The following is an account of the preliminary study of bionomics of Stenoperla prasina. Thus far investigations have been confined to the nymph, as adults are difficult to obtain alive; the former, however, is both the more interesting and the more important on account of the high specialisation attained in correlation with the aquatic habitat.
Swiftly flowing streams with a high percentage of oxygen in solution are preferred by the nymphs. Field collections made in Canterbury south of Mount Grey, Otago, and Southland have shown that the nymph inhabits the turbulent mountain streams which feed the tributaries of the large rivers, occurring in small numbers along the courses of the latter. In Otago and Southland collections were made in a number of the large rivers along their lower reaches, but no Stenoperla was obtained; indeed, these regions are relatively deficient in insect life.
They occur both in the small side branches and in the main courses of these mountain streams, being most plentiful in the former where there are plenty of rock fragments, beneath which the nymphs secrete themselves. Preliminary experiments were made to determine the velocity of such streams at different points, to discover those conditions which prevailed in the region inhabited by the nymphs. Difficulty was experienced at this stage in finding a piece of apparatus which would give an accurate measurement of spot velocities, and to eliminate the excess of air which is continually present in the more turbulent torrents. Finally, the following pitot tube was constructed and found to work efficiently.
It consists of two brass or copper tubes 17 inches long, the one one-eighth of an inch in diameter and the other one-sixteenth of an inch in diameter. (Pl. 32, Figs. 1 and 2.) The wider tube is cut off at an angle, which is determined when the instrument is calibrated; the other is bent at right angles so that its opening-projects in the same direction as the former (Pl. 32, Fig. 2). These two tubes are enclosed in an outer one which serves merely for protection; the distal portions have two wider tubes sweated on to serve for the insertion of rubber tubing which connects this portion of the apparatus with the head piece. This consists of two glass tubes one-quarter of an inch in diameter and 24 inches in length (Pl. 32, Fig. 1) enclosed in a brass case 1 inch in diameter, having an opening through which readings may be made. To one end of these glass tubes a mouth-piece is connected by means of a Y-piece. Three cocks, one to each limb and the other to the mouth-piece, were used, the reason for the three being given later.
To bring the instrument into operation, the basal portion is placed so that the openings of the tubes face upstream and is held in position by means of a clamp. With the stop-cocks open, water is sucked up until each glass tube and the mouth-piece are filled with water:
the cocks on each arm are then closed to hold the limbs in this condition whilst the mouth-piece is closed; then the former are opened. The whole instrument should be filled with water and quite free from air. The cock on the mouth-piece is then cautiously opened until the water runs back in each arm to about half the length of the gauge and is then shut. If now the head oscillates up and down so that readings are difficult, it can be rendered more steady by screwing up the cocks on each arm. The head will then take longer to reach a constant, but when this is attained it remains quite steady, and can readily be measured with a rule. This oscillation of the head was the chief and most difficult factor to overcome, as in other types of pitot tube it oscillates violently.
The difference in height of the two columns is the head (h), and from the following formula:—
v = k√2 gh
where k = constant determined on calibration
g = acceleration due to gravity
h = head the velocity can be determined at any particular point at any given time.
From observations made in this way it was found that around a stone, the region of greatest velocity was before and on top, but the region of lowest was behind and beneath; intermediate velocities were found to the sides, depending on the position of the stone. In some cases, negative velocities were recorded behind stones, but in no case was a region of high velocity found in this position. Clemens (1917), in a more extensive study on Chirotenetes, obtained similar results.
Now the nymphs are invariably found beneath the boulders with their heads upstream, where, from the above observations, it is evident that this is the optimum position in turbulent streams. Correlated with the velocity is the dorso-ventral flattening of the body, which, according to Clemens, presents the least surface to the current, and the very efficient adaptations of the legs, to be described later. These structures enable the nymph to hold on and to run across the substratum from boulder to boulder.
The location of the nymphs varies with the type of stream; in the one case it is beneath decaying plant detritus in rills which are abundantly oxygenated, and in the other beneath stones in turbulent streams where all plant detritus may or may not have been swept from the interstices.
The nymphs feed chiefly on the nymphs of mayflies, Deleatidium, Coloburiscus, Ameletopsis, and Oniscigaster, but may also prey upon Blepharocerids, other small stoneflies, and even upon other less fortunate fellows of their own species. In one instance it is reported that the nymphs come out at night and actively hunt the substratum. They will not eat dead food, but must have living prey, which is killed and devoured on the spot.
