1921.] Tillyard.—Neuropteroid Insects as Trout-food. 275 
being those of Procordulia smithii, P. grayi, and Xanthocnemis zelandica. 
No doubt, later on in the season, the trout would also feed upon the 
imagines, as in Tasmania. 
In certain parts of New Zealand I found dragon-fly larvae to be the 
principal food of the trout. In the Hot Springs region the trout seem to 
have considerably diminished the number of larvae, and they do not form 
so important an article of diet, perhaps because they are more protected 
by their peculiar coloration and habits than are some other aquatic insect 
larvae, such as those of may-flies. It seems clear that their numbers have 
been much decreased since the trout were freed in these lakes and rivers, 
although I am unable to estimate this reduction as clearly as in the cases 
of stone-flies and may-flies. 
The Caddis-flies. 
Observations in other parts of the world, as well as in other parts of 
New Zealand, show that caddis-fly larvae form one of the most important 
-articles of diet for the trout. Most of these larvae construct cases for 
themselves out of weeds, sticks, sand, or small pebbles ; and one would 
imagine that such habits as these would serve as efficient protection for 
them. But this is not the case. The trout know well the habits of the 
caddis larvae. They watch carefully for any suspicious movement amongst 
the weeds, sticks, &c., that strew the bottoms of the lakes and streams, 
and they pounce upon the larvae and swallow them whole in their cases. 
The substance of the case is usually indigestible ; but the larvae itself is 
.a succulent, fat morsel, and an excellent food for the fish. Those most 
sought after are the elongated, more or less cylindrical cases of the 
Leptoceridae and Sericostomatidae, the former usually made from weeds, 
pieces of leaves or sticks, the latter from grains of sand or a thin trans¬ 
parent substance secreted by the larva itself. In the Leptoceridae, the 
genera Notanatolica, Triplectides , and Oecetis are abundant throughout 
New Zealand ; in the Sericostomatidae the same is true of Olinga and 
Pycnocentria. Throughout the Hot Springs region the trout have most 
seriously diminished the number of these' and other caddis-flies. Only 
two species now remain at all common—viz., Oecetis unicolor, whose larva 
is still common, feeding in the green Niiella beds in the lakes, and Hydro- 
psyche colonica, whose larvae form fixed houses of small pebbles attached 
to rocks. This latter species still exists in great numbers in such places 
as the Okere Rapids, where the rush of water is too swift for the trout to 
search for it. Its comparative absence in other parts is strong evidence 
of the reduction" of the caddis fauna, due to the trout. 
The most striking instance of the almost complete loss of the original 
rich caddis fauna is afforded by the condition of Te Wairoa Stream, 
flowing into Lake Tarawera. Except in the rough water below the falls, 
where no trout exist, it is almost impossible to obtain any caddises in the 
.stream. But a short distance off there is a much smaller stream, rising 
from a hill near the lake. This stream has been dammed off by boards, 
and the water drawn off close to its exit into the lake by a force-pump. 
No trout pass up this small stream. On examining it I found that it was 
swarming with caddis larvae under every stone and stick, and upon the 
gravelly bed and sides of the stream these little creatures were most 
abundant. Yet a day’s search in Te Wairoa Stream yielded far less than 
I was able to pick out in the course of ten minutes in this tiny stream a 
mile away from it. 
