Position of the Larva on the Snare
In a previous paper (Gatenby, 1959), it was stated that a captive larva reposed on its horizontal runway ventral surface upwards. This was ascertained by examining with a binocular dissecting microscope the position of the head, which bends down somewhat during life. The position occupied by wild larvae in the field is not known for certain. The reflector is ventral to the light organs, and the latter lie in a hollow deep saucer-shaped shell made by the reflector. This reflector is not transparent, and can be seen as a somewhat opaque white body in the living larva. It does not seem that light could shine through this maze of tracheoles, and for the larva to make the best of the light coming from the luminescent cells, the position necessary would be ventral surface upwards. By lying in this position, light could be reflected down on the beads of mucus, strung on the vertical fishing lines; thus increasing the area of luminescence. It was believed by Norris (see Hudson, 1950), that the larva really lived in a tube (of mucus). It is true that in the newly made snare of captive larvae the latter are covered by mucus, which could be regarded as a tube (see Gatenby, 1959), but it is not known whether larvae in the field are completely covered with a mucus layer, or whether they lie free on the runway. The writer did not manage to ascertain the facts in this matter, but from the behaviour of the larvae watched with the naked eye, it appears unlikely that the finished snare has a tubular runway with strong walls. It seems certain that the silk threads of the runway do not form a tube, but the runway is always damp with mucus, and if there were really a silk tube as exists in some spiders' snares, the larva would not be free to bend over anywhere along the runway as it certainly can, and reach down to recover prey. The conclusion is that during the night the larva assumes the most favourable position to reflect its light down on to the beads of mucus, and to do this it must have its ventral surface upwards or sideways. There are, however, other points. Presumably to hold on to the runway, it must turn partly or wholly ventral surface down, because the hooked bands lie mainly on the ventral surface.
That the larva normally lives in a mucus bed ventral surface upwards has, therefore, been concluded for the following reasons: (1) In this position the light can be directed downwards on the the snare, since the reflector is ventral. (2) The hooklets on the ventral surface are directed forwards and would tend to interfere with free movements of the larva except backwards. (3) In cases where a binocular microscope could be used on captive larvae lying on the newly formed snare, the position was ventral surface upwards. (4) Some N.S.W. larvae examined alive appeared to move freely under a loose coverslip either dorsal or ventral surface upwards. (5) The dorsal surface of the larva is flattened posteriorly so as to bend into the horizontal runway.