Observations in the field revealed that three sizes occurred, and measurements on such material supported this. The smallest size on the average is about 1.5 cm. in length, the intermediate 2.4 cm.; this latter represents the size of the fully grown nymph prior to the appearance of wing-buds; the largest is 3.3 cm. long, this being the maximum size attained by the fully grown last nymphal instar. Since these three sizes are constantly found in the field, the life history is considered to extend over at least two years and probably extends to three.
The minimum temperature is below 0° C.; the optimum temperature ranges between 6° C. and 14° C., and the maximum temperature is 25° C., above which nymphs die very quickly.
The nymphs show decided body movements in relation to respiration. The body is moved up and down in relation to the substratum by means of the legs, the tarsi remaining fixed; by this means the gills are moved to and fro and in consequence are exposed to the maximum amount of water.
The nymphs actively run about by means of their legs, and if agitated while in still or relatively still water swim by means of lateral undulations of the body assisted at the same time by their legs, which are used as paddles by virtue of their possessing natatory hairs later described.
Reactions to Light.
(Pl. 43, Fig. 41.) The nymphs are decidedly negatively phototactic, and when placed in the apparatus in the lighted compartment rapidly make their way into the darkened one. The last nymphal instar is less markedly negatively phototactic and takes a much longer time to react. The imagines, likewise, are negatively phototactic.
The chief enemy of the nymphs is Archichauliodes dubitatus (Neuroptera), and trout which possibly also feed on the imago.
Thus far these number three. The first of these is a Gregarine found in the mesenteron (Plate 32, Fig. 3). It has no epimerite, but has a well-marked protomerite and deuteromerite. Large cysts of this parasite were taken from the hind-gut of the same specimen, a last nymphal instar.
Another unidentified parasite occurs in the mesenteron muscle layers of both nymph and adult, and has been observed in the fat body. It occurs in either muscle layer of the gut and forms a cavity in which it lies and increases in size.
Finally, an unidentified mite has been found on the nymph.
Habits of the Adult.
The adults are crepuscular and are never found very far distant from the streams. Such adults as have been taken were caught about the middle of the day beneath large boulders bordering the streams. Of these, four brachypterous males, all sexually mature, were captured associated with a normal winged female. According to Tillyard (1926) brachypterous males usually associated with normal females occur in other genera.
The adults are all negatively phototactic and immediately secrete themselves beneath stones when placed in the light. They have not been observed to feed, and evidence is given later to support the view that they do not.
A study was made of four particular regions, but as has already been mentioned, collections have been made from Mount Grey to Invercargill, these collections clearly showing that Stenoperla occurs to a very much less extent in the lower reaches of the largest rivers, and in many cases is entirely wanting. The four regions mentioned above may be divided into two groups according to the cycle of erosion, one of which may be further subdivided into two according to the origin of the rocks.
1. Early maturity textures of dissection fine.
(a) Sedimentary Greywackes. Mount Grey.
(b) Volcanic. Kaituna, Purau.
2. Young river valleys texture of dissection fine.
Cass-Arthur's Pass Region.
Mount Grey.—In the lower portion of this region the floor of the stream-bed is formed of water-worn pebbles covered with a profuse growth of algae and diatoms, and is inhabited by Archichauliodes, Caddis, Gobiomorphus (Pisces), and Hydropsyche. Here no Stenoperla was collected. Passing up towards the source, the stream gradually became more turbulent, the visible algae on the stones decreased, and finally disappeared altogether, whilst the banks were clothed with native bush. Here nymphs were taken associated with Deleatidium, Caddis, and Hydropsyche.
Kaituna.—The stream in this locality descends a valley of early mature dissection to discharge into Lake Ellesmere. It is turbulent in regions, with falls and pools, in the latter of which plant detritus has accumulated; the stones vary in size from 1 cm. to 1 metre in diameter and are all water-worn.
Association.—Hydropsyche sp. and other caddis; Deleatidium sp., Austrosimilium sp., Coloburiscus sp., Chironomids, Potamopergus sp., Helicopsyche sp., and Austroperla cyrene.
Most nymphs were collected in the turbulent portions beneath boulders, but a few were taken in the plant detritus.
Purau (Pl. 44, Fig. 43).—The Purau stream is a small water-course draining the inner western slopes of the caldera which forms Lyttelton Harbour, and arises high up on Mount Herbert (3014 feet). It runs in a north-westerly direction and discharges into the sea.
Grade.—Coarse sand to large boulders. In its lower reaches the stream bed drops very gently to sea level, and the flow is much reduced in speed.
