The Sensory Setae of the Chordotonal Organs

The setae have remained intact in only one of twenty-four whole stained mounts of the N.Z. larva. The photograph in Pl. 42, fig. 16S, of the curved seta, was made from a N.Z. larva which had been kept in alcohol. In fact, these setae are present in Dr. Evans' Tasmanian larvae kept in alcohol for nearly a quarter of a century. The setae apparently will become detached during the late stages of the staining and mounting routine, and must be examined in the fresh, or with flattened formalin and alcohol specimens. Owing to the lack of sufficient larvae of different ages of the three types, it is not possible to say for certain whether the Australian larva has longer setae than the N.Z. form, but it is believed to be so.

The position of the short (Y) and longer (X) curved setae are shown in Pl. 40, fig. 8A. The longer seta curves upwards in this specimen, but in the opposite direction in Pl. 42, fig. 16, where the palp has been photographed ventral surface upwards.

In all dead specimens, no matter how fixed, these papillae slope downwards as shown in the profile drawing of a Tasmanian lava in Pl. 40, figs. 8 and 8A. In other words, if the larva normally rests ventral side upwards, the papillae do not touch the runway, but are protruded upwards into the mucus in which the animal lies. The special advantage of this position is not known, but it may be suggested that the larva could notice disturbances in the snare over a wider area in a liquid medium dispersed over the runway. The runway is always wetted with mucus, so presumably this medium which extends out to the upper ends of the vertical lines acts as the carrier of vibrations.