[Illustration: _The Air-tubes magnified._]
[Illustration: _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.