1931-1936
Cachoeira Bulhões, no município de Petrópolis (RJ), em 1869. Nas cachoeiras da parte serrana do Brasil, Lutz realizou pesquisas sobre simuliídeos e blefarocerídeos. óleo sobre tela de Nicolao Antonio Facchinetti. In. Martins, Carlos (Org.) Brasiliana: revelando um acervo. São Paulo: Bei Comunicação. 2000, p.51.
Cachoeira Bulhões, in the municipality of Petrópolis (RJ), in 1869. In the waterfalls of Brazilian mountains, Lutz researched on simuliidae e blepharoceiides. Oil on canvas by Nicolao Antonio Facchinetti. In. Martins, Carlos (Org.) Brasiliana: revelando um acervo.São Paulo: Bei Comunicação. 2000, p.51.
Biology of Torrential Waters and Rapids*
It is generally supposed that freshwater organisms descend from marine ones, after going through an intermediary stage in brackish water. This explanation wellfits fihes, worms, molluscs and coelenterates, but not insects, very poorly representedin the sea but very well in freshwater, where they live especially during their firstphases. Freshwater organims are more abundant in ditches, moors and still-waterlakes than in current water, which the frequency of rain increases. A moderatecurrent may be swum up by fishes and crustaceans, which, for their rest, look outfor more protected backwaters and hiding places and need not fix themselveseither by the locomotion of their legs or, sometimes, by suckers, which also servefor fixation during rest. Wherever the bottom is formed by earth, some phanerogamsof the genera Ramunculus and Potamogeton succed in fixing themselves by theirroots in very rapid but shallow waters, abandonning the stem and leaves to thewater movement just like fucoids in the sea. There are also many swamp plants,as reeds and rushes, which are observed there.
In torrential waters and rapids, rivulets and creeks with a gravel bottom, and in the very falls and rapids that flow over rocks and stone walls, conditions seem toprohibit permanently fixed organic life. There are some vegetable and animalspecies, notwithstanding, which, in part, have become so well adapted to thoseconditions that cannot be kept alive in still or weakly agitated water. The algae, aswell as the mosses, found in this situation, have not yet been sufficiently studied,but there is a sufficiently known family of phanerogams growing almost exclusivelyunder these special conditions, thus serving as support, if not as food, for animalforms that will be dealt with later. These are the Podostemonaceae, very wellrepresented in Brazil.
The mountainous part of Brazil is very rich in rapids, and during my studies on simuliids and blepharocerids (whose larvae can only live in very agitated waters) I had the occasion to become familiar with that fauna, principally formed by insect larvae: Trichoptera, Plecoptera and Diptera, and could study some questions which will be presented in the sequence.
One of the first impulses to my studies of the rhyacophilous (as the study of the fauna of torrential faunas may be called) was given by the works of Fritz Mueller,who has described larvae of Bleharoceridae and Trichoptera living under these conditions, as well as the larva of a psychodid under the name Maruina. He has also seen a coleopterous larva of the genus Psephenus. As regards botany, the family Podostemaceae was well studied by Tulasne and Warming, with Brazilian material. Other rhyacophilous larvae and nymphs, belonging to the genera Simulium and Blepharocera, have been studied, especially in North America, by Johannsen. I have made extensive observations about blepharocerids and simuliids, whose larvae live exclusively in torrential, or at least agitated, waters.
The first question that presents itself is to know how rhyacophilous organisms can keep themselves amidst strong and continuous torrents, in addition increased by frequent floods and showers. The Diatomaceae existing under those conditions may glue themselves to stones or other objects by means of their gelatinous peduncles. Other algae may also be glued to stones where water falls in small volume. The Podostemonaceae have adhesive disks and, oftentimes, the entire body of the plant forms a kind of stalk adherent to the stone and may be confounded with mosses. In other species the vegetable body is divided into so fine branches that they offer little resistance to the waters in which they float.
Animals that openly live in torrential waters maintain themselves through different means. Their body is generally small and frequently flattened. They may fix themselves to the substrate, either by means of silk or by the means of suckers. This last mode of adhesion is the most perfect and excludes locomotion. Their body is generally small and frequently flattened.
