The delicacy of the sense of taste in man has been the subject of investigation by Messrs. E. H. S. Bailey and E. L. Nichols.[EP] They give the following table:—
| I. | Quinine— | ||
| Male observers detected 1 part in | 390,000 | parts of water. | |
| Female""1" | 456,000 | "" | |
| II. | Cane-sugar— | ||
| Male observers"1" | 199 | "" | |
| Female""1" | 204 | "" | |
| III. | Sulphuric acid— | ||
| Male observers"1" | 2,080 | "" | |
| Female""1" | 3,280 | "" | |
| IV. | Bicarbonate of sodium— | ||
| Male observers"1" | 98 | "" | |
| Female""1" | 126 | "" | |
| V. | Common salt— | ||
| Male observers"1" | 2,240 | "" | |
| Female""1" | 1,980 | "" | |
The above figures represent means or averages of a great number of individuals. There was very considerable variation for some tastes. In the case of the bitter of quinine, the maximum delicacy was the detection of 1 part in 5,120,000 parts of water; the minimum 1 part in 456,000 parts of water. Except in the case of salt, the sense was more delicate in women than in men. It is not stated whether the men tested were smokers.
It does not seem necessary to say anything concerning the sense of taste in the lower mammalia.
In birds and reptiles the sense of taste does not appear to be highly developed. Parrots are, perhaps, better off in this respect than the majority of their class; and the ducks have special organs on the edges of the beak, which seem to minister to this sense. A python at the Zoological Gardens, partially blind owing to a change of skin, is said to have struck at an animal, but to have only succeeded in capturing its blanket. This, however, it constricted, and proceeded to swallow with abundant satisfaction.
It may here be mentioned that the scales and skin of many fishes are provided with sense-organs which very closely resemble the taste-buds of higher animals. They occur in the head and along the "lateral line" which runs down the side of the fish, and may be readily seen, for example, in the cod. Mr. Bateson's[EQ] careful observations at Plymouth gave, however, no indication of the possession of an olfactory or gustatory function, and their place in the sensory economy of the fish remains problematical. In or near the mouth similar end-organs are found to be somewhat variously developed in different fishes—on the palate and lips, on the gill-bars, more rarely on the tongue, and on the barbels of the rockling and the pout. How far any or all of these have a gustatory function remains to be proved.
Anglers and fishermen, however, from their everyday experience, and naturalists from special observations, do not doubt that fishes have a sense of taste. Professor Herdman's recent experiments on feeding fishes with nudibranchs[ER] (naked molluscs) seem to show, for example, that the fishes concerned, including shannies, flat-fish, cod, rockling, and others, have a sense of taste leading them to reject these molluscs as nasty. They show, too, that some of the nudibranchs (Doris, Ancula, Eolis) are protected by warning coloration.
Our knowledge of the sense of taste among the lower (invertebrate) animals is imperfect, and is largely based rather on observation of their habits than on the evidence of anatomical structure. Here, again, comes in the difficulty of distinguishing between taste and smell. But even if the caterpillars which refuse to eat all but one or two special herbs, or the races of bloodsuckers which seem to have individual and special tastes, are guided in part by an olfactory sense, there is much evidence which seems to admit of no alternative explanation. Moisten, for example, the antennæ of a cockroach with a solution of Epsom salts or quinine, and watch him suck it off; or repeat F. Will's experiments on bees, tempting them with sugar, and then perfidiously substituting pounded alum. The way in which these little insects splutter and spit suggests that, whatever may be the psychological effect, the physiological effect is analogous to that produced in us by an exceedingly nasty taste. Here smell would seem to be excluded. Forel, moreover, mixed strychnine with honey, and offered it to his ants. The smell of the honey attracted them, but when they began to feed, the effect of the taste was at once evident.
The organs of taste in insects are probably certain minute pits, in each of which is a delicate taste-hair, which, in some cases, is perforated at the free end. They occur in the maxillæ and tongue in ants and bees, and on the proboscis of the fly.
In many of the invertebrates, the crayfish and the earthworm, for example—to take two instances from very different groups—observation seems to show that a sense of taste is developed, for they have marked and decided food-preferences. Nevertheless, the existence of special organs for this purpose has not been definitely proved.