Remarkable hydropsyche

Press, Volume CXII, Issue 33101, 16 December 1972, Page 11

Remarkable hydropsyche

(Contributed by the Canterbury Museum)

The net illustrated in a recent Museum of Nature article (December 2) belonged to the smallest and rarest of our hydropsyches (a genus of caddis fly). The illustration which accompanies this article is one of the South Island giants having a wing spread in fully winged females of a little more than an inch.

Its name. Hydropsyche tipua—the remarkable hydropsyche—has been given to it because of a number of remarkable features. First, where it inhabits torrential waters, such as Roaring Meg in the Kawarau Gorge or in the gorge section of the Ahuriri, the adults are flightless, the wings being mere vestiges. By contrast, when it inhabits calmer waters the wings are fully developed. Second, the almost wingless forms are able to run on the torrents like waterspiders, and to break through the surface and swim to the bottom. This is a remarkable accomplishment for so light an insect in such violent waters. Females of this

genus have the middle pair of legs modified as paddles to enable underwater swimming; they also have thigh muscles that any athlete might envy. Retractable gills Eggs are laid on the under sides of stones in a mass one layer deep. A considerable time is needed for this process. How then does the insect avoid drowning? The answer to this is also astonishing. Alone, among insects, as far as the writer can discover, the adults have retractable gills resembling those which they used when larvae. These they project and obtain oxygen from the water during their long immersion.

It is true that caddis flies of other families have similar egg-laying habits but these other kinds are equipped with hair arrangements for trapping air to form a bubble around the insect during the egg-laying process. These kinds walk into the water where the current is slow and weave their way along the bottom between the stones and find an egg-laying site and there get on with the business of laying their eggs surrounded by their own imported atmosphere.

The adaptation of vestigial wings in torrential water conditions is interesting. Trailing four hairy wings while swimming through turbulent currents would be like trying to swim fully clothed with a heavy overcoat into the bargain. The adults run across the surface like spiders and unless one knew otherwise they would be regarded as such. Yet in easier water the adults develop normal wings and strong flight. Built in crevice The net shown here had been built in a crevice between two large boulders. The webbing was fastened to the rock and the free edge reinforced by binding together scraps caught in the current. The boom is straplike and set at right angles to the plane of the net.

The mesh of this net is not so neat as that of Philpott’s Hydropsyche. The net is much bigger and more strongly constructed. Note the fine material spoiling the clean outline of the netting. This material will be the subject of a future article. In spite of its displeasing appearance it is of tremendous importance to the life of aquatic animals.—A. G. McF.