CRUSTACEA-INSECTS. 23 suspects to be ganglionic cells, and he suggests that the organ is one of taste. Even in the Crustacea (crabs, lobsters, etc.), though we can scarcely doubt that they possess the sense of taste, no organs have been yet described to which it can be with any confidence ascribed. Huxley, for instance, in his work on the Crayfish,* says, "It is probable that the crayfish possesses something analogous to taste, and a very likely seat for the organ of this function is in the upper lip and the metastoma, but if the organ exists it possesses no structural peculiarities by which it can be identified." As regards insects, the possession of the sense of taste cannot be questioned, though, except perhaps in many Hymenoptera and certain phytophagous insects, it may not be of great importance. No one who has ever watched a bee or a wasp can entertain the slightest doubt on the subject. It is, again, probably by taste that caterpillars recognize their food-plant. Moreover, this is partly the effect of individual experience, for, when first hatched, caterpillars will often eat leaves which they would not touch when they are older, and have become accustomed to a particular kind of food.† Special experiments, moreover, have been made by various entomologists, particularly by Forel and Will. Forel mixed morphine and strychnine with some honey, "The Crayfish: an Introduction to the Study of Zoology." † A remarkable case is afforded by those species in which the food of the larva and perfect insect is different, so that the mother has to select and find for her offspring food which she would not care to touch herself. Thus while butterflies and moths themselves feed on honey, each species selects some particular food-plant for the larvæ. Again, flies, which also enjoy honey themselves, lay their eggs on putrid meat and other decaying animal substances. 24 SENSE OF TASTE IN INSECTS. which he offered to his ants. Their antennæ gave them no warning. The smell of the honey attracted them, and they began to feed; but the moment the honey touched their lips, they perceived the fraud. Will tried wasps with alum, placing it where they had been accustomed to be fed with sugar. They fell into the trap, and ate some, but soon found out their error, and began assiduously rubbing their mouth parts to take away the taste. Will found that glycerine, even if mixed with a large proportion of honey, was avoided; and to quinine they had a great objection. If the distasteful substance is inodorous and mixed in honey, the ant or bee commences to feed unsuspiciously, and finds out the trick played on her more or less quickly according to the proportion of the substance and the bitterness or strength of its taste. The delicacy of taste is, doubtless, greater in bees and ants than in omnivorous flies or in carnivorous insects. At the same time, the sense of taste in ants is far from perfect, and they cannot always distinguish injurious substances. Forel found that if he mixed phosphorus in their honey, they swallowed it unsuspectingly, and were made very unwell. Some workers, he says, " de Formica pratensis se gorgèrent de miel au phosphore que je leur donnai. Après cela elles demeurèrent pendant de nombreuses heures immobiles, les mandibules écartées, la bouche ouverte, avec l'air très obsédées. Celles qui en avaient le plus mangé pérìrent, les autres guérirent peu à peu." It cannot, then, be doubted that insects possess a sense of taste, the seat of it can hardly be elsewhere than in the "Receuil Zool. Suisse." 1887. ORGANS OF TASTE IN INSECTS. 25 mouth or its immediate neighbourhood; and in all the orders of insects there are found on the tongue, the maxillæ, and in the mouth, certain minute pits which are probably the organs of taste. In each pit is a minute hair, or rod, which is probably perforated at the end. On this point there is, indeed, some difference of opinion. Will, for instance, maintains that to convey the sense of taste the food must come into direct contact with the termination of the nerve of taste, so that those hairs, or bristles, on the mouth parts which present no perforation cannot be regarded as true taste-organs, and probably serve rather as guards. Forel, on the contrary, considers this as an error. He observes, with justice, that the secretions are able to pass through the chitinous membrane which terminates the excretory canals of the glandular cells, and he maintains that the chitin is so thin and delicate-as well on the surface of the taste cones and hairs as on the olfactory hairs and plates of the antennæ of bees and other insects-that endosmosis through this fine membrane may sufficiently explain the sensation. * In 1860 Meinert described, on the maxillæ and tongue of ants, a series of chitinous canals, connected with ganglion cells, and through them with the nerves, and suggested-though with a note of interrogation-that they might be the organs of taste. Forel, in 1874, confirmed these observations of Meinert's, and described, at the point of the tongue of Formica pratensis, a series of seven such chitinous tubes. In the following year Wolff published his work, "Das Riechorgan der Biene," which contains a number of valuable observations, * "Bid. til. de Danske Myrers Natur Hist." 1860. though I am unable altogether to concur in his conclusions. He described a group of minute pits at the base of the tongue of the bee, and considered them as the organs of smell. It seems to me, however, more probable that they serve as organs of taste. Forel* also is disposed to regard these as constituting, perhaps, the most important part of the organ of taste, but considers that this sense resides also in certain organs scattered over the tongue and the maxillæ. Will regards the maxillæ and tongue as the only organs of Fig. 25.-Taste-organ of the bee (after Wolff). B, Horny ridge; R, R, sensory pits; C, C, skin of the mouth; L, muscular fibres; 4, A, muscular fibres; S, S', a b c def, section of skin of esophagus. taste in the bee. He maintains † that the organs of Wolff are deficient in the first requisite of an organ of taste, for that there is no orifice through which the food could directly enter into relation with the nerve. No doubt, moreover, the taste-organs on the tongue and maxillæ might be of themselves sufficient, so that à priori we need not seek for any others. At the same time, as to the existence of the "Sensations des Insectes," Receuil. Zool. Suisse, 1887. Kraepelin also regards them as the organ of smell. ↑ Will," Das Geschmacksorgan der Insekten," Zeit. für Zool., 1855. organs described by Wolff there is no doubt, and their position certainly seems to indicate that they are organs of taste. Moreover, we are not, I think, sufficiently acquainted either with the essential requisites of an organ of taste, on the one hand, or, on the other, with the minute structure of these organs, to feel justified in concluding that this is impossible. It must be remembered that these pits are very minute, being only from 003 to 006 of a millimeter in diameter, so that it is hazardous to assert that they are certainly imperforate, while even if they are, this would not necessarily prove that they cannot be organs of taste. Fig. 26 shows three of Wolff's cups, each with a central hair, a chitinous ring, and a double ganglionic swelling terminating in a nerve-fibre, magnified 500 times. Fig. 26. Shows three of 500 An additional reason for supposing that the Wolffian pits are really sense-organs arises from the fact that they are fewest in those insects which we may reasonably suppose to have the sense of taste least developed, and increase in number where, on other grounds, we may fairly regard it as being probably more highly developed. Thus the Chalcididae have often only one or two; the Evaneadæ, seven; the Proctotrupidæ, fifteen; the Tenth redos, twelve to twenty-four; the common wasp, twenty: some of the great tropical wasps, forty; while in the hive bee, the drone has fifty, the queen about one hundred, and the worker rather more still, say one hundred and ten. |