In the lobster or crayfish the auditory organs are found at the base of the smaller feelers or antennules. They are little sacs formed by an infolding of the external integument (see Fig. 26, p. 259). Beautifully feathered auditory hairs project into the sac along specialized ridges, and the sac in many cases contains grains of sand which play the part of otoliths. Hensen seems to have proved that shrimps collect the grains of sand and place them in the auditory sac for this purpose. The curious shrimp-like _Mysis_ has two beautiful auditory sacs in its tail. These are provided with auditory hairs. Hensen watched these under the microscope while a musical scale was sounded, and found that the special hairs responded each to a certain note. When this particular note was sounded the hair was thrown into such violent vibration as to become invisible, but by other notes it was unaffected. [Illustration: Fig. 28.--Tail of _Mysis_. au., auditory organ.] [Illustration: Fig. 29.--Leg of grasshopper. ty., tympanic membrane.] Passing now to insects, we may first note that grasshoppers and crickets have an auditory organ on the front leg. These are provided with tympanic membranes, and the breathing-tubes, or tracheæ, are so arranged that the pressure of the air is equalized on the two sides of the membrane--just as in us and other vertebrates the same end is effected by a tube which runs from the interior of the drum of the ear to the mouth-cavity (see Fig. 27). In the organ within the leg there is a group of cells, followed by a row of similar cells which diminish regularly in size from above downwards. Each is in connection with a nerve-fibril, and contains a delicate auditory rod. It has been suggested that the diminution in size of the cells may have reference to the appreciation of different notes, but nothing definite is known on the matter. Ants, too, have an auditory organ, as shown by Sir John Lubbock, in the tibia of the front leg. But in locusts it is situated on the first segment of the abdomen. In flies there are a number of vesicles, generally regarded as auditory (but by some as olfactory), at the base of the rudimentary hind wings--the so-called halteres, or balancers.