2. Of a sensitive apparatus at the end, which consists of staff-like cells, which are modified hypodermis cells, with which the fibres of the nerves connect. 3. Of a supporting and accessory apparatus, consisting of pits, or peg- or tooth-like projections filled with a serous fluid, and which may be regarded as invaginations and outgrowths of the epidermis. Hauser adds a remark on the distribution of the pits and teeth in the larvæ of insects, saying that his observations are incomplete, but that it appears that in the larvæ the teeth are most generally distributed, and that they occur not on the antennæ alone, but on the palpi; but in very many larvæ neither pits nor teeth[47] occurred. In the Myriopoda teeth-like projections occur on the ends of the antennæ. In Lithobius they form very small, almost cylindrical, pale organs. [Illustration: FIG. 268.—Olfactory organ of Caloptenus. ] [Illustration: FIG. 269.—Olfactory pits of the antenna of Stenobothrus. This and Fig. 268 after Hauser. ] LETTERING FOR FIGS. 268, 269, 273, 275, 276, 278–281.—_a_, _a_, circular thickening of the skin surrounding the opening of the olfactory pit; _ax_, thread-like continuation of the nerve-cell; _b_, vesicle-like bottom of the olfactory pit, through which the olfactory style passes; _br_ bristle in Fig. 283, stout, and protecting the olfactory pit; _bs_, bent bristle or seta; _ch_, chitinous integument of the antennæ; _d_, seen in section; _f_, invaginated pit; _Fv_, Forel’s flask-shaped organ; _Fvo_, its opening seen from the surface; _gl_, gland-like mass of cells; _hyc_, hypodermic cells; _i_, entrance into the canal belonging to the pit; _m_, olfactory membrane; _m′_, _m″_, _mc_, membrane-forming cell; _n_, nerve of special sense; _nc_, nucleus of the sense- or ganglion-cell; _o_, opening into the olfactory pit; _p_, olfactory pit; _cp_, compound pits; _pw_, wall of the pit; _s_, a large seta; _sc_, sense- or ganglion-cell; _st_, olfactory or sense-style, sometimes peg-shaped; _tb_, tactile bristle. [Illustration: FIG. 270.—_A_, _b_, sense-organ on the abdominal appendages of a fly (Chrysopila); _c_, sense-organ on the terminal joint of palpus of Perla. ] [Illustration: FIG. 271.—Longitudinal section of part of cercus of _Acheta domestica_: _ch_, cuticula; _hyp_, hypodermis; _n._ nerve; _h′^1_, integumental hairs, not sensory; _h^2_, ordinary hair; _h^3_, sensory hair; _h^4_, bladder-like hair; _sz_, sense-cell.—After Vom Rath, from Sharp. ]