Art. XIX.—Notes on an Insect found in some Hot Springs at Taupo.
[Read before the Wellington Philosophical Society, 2nd September, 1903.]
About a mile from Lake Taupo, close to the Taupo-Napier Road, are the well-known hot springs locally named the “Black Terraces,” from the terrace-formations of dark colour formed by the principal springs.
The terraces are of the usual kind. There is a pool of water confined by a rim of siliceous deposit. The hot water flows over this rim at the lowest part and falls into another basin below. There are perhaps a dozen of these small
cascades from the source of the largest springs to the bottom where its waters flow into the creek. As the water spreads out in several directions after issuing from the earth there are different series of these basins, and altogether they would cover about a quarter of an acre. Where there is water on the rims it is quite shallow, but a considerable part of most of them is exposed. A few inches from the edge the water deepens to 6 in. or 8 in. Covering the bottoms of these pools or basins, which vary in area from less than a square foot to several square yards, there is a slimy vegetable growth of dark-green colour of varying thickness.
While on a visit to these springs last March I noticed great numbers of small flies hovering about the pools or on the silica at their edges. They were really in millions, some of the dry patches of silica being covered by them to a depth of an inch or more. Their size was about half that of the common house-fly. In shape they were something similar, but more slender and graceful, and of a dark colour, almost black.
Their power of flight was feeble. I asked a resident of the locality if they appeared in the autumn only, but he assured me that they were there all the year, and that in the spring-time numbers of native birds gathered about the springs to feed upon them.
It occurred to me that they might breed in the hot water, and so I examined the organic matter at the bottom of the basins. It was alive with larvæ about one-half as large as that of the blow-fly, of a dark greyish-green colour, shaped curiously like a shark with its double-lobed tail. In still pools this slime could be seen all in motion, so numerous were these larvæ. The green matter was evidently their food, and supported them generously, judging by their numbers. On the surface of the water a great many pupa-shells were floating. These shells drifted to the lips of the basins, and in a great many places, becoming entangled, they formed small obstructions across the little depressions through which the water flowed. As silication proceeds rapidly in most of these springs, it is obvious that these rafts of shells, being cemented together and to the basin-edges by the silica, will in time raise the level of the water, compelling it to find an exit elsewhere, and this in turn will get blocked too. This may afford an explanation of the remarkably level nature of the walls of these small reservoirs. I broke off a piece of the top of one of these dams, and the outlines of numerous shells imbedded in the silica were clearly visible.
I regret very much that my visit. which was limited to about an hour, prevented me from making further observations. Some specimens of the fly were sent by Mr. Hudson
to Captain Hutton, who said it was an undescribed species of Opomyza.
Some very interesting questions are presented for solution by this little creature: Has it any enemies in the hot water where it develops? If not, it is not surprising that in its mature form it appears in such numbers all the year round. Has it by long residence in the hot water changed its nature, so that it no longer breeds at certain times of the year? Where food is abundant, and the temperature of the water is practically the same the year through, there would be no necessity for a breeding season. If the nature and habits of the insect have been much changed from those of its nearest relatives, these facts may indicate a longer period of existence for our thermal springs than they are generally credited with.
The statement as to the gathering of the birds might also be investigated. There is no natural bush of any extent within miles of these springs, and the congregation at a particular time of the year of many native birds may be due to a survival of an ancestral habit, as some avian migrations are known to be.
It is a curious thing that the cast-off shells should materially assist in building these natural incubators, as I am certain they do. Complete sections of different basin-walls should be made in order to ascertain if this process has been in operation since the beginning of the terrace-formation.
These questions are deserving of further study by a competent scientist.