Fig. 283.—Vertical section through a single olfactory pit in the antenna of the horse-fly (Tabanus bovinus). For lettering see p. 272.—After Hauser.
The distribution of the olfactory peg or tooth-like projections seems to be much more limited than that of the pits in the Ichneumonidæ. Hauser could not find any. Apis mellifica possesses on each antennal joint only about twenty slender pale teeth, scarcely a third as many as in Vespa crabro; on the other hand, Formica, of which genus several species were examined, seems to have far more teeth than pits; they are relatively long, pale, transparent, and somewhat clavate; they are not unlike those of Chrysopa; on the terminal joint only occur the round openings (Fvo), which lead into a bottle-shaped invagination of the integument (Fv) and contain an olfactory style (Fig. 281). In the Tenthredinidæ only teeth and no pits were to be detected. Sirex has on the under side of the nine last joints of each antenna a group of from 200 to 300 small teeth, which resemble those of Vespa crabro; Lyda has on the terminal joints about 100 teeth. We may add that supposed organs of smell occur on the antennæ of Campodea (Fig. 282).
Kraepelin also thus briefly summarizes Hauser’s statements as to the forms of the different organs of smell.
The manifold nature of the antennal organs has, by Hauser, from thorough studies of the nerve-elements belonging to them, been not simplified but rendered more complicated. According to this naturalist we may distinguish the following forms which the olfactory organs may assume: 1. “Pale, tooth-like chitinous hairs on the outer surface of the antennæ, which are perforated at the end; nothing is known as to the relation of the nerve passing into it (Chrysopa, Anophthalmus). 2. In pit-like depressions of the antennæ arise nerve-rods (without a chitinous case) which stand in direct relation with a ganglion-cell lying under it. These pits are either simple, viz. with only an ‘olfactory rod’ (Tabanus, Fig. 283, and other Diptera, Vanessa), or compound (Muscidæ, and most other Diptera, and Philonthus). It is important that these pits are partly open (in the above-named groups of insects), and partly closed and covered with a thin membrane, under whose concavity the olfactory rods end (Orthoptera, Melolontha, and other lamellicorns). 3. Short, thick pits sunken slightly into the surface of the antennæ, and over this a chitinous peg perforated at the end, in whose base, from the interior, projects a very singular nerve-peg, which is situated over an olfactory ganglion-cell, and provided with a slender crown of little rods, and flanked on each side by a flagellum-cell (Hymenoptera). 4. Round or crevice-like pits covered over by a perforated chitinous membrane with nerve-rods like those in 3, but in place of the flagellum-cell with ‘membrane-forming’ cells spread before it. Hauser finally mentions further differences in the ganglion-cells sent out into the nerve-end apparatus. These exhibit in Diptera and Melolontha only one nucleus, in Hymenoptera a single very large one (with many nucleoli) and three small ones, in Vanessa six, in Orthoptera a very large number of nuclei, etc.”
LITERATURE OF THE ORGANS OF SMELL
Réaumur, Réné Ant. de. Mémoires pour servir à l’histoire des insectes. Paris, 1734–42. (i, 283; ii, 224).
Lesser, F. C. Insecto-theologia, 1740, p. 262.
Roesel, A. J. Insektenbelustigungen, 1746, ii, p. 51.
Reimarus, H. S. Allgemeine Betrachtung ueber die Triebe der Thiere hauptsächlich ihre Kunsttriebe (Instinct). Hamburg, 1760, p. 355.
Sulzer, J. H. Die Kenntzeichen der Insecten. Zürich, 1761.