Returning to Plate VI., in Fig. 21 we see a curiously branched appearance, something like the hollow root of a tree, and covered with delicate spiral markings. This is part of the breathing apparatus of the silkworm, extracted and prepared by myself for the purpose of showing the manner in which the tubes branch off from the “spiracle” or external breathing-hole, a row of which may be seen along the sides of insects, together with the beautiful spiral filament which is wound round each tube for the purpose of strengthening it. One of these spiracles may be seen in the neck of the gnat (Fig. 27). Another spiracle, more enlarged, may be seen on Plate VII. Fig. 34, taken from the wireworm, _i.e._ the larva of the skipjack beetle (_Eláter_), to show the apparatus for excluding dust and admitting air. The object of the spiral coil is very evident, for as these breathing-tubes extend throughout the whole body and limbs, they would fail to perform their office when the limbs were bent, unless for some especial provision. This is achieved by the winding of a very strong but slender filament between the membranes of which the tube is composed, so that it always remains open for the passage of air throughout all the bends to which it may be subjected. Flexible tubes for gas and similar purposes are made after the same fashion, spiral metal wire being coiled within the india-rubber pipe. A little piece of this thread is seen unwound at the end of a small branch towards the top, and this thread is so strong that it retains its elasticity when pulled away from the tube, and springs back into its spiral form. I have succeeded in unwinding a considerable length of this filament from the breathing-tube of a humble bee. Fig. 28 represents the two curious tubercles upon the hinder quarters of the common green-blight, or Aphis, so very common on our garden plants, as well as on many trees and other vegetables. From the tips of these tubercles exudes a sweet colourless fluid, which, after it has fallen upon the leaves, is popularly known by the name of honey-dew. Ants are very fond of this substance, and are in the habit of haunting the trees upon which the aphides live, for the purpose of sucking the honey-dew as it exudes from their bodies. A drop of this liquid may be seen on the extremity of the lower tubercle. The head of the same insect may be seen in Fig. 24, where the reader may observe the bright scarlet eye, and the long beak with which the aphis punctures the leaves and sucks the sap. Fig. 29 is the head of the sheep-tick, exhibiting the organ by which it pierces the skin of the creature on which it lives. Fig. 25 is the head of another curious parasite found upon the tortoise, and remarkable for the powerful hooked apparatus which projects in front of the head. Turning to Plate VII. Fig. 4, we find the head of a ground beetle (_Cárabus_), valuable as exhibiting the whole of the organs of the head and mouth.