Thus four different views, confused, were held at the opening of this century; the Hamburg zoölogist, M. C. S. Lehrman, in three different treatises, brought together all the hitherto known observations and arguments, treated them critically, and completed them by his own extended studies. Lehrman adopted the opinions of Reimarus, Baster, Duméril, and Schelver, that the stigmata presented the most convenient place for the site of the organs of smell. Cuvier followed throughout the lead of Lehrman, but Latreille returned to the view of the perception of smell by the antennæ, while Treviranus considered the mouth of arthropods as the probable site of the sense of smell, an opinion which, before his time, Huber, in his experiments on bees, had thought to be correct. Marcel de Serres (1811) returned again to the palpi, and asserted—at least in the Orthoptera—their functions to be olfactory, while Blainville, ten years later, again expressed anew the old opinion that the antennæ, or at least their terminations, were organs of smell. Up to that date there was an uncertainty as to the seat of the organs both of smell and hearing. Fabricius, indeed, had already, in 1783, thought he had found an organ of hearing at the base of the outer antenna. In 1826 J. Müller mentioned an already well-known organ in the abdomen of crickets as an organ of hearing. Müller, however, was doubtful, from the fact that the nerve passing to this organ arose, not from the brain, but from the third thoracic ganglion; but, notwithstanding, he remarks: “Perhaps we have not found the organ of hearing in insects because we sought for it in the head.” This discovery was afterwards considerably broadened and extended by Siebold’s work, for the views of these naturalists on the seat of both organs had a definite influence, especially in Germany. For awhile, indeed, Müller’s hypothesis stood in complete contradiction, so that during the following decennial was presented anew the picture of opposing observations and opinions as to the nature of the organs of smell. While Robineau-Desvoidy, at the end of the twentieth year, and also later, in different writings, strove energetically for the olfactory nature of the antennæ, Straus-Dürckheim held fast to the view that the tracheæ possessed the function under discussion. At the same period Kirby and Spence, in their valuable Introduction to Entomology, maintained that “two white cushions on the under side of the upper lip” in the mouth of biting insects formed a nose or “rhinarium” peculiar to insects. This opinion was afterwards adopted by Lacordaire (Introduction à Entomologie), and also by Oken in his Lehrbuch der Naturphilosophie, while Burmeister, rejecting all the views previously held, believed that insects might perhaps smell “with the inner upper surface of the skin.” Müller’s locust’s ear he regarded as a vocal organ.

Besides these occasional expressions of opinion, the French literature of the thirtieth and fortieth years of this century recorded a long series of special works, with weighty experimental and physiological contents, on this subject. Thus Lefebre, in 1838, described the experiments which he made on bees, and which seemed to assign the seat of the sense of smell to the antennæ. Dugès reported similar researches on the Scolopendræ, and Pierret thought that the great development of the antennæ in the male Bombycidæ might be similarly interpreted. Driesch sought to give currency to the views of Bonsdorf, Lamarck, and Marcel de Serres, that the sense of smell was localized in the palpi, though Duponchel went back to the old assertion of æroscepsis of Lehrman, i.e. of the air-test through the antennæ, and Goureau again referred the seat of the sense of smell to the mouth. In England, Newport at this period put forth a work in which he considered the antennæ as organs of touch and hearing, and the palpi as organs of smell—a view which, as regards the antennæ, was opposed by Newman.

Thus the contention as to the use of the antennæ and the seat of the organs of smell and hearing fluctuated from one side to the other, and when in 1844 Küster, by reason of his experiments on numerous insects, again claimed that “the antennæ are the smelling organs of insects,” he argued on a scientific basis; yet v. Siebold and Stannius (1848), in their valuable Lehrbuch der vergleichenden Anatomie (p. 581), remarked that “organs of smell have not yet with certainty been discovered in these animals.”

The following decennial was of marked importance in the judgment of many disputed questions. Almost contemporaneously with Siebold and Stannius’ Lehrbuch appeared an opportune treatise by Erichson, in which this naturalist first brought forward certain anatomical data as to the structure of the antennæ of insects. In a great number of insects Erichson described on the upper surface of the antennæ peculiar minute pits, “pori,” which, according to him, were covered by a thin membrane, and to which he ascribed the perception of smell. A still more thorough work on this subject was published in the following year by Burmeister, who recognized in the pits of lamellicorns many small tubercles and hairs; and about the same time Slater, as also Pierret and Erichson before him had done, out of the differences of the antennal development in the males and females in flesh and plant-eating insects, brought together the proof of the olfactory function of the antennæ. But the most valuable work of this period is that of Perris, who, after a review of previous opinions, by exact observations and experiments, a model of their kind, sought to discover the seat of the sense of smell. He comes to the conclusion that the antennæ, and perhaps also the palpi, may claim this sense, and finds full confirmation of Dufour’s views, and adopts as new the physiological possibility expressed by Hill and Bonnet, that the antennæ might be the seat of both senses—those of smell and hearing.

