THE SENSE OF TASTE Among invertebrates, as we have already mentioned, a separate sense of taste has been demonstrated only in insects. In these creatures, we find spatially distinct olfactory and gustatory organs, which respond to different stimuli. Up to the present time, the insects which have been most studied are the honey-bee and the water beetle, Dytiscus (Schaller). The organs of taste are situ ated in the mouth cavity itself, as well as on the maxillary and labial palps, but not, however, on the olfactory antennae.
In the sense of taste, also, a remarkable agreement between that of man and that of insects is to be observed. The substances appreciated by the sense of taste are substantially the same in the two widely differing organisms. Sugar exercises a strong attrac tion. According to Minnich, the admiral butterfly (Pyrarneis atlanta) is about 256 times as sensitive to cane sugar as man. It is a striking fact that insects do not accept all the kinds of sugar which taste sweet to us. The honey-bee responds only to cane sugar, the analysis products of which are d-glucose and d-fructose, as well as maltose.
Other sweet substances such as mannose, galactose or man nite and glycocoll are declined. In the same way, the bee-keeper has long known that bees will not take saccharine. Salt, bitter and sour substances, as well as lyes, always repel them ; this is known to apply also to those animals which are without a separate sense of taste (Crustacea and gastropods).
It is of interest that the admiral butterfly, already mentioned several times, bears its organs of taste, not in the mouth, but on the tips of the feet (tarsi).
A marked reaction to tastes can be proved also in numerous animals which possess no separate gustatory sense. Bitter sub stances, in particular, are very generally declined (worms, gas tropods, ascidians and holothurians). It is characteristic that the threshold of stimulation, that is to say, the minimum concentra tion to which the animal responds, is considerably lower in in vertebrates than in fishes. Hanstroem connects this with the fact that the receivers of chemical stimuli in invertebrates are primary sensory cells, while those of vertebrates are of the type known as secondary sensory cells, in which the cell is in connection with the nerve only by contact.
The attraction exercised by sugar may be observed very fre quently. Gastropods, in particular, are very greedy for sugar. It can be proved, however, that in these animals the sense-organs by means of which they perceive it are not situated in the mouth itself, for when they are crawling on the surface of the water they will eat anything, impartially, which may be placed directly on the mouth. The gustatory cells are situated, apparently, on the anterior margin of the foot, which, in crawling, is the first part to come in contact with the substratum. Leeches, which have been studied by Loehner, also react very plainly to tastes. The experiments were so arranged that the animal first of all sucked blood which it found in a test-tube closed by an animal membrane. As soon as it had commenced to suck, the blood was replaced by some other liquid of the same degree of warmth. Pure water calls forth no reaction; if, however, too highly flavoured, concen trated substances are placed bef ore it, the animal immediately leaves off. In the case of common salt this happened first when it was present in the proportion of 7%; with cane sugar when present in the proportion of 5%, with quinine sulphate at 1%. Salts and sugar are naturally contained in blood, but in a much lower degree of concentration. (W. v. BUD.) SMELT (Osmerus eperlanus), the typical fish of the sal monoid family Osmeridae, which includes also the capelin (Mal lotus villosus) and the candlefish (Thaleichthys pacificus). Ex cept the curious Plecoglossus of the rivers of Japan, the Osmeridae are northern marine fishes, small and predacious. It breeds, unless land-locked, in salt or brackish water, and though it often enters rivers it does not ascend beyond tidal influence. Like other British salmonoids it spawns in winter. The smelt ranges from Scandinavia to the English Channel; it is a slender, silvery fish, with olive green back. It grows to a length of 13 inches.