The circulation in an Ephemera. The arrows represent the direction of the blood-current.
As the reader has laid his finger on his pulse, or felt his heart beating within him, or seen a drop of blood ooze from his wounded hand, have the questions never occurred to him,—Have insects blood-vessels, and a beating heart? And if so, have they blood like ours? Many a wise man of old, even the great Baron Cuvier himself, has supposed that there was no circulation of the blood in insects. But, with a magnifying glass and a caterpillar before us, we shall soon be satisfied that this is a great mistake, for by carefully looking along the insect's back we shall perceive in that part a very evident pulsation, as though a fluid were pushed at regular intervals towards the head. The cut represents the circulation in the vessels of the larva of an Ephemera. But insects have no heart, like that of man and animals. What, then, can this beating organ be which we behold? Although it is not like our heart, it is nevertheless an organ which serves all the purpose of ours, and pumps on the blood with great regularity. Instead of being a muscular organ of small size, but great thickness, like our heart, it is in reality a long tube, called the dorsal-vessel, running along the abdomen and thorax of the insect, into which the blood is poured by the veins, and out of which it is forced on towards the head by the regular contractions of its side. This causes the beating movement which we saw in the caterpillar. Perhaps there may be some curiosity to know whether an insect's pulse beats faster or slower than ours. John Hunter counted thirty-four beats in a minute in a silk-worm. Another observer counted thirty a minute in a caterpillar of the pine-moth. But in the imago state the pulse appears to move much quicker: and it is easy to make an insect's beat extremely fast, by exciting or provoking it. Mr. Newport having done so to a bee, counted as many as 142 beats a minute. In a middle-aged man the pulse beats about seventy times in a minute; in a child seven years old about eighty-five times; and at the age of fourteen about eighty times. Hence it follows that our pulse beats about twice as fast as that of an insect in the larva state: in the imago state it is probably, as a general rule, at least in winged, active insects, higher than ours.
FOOD AND DEATH OF THE IMAGO.
But we have something to say about the food of insects. Although it has been already laid down as a general rule, that insects in their perfect state, do not eat in any degree with the voracity they exhibit in the larva form, it is nevertheless true that they do both require and devour food in considerable quantities, and of various kinds. Some, for example, are exclusively vegetable feeders. They attack all the parts of plants, not excepting even the root and the bark. Some, with an elegant taste, select the yellow pollen of flowers for their dainty and delicate food. And others, more refined still in their appetites, will have nothing but the fresh distilled honey that lies hid at the bottom of the flowers, pumping it up by the beautiful spirally-coiled tube which forms a part of their mouth. Need we say these are the butterflies? The fly loves a grain of sugar, or a savoury joint of meat: and to other insects, to use a quotation of Mr. Spence's, which prefer the paper of our Atlases, or maps,—
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----"a river and a sea Are a dish of tea, And a kingdom bread and butter." |
A large number also are carnivorous creatures: need we mention the spider-tribe? a name of terror to myriads of our summer insect-friends. The beautiful cicindelæ, called by Linnæus the "tigers to insects," prey upon the whole insect race, and are endowed with powers of offence and destruction, to a degree sufficient fully to justify this title. In France, we are told, the butchers are very glad to have wasps attend their stalls; since they drive away, and undoubtedly prey upon, the numerous flies which frequent these places. The larger species of ants are equally ferocious, attacking any small soft-bodied insect they may meet with, and when killed dragging it to their nest. The beautiful lady-birds, which we look upon with so much tenderness, remembering the ditty,