To inhale atmospheric air, which is necessary for respiration, above the water, certain insects employ their elytra[2] as a sort of reservoir; others make use of their antennæ, the hairs of which retain the globules of air. In this case it is brought under the thorax, whence a groove carries it to the spiracles. Sometimes the same result is obtained by a more complicated arrangement, consisting of respiratory tubes which can be thrust into the air, which it is their function to introduce into the organisation. [2] The horny upper wings with which some insects are provided are called elytra.--ED. Insects which breathe in the water without rising to the surface are provided with gills--organs which, though variable in form, generally consist of foliaceous or fringed expansions, in the midst of which the tracheæ ramify in considerable numbers. These vessels are filled with air, but it does not disseminate itself in them directly, and it is only through the walls of these tubes that the contained gas is exchanged for the air held in suspension by the surrounding water. The oxygen contained in the water passes through certain very permeable membranes of the gill, and penetrates the tracheæ, which discharge, in exchange, carbonic acid, which is the gaseous product of respiration. [Illustration: Fig. 14.--Branchiæ, or gills, of an aquatic larva (_Ephemera_). A, foliaceous laminæ, or gills.] Fig. 14 represents the gills or breathing apparatus in an aquatic insect. We take as an example _Ephemera_.[3] It may be observed that the gills or foliaceous laminæ are placed at the circumference of the body, and at its smallest parts. [3] May-fly family.--ED. We have now seen that the respiratory apparatus is considerably developed in insects; it is, therefore, easy to foresee that those functions are most actively employed by them. In fact, if one compares the oxygen they imbibe with the heavy organic matter of which their body is composed, the amount is enormous. Before finishing this rapid examination of the body of an insect, we shall have to say a few words on the nervous system. This system is chiefly composed of a double series of ganglions, or collections of nerves, which are united together by longitudinal cords. The number of these ganglions corresponds with that of the segments. Sometimes they are at equal distances, and extend in a chain from one end of the body to the other; at others they are many of them close together, so as to form a single mass.