Ant colonies > Internal structure of ants > Olfactory and Gustatory Sensillae

Olfactory and Gustatory Sensillae

Olfactory and Gustatory Sensillae

It seems to be impossible to distinguish between these organs in insects, although it may be asserted that the organs of smell are situated mainly or exclusively on the antennae, whereas, those of taste are found on the mouth-parts, espe­cially on the maxilloe and labium and their paipi. The antennary sensillae of ants have been studied by Hicks (1859), Leydig (1860), Forel (1874, I884), Lubbock (1877), Kraepelin (1883), and more recently by Krause 0907). From the researches of these authors it appears that in addition to numerous tactile hairs like those described above, there are four more modified types of sensillae which have been more or less definitely connected with an olfactory function. These do

FIG. 32. Subdiagrammatic section of the antennal sense-organs of an ant. (Kraepelin.) a, Basiconic sensilla; b, trichodeal sensilla, or tactile hair; c, coeloconic sensilla ; d, ampullaceous sensilla ; f, flask-shaped sensilla ; g and Iti, openings of same on surface of antenna; i, gland cells; k, chitinous integument.

not occur on the scape and first funicular joint of the antennae, but only on the remaining joints and especially on the enlarged terminal joint, which possesses by far the greatest number of all the various sensillae. The following is a very brief description of these extra­ordinary structures:

(a) Clubs of Foyel (now called basiconic sensillae by Berlese)

­These resemble the tactile hairs, but are conical and immovable at the base and their chltlllotls investment is exceedingly thin (Fig. 32, a).

What corresponds to the cavity of the hair contains a dense bundle of delicate protoplasmic threads, which are prolongations from as many large elliptical cells situated in the hypodermis. These cells form a compact mass, formerly supposed to be a ganglion, but now interpreted as a cluster of unicellular glands that secrete a liquid through the thin chitinous cap of the organ onto the surface of the antennae.

It is, indeed, difficult to conceive such sensillae as having an olfactory function unless their exposed surfaces are moist like the olfactory organs in the mucous membranes of vertebrates. The nerve termina­tion to the basiconic sensillae applies itself to the cluster of gland cells and then breaks up into delicate branches that pass around and between the latter and up into the conical portion of the organ.

(b) Clubs Lying in Elliptical Pits ( coeloconic sensillae of Berlese )

These may be derived from the preceding type by supposing that the conical hair has come to lie horizontally and to be' enclosed in an elon­gated cavity in the chitinous integument (Fig. 32,c). The cellular structure of the organ is essentially the same as that of the basiconic sensillae.

(c) Champagne-cork Organs of Forel (ampullaceous sensillae of Berlese)

These evidently represent a further modification of the ceeloconic type, on the supposition that the hair becomes smaller and more erect and the pit in which it is enclosed becomes circular, much deeper and opens on the surface of the body by means of a small pore (Fig. 32,d).

(d) Flask-shaped Organs of Lubbock and Forel.

Hicks (1859) was the first to describe these extraordinary organs in Myrnnica, but Forel and Kraepelin have given a more detailed account of their struc­ture. They are really an extreme form of the ampullaceotis sensilla, and may be derived from this by supposing that the chitinous ampulla has become enormously lengthened and attenuated till it forms a narrow sac enclosing the conical hair and connected with the pore in the integu­ment by means of a slender tube running more or less parallel with the surface of the antenna (Fig. 32, g, .h ) . That these sensillae have devel­oped from those of the preceding type (c) is shown by the existence of transitional forms both in ants and in other Hymenoptera. The cellular portions of all these forms of ampullaceous sensillae are essen­tially the same as those of types a and b.

The gustatory sensillae, situated on the mouth-parts and including those in the terminal joints of the palpi, though resembling the anten­nary sensillae, in general reproduce only the more primitive of the above-lletitioned types, that is, those most like the typical tactile and basiconic sensillae. The more specialized ampullaceous types are found only in the antennae.