[Illustration: Fig. 40.--Antennary structures of hymenoptera. (After Lubbock.) a., cuticle; b., hypodermis; c., ordinary hair; d., tactile hair; e., cone; f., depressed hair lying over g. cup with rudimentary hair at the base; h., simple cup; i., champagne-cork-like organ of Forel; k., flask-like organ; l., papilla, with a rudimentary hair at the apex.] Sir John Lubbock has an interesting chapter on problematical organs of sense. In the antennæ of ants and bees there are modified hairs and pits in the integument (at least eight different types, according to Sir John Lubbock), the sensory nature of which is undoubted. But what the sensory nature in each case may be is more or less problematical. Many worms have sense-hairs or bristles of the use of which we are ignorant. Some organs described as tactile or olfactory in the lower invertebrates are so described on a somewhat slender basis of evidence. The sense-value of the bright marginal beads of sea-anemones is unknown. Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs--the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate; by others, as of importance for the equilibration or balancing of the fish. It will thus be seen that, apart from the possibility of unknown receptive organs as completely hidden from anatomical and microscopic scrutiny as the end-organs of our temperature-sense, there are in the lower animals organs which may be fitted to receive modes of influence to which we human folk are not attuned. And what are the physical possibilities? We have seen that, through the telæsthetic senses--hearing, vision, and the temperature-sense--we are made aware of the vibrations of distant bodies, the effects of which are borne to us on waves of air or of æther. The limits of hearing with us are between thirty and about forty thousand (or perhaps, in very rare cases, fifty thousand) vibrations per second. But these are by no means the limits of vibrations of the same class. By experiments with sensitive flames,[FN] Lord Rayleigh has detected vibrations of fifty-six thousand per second; and Mr. W. F. Barrett has shown that a sensitive flame two feet long is sensitive to vibrations beyond the limit of his own hearing and that of several of his friends who were present at the experiment. We have some reason to suppose that vibrations too rapid to be audible by man are audible by insects, but not much is known with regard to the exact limits. The following table shows what is known concerning the æther-vibrations. The figures are those given by Professor Langley:--