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.

In the course of a special description of these sense-organs in the Orthoptera, Hauser describes at length those of Œdipoda cœrulescens and Caloptenus italicus. On one antennal joint of Caloptenus (Fig. 268) was often counted 50 pits; on the anterior joints the number diminishes to about 30. Hauser thinks that in all Orthoptera whose antennæ are like those of Caloptenus occur similar pits, as he found them in Stenobothrus (Fig. 269) as well as in Œdipoda. Gryllotalpa possesses similar pits,—four to six on each antennal joint, making between 300 and 400 pits on each antenna.^[48] In Mantis religiosa the pits were not detected, but on each joint, except the eighth basal, there are about 200 small, hollow, curved teeth with a fine opening in front.

In the Neuroptera (Chrysopa) there occur on the antennæ, besides numerous very long tactile bristles, small pale, transparent teeth. No pits could be detected.

In the Hemiptera (two species of Pyrrhocoris only were examined) only two kinds of tactile bristles occurred, but Hauser detected no pits, though Lespès states that they are present.

Of the Diptera, Hauser examined more than 60 species. The pits in the Diptera brachycera (Muscidæ, etc.) are unexceptionally confined to the third antennal joint. Their number varies extraordinarily in the different species. Helophilus florens has on each antennal disk only a single pit, while Echinomyia grossa possesses 200 of them. In flies of certain families the pits are compound, and contain 10, 20, and often 100 olfactory hairs, partly arising from the coalescence of several pits. Such pits are usually divided by lateral walls into several chambers, whose connection is only indicated by their common outlet. Simple olfactory pits with a single olfactory style were observed only in the Tabanidæ, Asilidæ, Bombylidæ, Leptidæ, Dolichopidæ, Stratiomyidæ, and Tipulidæ. In the last the compound forms do not occur at all, but in the other families mentioned also occur compound pits, receiving from two to ten nerve-terminations.

The antennal pits of flies are always sac-like invaginations of the external chitinous integument, of manifold shapes, opening externally and never closed by a membrane. The pits differ but slightly in the different species, and that of Cyrtoneura stabulans (Fig. 273) is described at length as typical of those of brachycerous flies in general.

The olfactory pits of the Tipulidæ seem to have a somewhat different structure, since the external passage is closed. It is circular, surrounded with a slight chitinous wall, and not covered with bristles. Such pits in their external appearance are like those of the locust (Caloptenus) and many Hymenoptera. They are situated usually on the third antennal joint. Pachyrhina pratensis L. has about 60 of them, as have Tipula oleracea L. and Ctenophora.

In the Lepidoptera, olfactory pits are much like those of flies. Hauser describes in detail those of Vanessa io. Those of the moths were not examined, but they can be readily and satisfactorily proved to be the site of the olfactory sense.

Historical researches in respect to the Coleoptera generally gave a very unfavorable result, contrary to Lespès’s views. That author states that in the Carabidæ the pits are found on the four first joints, but Hauser could discover them in none which he examined. Usually only tactile bristles occur, so also in the Cerambycidæ, Curculionidæ, Chrysomelidæ, and Cantharidæ. In a blind silphid beetle (Adelops hirtus) of Mammoth Cave we have found well-marked olfactory organs (Fig. 274). Similar organs occur in the antennæ of the Panorpidæ.

Olfactory pits, however, without doubt occur in Silpha, Necrophorus, Staphylinus, Philonthus, and Tenebrio. The openings of the pits are small and surrounded with a small chitinous ring; in Silpha, Necrophorus, and Tenebrio they cannot easily be distinguished from the insertion-cavities of the bristles, but in Philonthus and Staphylinus they are less like them, being distinguished by their somewhat larger size and their often more oval form. In Philonthus æneus about 100 such small pits occur irregularly on the terminal joints; besides, in this species on each side of the terminal joint is an apparatus which is like the compound pit generally occurring in the Diptera.

Very remarkable pits occur in the antennal lamellæ of Melolontha vulgaris (Fig. 275) and other lamellicorns. On the outer surface of the first and seventh (in the female the sixth) antennal leaf, as also on the edges of the other leaves, only arise scattered bristles; on the inner surface of the first and seventh leaves, as also on both surfaces of the second to sixth leaves, are close rows of rather shallow depressions of irregular form, some circular, others regularly hexagonal. Their number is enormous: in the males 39,000, in the females about 35,000, occur on each antenna.

The antennal pits and teeth of Dyticus marginalis are morphologically and physiologically identical with those of bees and wasps. In Anophthalmus bilimekii, Hauser found on the last antennal joints about 60 teeth, which essentially differ in form from those previously described; they are very pale, transparent, cylindrical, elongated, and bent elbow-shaped on the first third, so that the last two-thirds run parallel with the antenna. The length of these remarkable teeth is 0.035 mm., their breadth 0.005 mm. He only found them in Anophthalmus, and in no other species of Carabidæ; they must resemble the teeth described in Chrysopa. Our species possesses similar processes (Fig. 277). Similar teeth occur on the maxillary and labial palpi of beetles. Dyticus marginalis possesses at the end of each terminal palpal joint a group of very small teeth, which were also detected in Anophthalmus bilimekii, Melolontha vulgaris, etc. In Carabus violascens were detected on the maxillary palpi large, plainly microscopical, white disks, which are surrounded with a great number of extremely small teeth.

Whether the above-described organs on the palpi of beetles should be considered as olfactory or gustatory in their nature can only be determined by means of physiological experiments; they probably receive taste-nerve terminations.

The Hymenoptera furnished very good material for histological purposes, so that Hauser could not only study the terminal apparatus of the olfactory nerves in the perfect insect, but also in three different stages of the pupa. These are described at length, as regards the distribution of the pits and teeth, in Vespa crabro; each joint of the antenna (flagellum) possesses between 1300 and 1400 pits, nearly 60 teeth, and about 70 tactile hairs; on the terminal joint there are more than 200 teeth, so that each antenna has between 13,000 and 14,000 olfactory pits and about 700 teeth (Kegeln). Fig. 278 represents a cross-section through the penultimate antennal joint of Vespa crabro; we can see how thick are the series of openings on the surface of the antennæ, and how regular is the distribution of the teeth.

The distribution of the olfactory pits and olfactory teeth is thus seen to be very general; the deviations are so insignificant that there is no reason for the establishment of more than one type.

Antennal pits with a small crevice-like opening occur in genera nearly allied to Vespa and also in most Ichneumonidæ, Braconidæ, and Cynipidæ. But the crevice-like openings in these families are considerably longer and often are of a somewhat twisted shape. In all the species with translucent antennæ we can recognize the inner mouth of the pit as a round or nearly round disk situated usually under the middle of the opening. The antennal pits of Apis mellifica, as well as those of Bombus (Fig. 280) and allied genera, differ from those of the Ichneumonidæ in being not like crevices, but circular openings.