BY R. J. TILLY ARD. 207 



dant in most parts of New Zealand, it appears to have been exterminated by the 

 trout in most parts of the Hot Springs Keaion, thouali it is still fairly al)undant 

 on the Tongariro Biver and tributaries. 



The other four Orders, Stone-flies, May-flies, Dragon-flies and L'adilis flies, 

 are of the gi-eatest importance as trout-food, and we liave to consider them in 

 somewhat greater detail. 



The Stone-flies. 



These insects confine themselves to running watei-, preferring rocky streams, 

 with a fairly fast current. Their larvae live on rocks and stones, where they 

 wander freely in search of food. The perfect insects, or imagines, are sluggish, 

 and seldom fly, preferring to sit about on the vegetation overhanging the streams. 

 Nevertheless, they frequently fall into the water, and are eagerly seized by the 

 trout. Both larvae and imagines are soft-bodied insects with a plentiful supply of 

 fat, and form an ideal and easily digested food for trout. 



Except along the Tongariro River and its tributaries, where the Stone-By 

 fauna is still abundant, these insects are very rare in the Hot Springs Region ; 

 and it is very clear that they have been greatly I'educed in number by the trout. 



The large green Stone-fly, Stenoperla prasiiia, generally abundant tlirough- 

 out New Zealand, and one of the best of trout-foods, was only to be found in 

 streams above high waterfalls, where trout were absent. It has been almost 

 completely annihilated in the Hot Springs Region. 



The Black Stopc-fly, Austrnperhi ci/re>ie, the most abundant Stone-lly through- 

 out New Zealand, has likewise been almost eliminated by the trout. A colony of 

 larvae was discovered at the extreme head of one of the small streams at Hamu- 

 rana. There are also a few larvae left here and there in the Tongariro River. 



Of the slender Stone-flies (family Leptoperlidae) a number were found on 

 the Tongariro River and its tributaries. Some of these are new to science, and 

 will be described later. But, generally speaking, it may be said that these valu- 

 able flies, like the rest of the Perlaria. are on the verge of extinction in the Hot 

 Springs Region. 



It would, I think, be well within the mark to estimate that more than 80 % 

 of the original Stone-fly fauna of the Region has already been destroyed by the 

 trout. 



The May-flies. 



These insects inhabit both streams and lakes. In all parts of the world they 

 form one of the most important articles of diet for the trout. Both the larvae 

 and the imagines are soft-bodied and easily digested. In the Northern Hemi- 

 sphere, where the evolution of the May-flies has taken place alongside that of the 

 native trout, the larvae have resorted to many cunning devices in order to escape 

 their rapacious enemies. Those of the larger species mostly burrow into the 

 banks of streams, and thus secure immunity from attack; while the smaller forms 

 hide under rocks and stones, and evade attack by their quick running powers. 

 Thus the trout only secure the May-flies in their winged stages (subiniago and 

 imago ) . 



In New Zealand, the native May-fly fauna has been evolved without exposure 

 to the attacks of rapacious fish such as the trout. There is only one large species 

 whose larva burrows into the river-banks, viz., Ichthybotus hudsoni; and it is 

 clear, for many reasons, that this May-fly did not evolve this habit in New Zea- 

 land, but migrated thither long after the adoption of it. All the rest of the 



