A modification of this apparatus is found throughout a very extensive order of insects--the Hymenoptera; but in the majority of cases it is not connected with purposes of warfare. Wherever it occurs it is always confined to the female sex, or (as in the case of some social insects) to the neuters, which are undeveloped females. When it is not accompanied by a poison-reservoir it is ancillary to the deposition of the eggs, and is hence called an ovipositor, though in many cases it performs a part much more extensive than the mere placing of the ova. A very wide field of observation, and one easily cultivated, is presented by the organs of sense in the insect races, and in particular by those curious jointed threads which proceed from the front or sides of the head, and which are technically called antennæ. These may sometimes be confounded with the palpi; for in a carnivorous beetle, for instance, both palpi and antennæ are formed of a number of oblong, polished hard joints, set end to end, like beads on a necklace. And it is probable there may be as much community in the function as in the form of these two sets of appendages; that both are the seats of some very delicate perceptive faculty allied to touch, but of which we can not, from ignorance, speak very definitely. It is likely, indeed, that sensations of a very variable character are perceived by them, according to their form, the degree of their development, and the habits of the species. It is not impossible, judging from the very great diversity which we find in the form and structure of these and similar organs in this immense class of beings, compared with the uniformity that prevails in the organs of sense bestowed on ourselves and other vertebrate animals, that a far wider sphere of perception is open to them than to us. Perhaps conditions that are appreciable to us only by the aid of the most delicate instruments of modern science may be appreciable to their acute faculties, and may govern their instincts and actions. Among such we may mention, conjecturally, the comparative moisture or dryness of the atmosphere, delicate changes in its temperature, in its density, the presence of gaseous exhalations, the proximity of solid bodies indicated by subtile vibrations of the air, the height above the earth at which flight is performed, measured barometrically, the various electrical conditions of the atmosphere; and perhaps many other physical diversities which can not be classed under sight, sound smell, taste, or touch, and which may be altogether unappreciable, and therefore altogether inconceivable, by us. It is probable, however, that the antennæ are the organs in which the sense of _hearing_ is specially seated.