c. The organs of taste
The gustatory organs of insects are microscopic pits or setae, either hair-like or resembling short pegs, which form the ends of ganglionated nerves. They are difficult to distinguish morphologically from certain olfactory structures, and it is owing to their position at or very near the mouth that they are supposed to be gustatory in nature.
Meinert was the first (1860) to suggest that organs of taste occurred in ants. He observed in the maxillæ, and tongue of these insects a series of canals in the cuticula of these organs connected with ganglion-cells, and through them with the nerves, and queried whether they were not organs of taste. Forel afterwards (1874) confirmed these observations. Wolff in an elaborate work (1875) described a group of minute pits (Fig. 284) at the base of the tongue of the honey bee, which he supposed to possess the sense of smell, but Forel and also Lubbock attributed to these sensory pits the function of taste. Ten years afterward Will showed conclusively, both by anatomical studies and by experiments, that Diptera and Hymenoptera possess gustatory organs. He, however, denied that the organs of Wolff were gustatory, and maintained that organs of smell were confined to the maxillæ, paraglossæ, and tongue. As we shall see, however, what appear to be with little doubt taste-pits, with hairs or pegs arising from them, are most numerous on the epipharynx of nearly all insects, and situated at a point where they necessarily must come in contact with the food as it enters the mouth and passes down the throat.
Fig. 284.—Taste-pits on the epipharynx (C) of the honey-bee: B, horny ridge; R, R, taste-pits; L, A, A, muscular fibres; S, S′, a b c d e f, section of skin of œsophagus.—After Wolff.
Fig. 285.—Tip of the proboscis of the honey bee, × 140: L, terminal button or ladle: Gs taste-hairs; Sh, guard-hairs; Hb, hooked hairs.—After Will.
Kraepelin (1883) discovered taste-organs on the proboscis of the fly, and taste-hairs at the end of the tongue of the humble-bee (Fig. 285), and afterwards Lubbock critically discussed the subject, and concluded that the organs of taste in insects are situated “either in the mouth itself, or on the organs immediately surrounding it.”
Structure of the taste-organs.—The organs have been best studied by Will, who, besides describing and figuring the chitinous structures, such as the pits or cups, hairs and the pegs, showed that they were the terminations of ganglionated nerves.
Figure 286 represents the taste-cups on the maxilla of a wasp, and Fig. 287 the taste-cone or peg projecting from the cup or pit. The cell out of which the pit and projecting hair or peg are formed is a modified hypodermis cell; and the seta is apparently a modification of a tactile hair, situated at the end of a nerve, which just beneath the chitinous structures passes into a ganglion-cell, which sends off a nerve-fibre to the main nerve.