they form around the gills a kind of living moss, which at last kills the fish. There are different leeches which inhabit invertebrate animals. Rang mentions a little creature of this kind in Senegal, living as a parasite upon the respiratory apparatus of an anodont. Gay discovered in Chili one of the Hirudinidæ in the pulmonary sac of an Auricula, and another on the branchiæ of a crab (Branchiobdella Chilensis). Mons. Blanchard has noticed a malacobdella in the branchia of the Venus exoleta; and it was known in the last century that the Mya truncata of our coast also lodges a malacobdella which lies always under the foot of the animal. This is the hirudinean of which we have spoken above, which is allied transitionally to the trematoda. Together with the Hirudinidæ, we find very small worms, transparent, bristling with daggers and spikes of every form, which are found everywhere in fresh water. They are known by the name of Naïs. They are so completely transparent that we can see the action of all their organs through the substance of the skin. They have been the subject of several remarkable works. They live freely among the leaves of Lemna and other aquatic plants; but there is one species much more restricted in their habitat than the others; these seek assistance from the Lemneæ, and live at their expense. It is because of this kind, of which the genus Chatogaster has been formed, that we mention them here. Their long bristles are veritable halberds, which they employ with astonishing skill, both in attack and defence. Among free parasites are found many very important articulated animals, which neither the naturalist nor the physician ought to ignore. Some of these increase with frightful rapidity on the skin which harbours them, and their name alone is sufficient to inspire disgust, if not horror: others live like leeches at the expense of different animals, but without inhabiting them. There are many of these which follow their host everywhere, and which are dreaded not without just reason. Of this kind are gnats, fleas, lice, bugs, and a great many others, among which we ought not to forget the acaridæ, nor those singular parasites of bats, which bear no slight resemblance to spiders swimming in the midst of the fur. Volumes might be written concerning the organization and the habits of these parasites. These small creatures inspire the naturalist with no more disgust than the earth-worm of our flower-beds, or the salamanders of marshy places. Each one plays its part according to its conformation, and the most abject in appearance is not always the least useful. We will select among these parasites some two-winged insects, among which there are many which suck blood. Those which are generally called flies are divided into two groups, under the name of Nemocera and Brachycera; many of these live only on blood, and are more terrible than the lion and the tiger; in many countries man can defend himself against those fierce carnivora, but he is there completely Fig. 7.-Antenna of a Gnat. powerless and without defence against these insects. Among the Nemocera are found the gnats (Culex pipiens), those brilliant children of the air, with fine and slender claws, and delicate membranaceous wings, and wearing on their heads feathery antennæ of rare elegance. They are known in the Old as well as in the New World, and in southern regions it is necessary to guard against their nightly attacks by musquito curtains. In the Antilles they bear the name of Maringouins, and in hot countries they are generally known as musquitoes. They are also called gnats, midges, black-flies, zanzare, &c., in different localities, but as may be supposed, these names do not always designate the same insect. The musquitoes of the French colonies are often Simulia. At Madagascar and the Isle of France is found the gnat known by the name of Bigaye. In Davis's Straits, in lat. 72° N., Dr. Bessels, on board the Polaris, was obliged to interrupt his observations on account of these insects. A great number of them have been seen up to the 81st degree of latitude. Besides gnats, there were also found Chironomi, Corethræ, and Trichoceræ. As Dr. Bessels was able to save from the Polaris some small collections of insects, we shall socn know the names of the species which live in these high latitudes. It is said that the Esquimaux and the Lapps cover their skin with a coating of grease, not only to lessen the effect of the cold, but to defend themselves from the stings of gnats. "The gnat is a plague from June till the first frosts," says Mons. Thoulet, speaking of his abode among the Chippeways. "It renders the country almost uninhabitable; and one is so exhausted by this suffering, which does not cease by night or by day, and by the loss of blood through their bites, that we manage to get through our daily task only by the force of habit; we can neither speak nor think. When the musquitoes disappear, the black-flies' come: the musquito pumps up a drop of blood and flies away; the black-fly bites and makes a wound which continues to bleed." De Saussure has alluded to curious relations which exist in Mexico between a bird, a beast, and an insect. "Bulls bury themselves in the mud," says this learned traveller, "in order to avoid the attacks of gnats, leaving in the air only the tip of their nostrils, on which a beautiful bird, the Commander, posts himself; in this position the Commander watches for the Maringouin which is bold enough to enter the nostrils of the animal." Gnats are parasites in the same manner as leeches, since, like them, they suck the blood, and live at the expense of others. There is, however, this difference, that the females only are greedy of blood; if this fail them, they live, like the males, on the juices of flowers. Another difference is that they are completely harmless till they have wings, and though they live long under their first form, in damp earth or in water, the duration of their life as perfect insects is of short duration. We need not trouble ourselves about the active larvæ which swarm in stagnant water, nor the chrysalids which float immovable in their natural sepulchre. We give on the next page a representation of a larva of the gnat. The females alone pierce the skin by means of an auger with teeth at the end; they suck the blood, and before they fly away, distil a liquid venom into the wound. This bite seems to have an anesthetic effect, which does not cause it to be felt till some time after. |