Page:Jardine Naturalist's library Entomology.djvu/168

162 on different sides of the body generally communicate with each other in the manner indicated, there are not wanting instances where such a union cannot be traced. This, according to M. Léon Dufour, is the case with the majority of the Hemiptera, the respiratory arbuscles on the one side appearing quite independent of those on the other.

On examining the structure of the tracheal tubes, we observe, first, an outer membrane, smooth and transparent, and seemingly without fibres; then a very delicate cartilaginous filament of a pearly white colour, twined spirally round, and forming a tube like the windpipe of the superior animals, within which there is a thin membrane adhering closely to the spiral filament. The outer membrane is commonly colourless, but occasionally it is brown, red, or black, and, in such cases, the ramifications of the vessels can be followed more easily. The pearl-coloured filament, also, in recently dead specimens, affords a similar advantage, as it makes the vessels shine, while the air continues in them, like branches of arborescent silver.

Besides the tubular tracheæ, others are occasionally to be met with of an entirely different character, which have received the name of vesicular air-vessels.