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Nerve Endings and Tracheoles
The luminescent organ is supplied with nerves passing towards or originating in the 7th abdominal segment. Nerves from this ganglion also are connected to the hind genital region (adult) and chordotonal organs (larval). The malpighian tubes emerge from the gut at the upper third of the abdominal segments, and it is curious that nerves from the penultimate segment should innervate the distal ends of the malpighian tubules which constitute the light ogran.
Perusal of some of the relevant literature on insect nerve endings did not help the writer in regard to the possible relation of nerve fibres and luminescent cells in this larva. The material used had been fixed in Gilson or formalin, and the finer ramifications of the fibres could best be seen under phase contrast. The apparent coagulum (R) in the reflector is due to the use of phase contrast. By normal lighting there is seen to be a true cavity at (R).
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On Pl. 42, figs. 14 and 15, are two phase contrast photomicrographs of the light organ region to show a supposed nerve ending and probable tracheoles. The difficulty is to distinguish between connective tissue, the smaller nerve fibres, and the tracheoles. For example, in Fig. 14, from an adult male, it is certain that the fibre (AN) is a nerve, because it has been followed back as it thickens to join the ganglion in the seventh segment. But the smaller fibres (TX) which appear only possibly to originate from the nerve (AN) could be, and probably are, tracheoles. In fig. 15, from an adult female, the nerve (AN) can be followed branching to give rise to the object (NE) which has been tentatively interpreted as a nerve ending. Likewise, the spindle shaped body to the left of the numeral 15 is also traceable to the nerve (AN). In the same photograph the tubes (T) are interpreted as tracheoles. From these photographs it can be concluded that fibres (AN) from the seventh ganglion, pass down and branch in close contact with the cells of the light organ, but the nature of the nerve endings is still in doubt.
Text-fig. 1. — Plan of association of reservoir cavity (R), tracheoles (T), and supposed nerve endings (NE), with luminescent cell (LO). On the right the projections of the cell membrane (CL) into the cell (LO) are closed, on the left open (OP). (MIT), mitochondria; (AN), nerve fibre. The relative sizes of the diverticula from the cell membrane into the cell, and the other parts are considerably out of proportion. This hypothesis of light control is based on electron micrographs taken at the Dominion Physical Laboratory by Mr. W. Bertaud.
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In Text-fig. 1 is a diagrammatic interpretation of the relations of nerves and tracheoles to the light organ cells. From electron microscopy, it is known that the cell membrane has deeply penetrating tubes and infolds, which, in larvae fixed during the day appear closed, as on the right of Fig. 1 at (CL), but have been drawn open as at (OP) on the left. It is suggested that the larva controls luminescence by cutting off the oxygen supply by closing the apertures (OP) under stimulus from the nerve ending (NE). It is possible that this might be tested by killing larvae at night whilst they are luminescent. Since, however, these infoldings of the cell membrane cannot be resolved by optical microscopy, this is a future task for electron microscopists.