the annelids a complex system of tubes conveys water to the tissues, so in the insects all parts of the body, and the nutritive fluid, are bathed by air admitted through a complex tube-system. So complete is the aeration of the whole structure, that practically the blood of an
Fig. 17.—A, lava; B, pupa of a mosquito, d represents the water-line. (After Morse.
insect is wholly arterial. The necessity for this lies in the slow and imperfect circulation, coupled with an unparalleled activity.
The air-tubes, "tracheæ," traverse every part of an insect's body, even the brain and eye, and form the nervures or framework of the wings, where they are sheathed by another tube conveying blood. 
Fig. 18.—Grub of Chameleon-Fly. They are prevented from collapsing by a mechanism, one of the most admirable and exquisite found in nature. A delicate, elastic thread is wound in a close spiral between the two membranes of the walls of the tubes a contrivance which we have imitated in the flexible gas-tube of a drop-light.
The openings to the tracheæ, termed "spiracles" or "stigmata," are generally located on the sides, of the abdomen and thorax, a pair for each segment, and exhibit great variety and adaptation to their purpose. So constructed as to admit air, they exclude water and dust, and can be opened and closed at the will of the creature. Sometimes they are a mere slit, like a button-hole. In the soft-skinned larvæ they are kept open by a horny ring. The aperture is sometimes protected by interlacing hairs; sometimes it is grated, or in other species closed by a thin membrane pierced by innumerable small pores a real sieve. A drop of oil placed upon the abdomen immediately fills the spiracles and smothers the creature.
