The respiration of Insects is carried on by means of a system of vessels for the conveyance of air to all parts of the body; this system is most remarkably developed and elaborate, and contrasts strongly with the mechanism for the circulation of the blood, which is as much reduced as the air system is highly developed, as well as with the arrangement that exists in the Vertebrates. There are in Insects no lungs, but air is carried to every part of the body directly by means of tracheae. These tracheae connect with the spiracles—the orifices at the sides of the body we have already mentioned when describing the external structures—and the air thus finds its way into the most remote recesses of the Insect's body. The tracheae are all intimately connected. Large tubes connect the spiracles longitudinally, others pass from side to side of the body, and a set of tracheae for the lower part of the body is connected with another set on the upper surface by means of several descending tubes. From these main channels smaller branches extend in all directions, forking and giving off twigs, so that all the organs inside the body can be supplied with air in the most liberal manner. On opening a freshly deceased Insect the abundance of the tracheae is one of the peculiarities that most attracts the attention; and as these tubes have a peculiar white glistening appearance, they are recognised without difficulty. In Insects of active flight, possibly in some that are more passive, though never in larvae, there are air-sacs, of more than one kind, connected with the tracheae, and these are sufficiently capacious to have a considerable effect in diminishing the specific gravity of the Insect. The most usual situation for these sacs is the basal portion of the abdominal cavity, on the great lateral tracheal conduits. In speaking of the external structure we have remarked that the stigmata, or spiracles, by which the air is admitted are very various in their size and in the manner in which they open and close. Some spiracles have no power of opening; while others are provided with a muscular and valvular apparatus for the purpose of opening and closing effectually.