Association.—Blepharocerids, newly hatched Deleatidium sp. (extremely numerous), Hydrobiosis sp., Olinga feredeyi, Archichauliodes sp., Coloburiscus sp., Chironomus sp., Potamopergus sp. Its banks are grassy and the stones in the stream are water-worn and have an algal and diatomaceous growth, and it is the first stream in which Stenoperla was collected associated with Archichauliodes, which is usually confined to the lower portions, whilst the former frequents the upper. It is evident that a certain amount of overlapping occurs, and that Stenoperla is only able to inhabit the lower regions in very small numbers, by virtue of its more active habits, the slower moving Archichauliodes nymphs readily devouring such of the former as come within reach of their enormous jaws.
Cass-Arthur's Pass District (Map).—This district comprises the upper watershed of the Waimakariri River and its tributaries, many of the latter of which form large rivers in themselves. The region is situated far back in the Southern Alps and has an average annual rainfall of 50 inches. Thus the tributaries and their branches are subject to severe flooding and rise with great rapidity. Many of the streams are spring-fed, hence their temperature remains constant; in others, however, where the stream is merely a small tricklé, the temperature may vary from warm conditions in summer to 2 to 3° C. in winter.
In this region in mid-winter the snowline has descended to somewhere about the 2000ft. level, and falls are frozen, while the ground is frozen to a depth of many inches and the stones in streams are covered with ice to a depth of 3 to 4 inches on their upper surfaces. The streams are all turbulent mountain torrents carving out valleys and gorges in folded and faulted greywacke and phyllite. The Cass region itself is a glaciated valley, the terminal moraine of which retains three lakes—Lakes Sarah, Grasmere, and Pearson—into the latter of which discharges Ribbonwood Creek, to be described more fully later.
Misery Creek (Pl. 44, Figs. 44 and 45).—It is a typical spring-fed mountain torrent draining a sub-alpine fell-field, and descends along part of its course by a long series of cataracts to discharge into a swamp; thence it drains into the Cass.
Association.—Zealandoperla sp., Deleatidium sp., Blepharocerids, Austroperla cyrene, Ameletus sp., Caddis, Austrosimulium sp.
Plant.—Very little algae, chiefly diatoms. Beech on banks (Nothofagus cliffortioides).
It is interesting to note that a certain portion of the stream changed its course twice, finally returning to its old bed. Its fauna, however, remained constant. Stenoperla occurs only in its upper reaches, at an altitude of 2,750ft.
Ribbonwood Creek (Pl. 43, Fig. 42).—This is also a typical mountain torrent draining a similar but smaller basin to the Cass River at the back of Mount Baldy, emptying into Lake Pearson at its northerly end. About two and a-half miles from Lake Pearson, at an altitude of 2,750ft., there is a very small side trickle about 50ft. long arising from a scree. Most work has been accomplished on this stream, since it is very thickly populated and very interesting from many aspects.
Grade.—Coarse sand to large boulders, plant detritus abundant.
|17–ii–32||8.5° C. (cold spell)|
Association.—(a) Animal: Deleatidium vernale, Ameletus sp., Tipula sp., Olinga feredeyi, Austroperla cyrene, Zealandobius sp., Chironomus sp., Austrosimulium sp., Amoeba sp., Paramoecium sp., and Rotifers.
(b) Plant: The stream could be divided into three regions according to the Cryptogams.
1. Upper, about 15ft. in length.
Staurastrum (2 spp.)
Navicula (3 spp.)
Melosira sp. (predominant)
2. Middle, about 20ft. in length.
3. Lower, about 25ft. in length. The cryptogam flora resembles the upper region.
Now the interesting fact about this is that the upper and lower regions are plentifully supplied with plant food, especially Melosira sp., and that this genus enters into and forms an important part in the food relations of the herring in the North Sea. An analysis of the animal fauna shows that Deleatidium sp. and Ameletus sp. were abundant in the upper and lower reaches, but were absent from the middle, whilst the large Tipula sp. was very plentiful in this latter. It is known that mayflies eat diatoms and an analysis of a Stenoperla nymph in the crop region revealed fragments of mayflies of the above two genera, together with an enormous amount of plant food, which consisted chiefly of Melosira sp. together with filamentous algae. Now, the stoneflies feed on the mayflies, which in turn depend on the algae for their food, and it has already been shown that the middle portion was deficient in diatoms, which would explain the absence of mayflies and stoneflies. On the other hand, over two hundred nymphs were taken out of the upper position where Melosira sp. was very abundant.