The beautiful works of Erichson, Burmeister, and Perris could not remain long unnoticed. In 1857 Hicks published complete researches on the peculiar nerve-endings which he had found in the antennæ, also in the halteres of flies and the wings of all the other groups of insects, and which he judged to be for the perception of smell. But Erichson’s and Burmeister’s “pori” were by Lespès, in 1858, explained to be so many auditory vesicles with otoliths. This view was refuted by Claparède and Claus without their deciding on any definite sense. Leydig first made a decided step in advance. In different writings this naturalist had busied himself with the integumental structures of arthropods, and declared Erichson’s view as to the olfactory nature of the antennal pits as the truest, before he, in his careful work on the olfactory and auditory organs of crabs and insects, had given excellent representations of the numerous anatomical details which he had selected from his extensive researches in all groups of arthropods. Besides the pits which were found to exist in Crustacea, Scolopendræ, beetles, Hymenoptera, Diptera, Orthoptera, Neuroptera, and Hemiptera, and which had only thus far been regarded as sense-organs, Leydig first calls attention to the widely distributed pegs and teeth, also considering them as sense-organs. “Olfactory teeth,” occurring as pale rods, perforated at the end, on the surface of the antennæ of Crustacea, Myriopoda, Hymenoptera, Lepidoptera, Coleoptera, are easily distinguished, and besides the “olfactory pegs” of the palpi, may be claimed as organs of smell. The nerve-end apparatus first discovered by Hicks in the halteres and wings, Leydig thinks should be ranked as organs of hearing.

There was still some opposition to Leydig’s opinion that in the insects the sense of smell is localized in the antennæ (teeth and pits), and here the work of Hensen might be mentioned, which in 1860 had a decided influence upon the conclusion of some inquiries.

Thus Landois denied that the antennæ had the sense of smell, and declared that the pits in the antennæ of the stag beetle were auditory organs. So, also, Paasch rejected Leydig’s conclusion, while he sought to again reinstate the old opinion of Rosenthal as to the olfactory nature of the frontal cavity of the Diptera. In spite of the exact observations and interesting anatomical discoveries of Forel in ants, made in 1874, there appeared the great work of Wolff on the olfactory organs of bees, in which this observer, with much skill and acuteness, sought to give a basis for the hypothesis of Kirby and Spence that the seat of the sense of smell lay in the soft palatine skin of the labrum within the mouth (i.e. the epipharynx). Joseph, two years later, drew attention to the stigmata as olfactory organs, referring to the olfactory girdle, and Forel sought by an occasional criticism of Wolff’s conclusions to prove experimentally the olfactory function of the antenna; but Graber, in his widely read book on insects, defended the Wolffian “nose” in the most determined way, and denied to the antennæ their so often indicated faculty of smell. In 1879 Berté thought he had observed in the antenna of the flea a distinct auditory organ, and Lubbock considered the organs of Forel in the antennæ of ants as a “microscopic stethoscope.” In 1879 Graber described a new otocyst-like sense-organ in the antennæ of flies, which was accompanied by a complete list of all the conceivable forms of auditory organs in arthropods. In this work Graber described in Musca and other Diptera closed otocysts with otoliths and auditory hairs, as Lespès had previously done. But Paul Mayer, in two essays, refuted this view in a criticism of the opinion of Berté, referring the “otocysts with otoliths” to the well-known antennal pits into which tracheæ might pass. Mayer did not decide on the function of the hairs which extend to the bottom of the pits; while in the most recent research, that of Hauser, the author again energetically contended for the olfactory function of the antennæ. Both through physiological experiments and detailed anatomical investigations Hauser sought to prove his hypothesis, as Pierrot, Erichson, Slater, and others had done before him, besides working from an evolutional point of view. In a purely anatomical aspect, especially prominent are his discovery of the singularly formed nerve-rods in the pits and peg-like teeth of the Hymenoptera and their development, as well as the assertion that numerous hairs in the pits described by Leydig, Meyer, etc., should be considered as direct terminations of nervous fibres passing into the pits. In the pits he farther, with Erichson, notices a serous fluid, which may serve as a medium for the perception of smells. Among the latest articles on this subject are those of Künckel and Gazagnaire, which are entirely anatomical, while the latest treatise of Graber on the organs of hearing in insects opposes Hicks’s theory of the olfactory function of the nerve-end apparatus in the halteres, wings, etc., and argues for the auditory nature of these structures. Finally, according to Voges, the sense of smell is not localized, but spread over the whole body.

My own observations on different groups of insects agree, in general, with those of Perris, Forel, and Hauser, without being in a position to confirm or deny the varying relations of the Hemiptera. That irritating odorous substances (chloroform, acetic acid) cause the limbs to move in sympathy with the stimulus, I have seen several times in Acanthosoma; still it may be a gustatory rather than olfactory stimulus.

Turning now from speculation and simple observation to exact anatomical and histological data, the nerve-end apparatus seems to have a distinct reference to the perception of odors. It comprises a structure composed of nervous substances which are enclosed in a chitinous tube, and either only stand in relation to the surrounding bodies by the perforated point, or pass to the surface as free nerve-fibrillæ.

In insects there is a remarkable and fundamental difference in the structures of the parts supposed to be the organs of smell. Erichson was acquainted only with the “pori” covered by a thin membrane; but Burmeister, in his careful work on the antennæ of the lamellicorns, distinguished pits at the bottom of which hairs rise from a cup-like tubercle, from those which were free from hairs. Leydig afterwards was the first to regard as olfactory organs the so-called pegs (kegel), a short, thick, hair-like structure distinctly perforated at the tip, which had already, by Lespès in Cercopis, etc., been described as a kind of tactile papilla. Other very peculiar olfactory organs of different form, Forel (Fourmis de la Suisse) discovered in the antennæ of ants, which Lubbock incorrectly associated with the nerve-end apparatus found by Hicks in other insects.