polymorphism seems to be more apparent than real, although there is a considerable difference in their manner of life and of procuring nourishment. Is this difference the result of the different kinds of food taken from the roots and the leaves? There is one thing which may reassure us as to the future attacks of the phylloxera, that Mons. Planchon has just discovered in America the cat of the phylloxera, one of the acaridæ, its mortal enemy; and it is only necessary to multiply these in order to destroy this terrible pest of the vineyards. We thus see that we have only to imitate this so-called blind Nature, in order that we may arrest a misfortune against which man is unable to protect himself by his own powers.
We will here repeat what we wrote respecting aphides some years ago. Who does not know these small green bodies, of the size of a pin’s head, coming like a cloud upon the buds and leaves of the rose bushes, which shrivel and wither immediately? There are green ones on certain plants, and black ones on others, but whatever be their colour, they are living pearls which form garlands round the stalk. The world considers them as vermin, and they scarcely dare to touch them with the point of their fingers. To the naturalist they are a little world of wonders. Let us examine with a magnifying lens these walking grains of sand; each grain will reveal to us a charming insect, whose head is adorned with two little antennæ, and has globular projecting eyes glistening with the richest colours; behind these are two reservoirs of liquid sugar, elegantly mounted on a polished stalk, and always full; long and slender limbs support the globular body.
Much has been written about these small sugar manufactories, so well known by ants that they have procured for the aphis the name of ant-cow. Among the curious phenomena presented by these grains of animated dust, that which most interests us relates to the secret of their astonishing, we may say, their prodigious fecundity.
Nature requires millions of aphides in a few hours, to arrest the exuberance of vegetation, and as if she distrusted the assistance of the male insect, she dispenses with it, and the female brings into the world a daughter already prepared to produce a grand-daughter. Generations succeed each other with such rapidity, that if the daughter at her birth were to meet with any obstacle in her passage, the grand-daughter might come into the world before her mother; a single egg can produce in the course of one season milliards of individuals. Each plant has its own aphis, and in many localities the ravages of the Aphis laniger are but too well known, though it was unknown in Europe a quarter of a century ago.
The Gyrodactylus elegans, of which we have spoken above, contains embryos similarly enclosed, and if these facts had been known at an early period, the celebrated theory of the enclosure of germs, so warmly advocated by Bonnet, would have preserved still longer its intrepid defenders.
With but few exceptions, all the Hemiptera are parasites of the vegetable kingdom. There are only very few which attack animals. There is one species, the name of which may be readily guessed (Acanthia lectularia), which pursues us relentlessly everywhere, for it will wait for months and years, always equally greedy of our blood. It surprises us during the night, and does not wait till its digestion is complete before it attacks us again. Happily for us, another hemipterous insect, the masked reduvius (Reduvius personatus) penetrates like the preceding one into our apartments, and covers itself with dust, in order the more readily to fall upon its enemy; but man is not sufficiently acquainted with its habits, to make war in common with it on this miserable parasite. We ought for this purpose to place the masked reduvius under the protection of the law, to collect the various kinds together, and to offer premiums for the most vigorous races.