In those parts of torrents where the current is milder and the bottom muddy, larvae of tabanids, leptids and other Diptera may penetrate the latter and reach a relatively large size. Where there are larger stones, the more protected situations beneath them or the side not exposed to the current are sought after by larvae of Plecoptera, Neuroptera and Trichoptera. Plecopteran larvae (perlids) have a flattened, but very resistent body, and well-developed legs, with which they rapidly run over stones not exposed to currents. Trichopteran larvae usually live inside cocoons of the most varied forms, which they securely attach to stones by means of silk that resists strong currents. The fore part of the body, with well-formed legs, may emerge from the cocoon and drag it, when it is not attached to something. Some species spin funnel-shaped webs below the water, reminiscent of some spiders. Neuropterans of the family Sialidae have very large and strong larvae, living in running waters with loose gravel, under which they hide. Many running-water larvae are predatory of others, especially of those of Simuliidae, whose fixed nymphs constitute an easy prey. Dipterous larvae have no articulated legs, only false legs serving for locomotion. Among the Diptera appears a new and very efficient type of fixation, consisting of suckers adaptated not to avoid locomotion.
There are two families of Diptera whose larvae live exclusively in very turbulent water and even in the the most violent rapids and falls and which may tumble from great height. They are the simuliids and the blepharocerids. The latter took larval adaptation to the point of appearing fixed on a smooth stone in the midst of a current that takes them far way as soon as they are detached, in such a way that one must reccur to several expedients to collect them, as I have already described in a paper on the subject. These larvae are not only flattened, but their fixation apparatus consists of half a dozen suckers, placed on the ventral surface. This fixation is so efficacious that half the suckers suffice to resist the current, which allows a slow, bur perfectly efficient, lateral locomotion. They prefer very smooth stones to which they attach themselves. The cocoons are fixed and immobilized by total apposition of the ventral surface, preferably below prominences that somewhat enfeeble the current's strength, and there keep waiting until they become exposed, during dry weather, due to the lowering of the water. If the water is deviated, the larvae, up to now motionless, engage in lateral movements, searching for a more irrigated place. They generally favour rivers where the water layer is not so high, but during floods they may stay and live at greater depth. They fix themselves, preferentially, in slabs and stone-walls, closely tending to a vertical position, contrariwise to simuliid larvae, which, in the steps of waterfalls, seek more horizontal parts.
Simuliid larvae are not flattened, but have two means of fixation that permit locomotion. On the caudal end they have a sucker that is enough for definite fixation; on the thorax there is a false leg, furnished with another sucker, whose alternative action permits the larvae to walk as geometrid caterpillars, forming an arch or a loop with the body. Moreover, they possess the faculty of producing silken threads that permit them to attach the anterior part of the body and to let them be taken by the current till finding a suitable point for the fixation of the suckers. The silk also serves to construct a cocoon in the shape of paper funnel, fixed by the tip and open above, within which the larvae is transformed into pupa. They generally live in groups and preferably attached to vegetable substances – rushes, hanging branches of riparial plants, leathery leaves and dry branches stuck among stones forming little falls. A few species, among them Simulium pertinax, one of the worst pests of men, fix themselves on slabs, forming dense colonies united by silken threads. Two species choose Podostemonaceae of the Ligea-type to attach themselves to. One of these abounds in rock pools of the Pirapora waterfall. During rest, larvae are only fixed by the terminal sucker. The body is kept in continuous vibration, but maintains a near vertical position. In aquaria, where a water jet flows, they all fix themselves in the place of the strongest shock.
There are yet two species of larvae who fix themselves in a pneumatic way and have a very flattened body. Some belong to the psychodid genus Maruina and have suckers like blepharocerids. Others belong to a semi-aquatic Coleoptera of the genus Psephenus. In these, the entire body forms only one sucker, comprising the ventral surface with the three pairs of legs. The dorsal surface resembles an oval shield. These two kinds of larvae are preferently found on stones irrigated by a thin layer of water. Under these conditions are also found certain tadpoles of batrachians, which maintain themselves only through the adhesion of the ventral surface and know how to climb, especially using the tail muscles. They are adapted to this genre of life and are not found in still water.
Among freshwater molluscs, there are several species living in running water and that can even invade waterfalls. They fix themselves by the sole of their feet on rocks or plants, but do not show any phenomena of special adaptation.
The second question regarding the rhyacophilic fauna refers to the feeding of organisms. Some species are predatious and carnivorous, as many trichopterans, establishing great damages upon colonies of larvae and nymphs of simuliids, as well as the larvae of the neuropteran [now megalopteran] genus Corydalus. Others feed upon living or dead vegetable matter, found on the bottom or on the margins of the waters or upon crusts covering stones. Even on apparently clean slabs many Diatomaceae may occur, having even been observed covering larvae of Psephenus and blepharocerids. Intermingled with the Diatomaceae are also found Desmidiaceae and strings of green algae. Simulium larvae have fan-shaped brushes serving to facilitate feeding on the debris found even in pure waters, but especially in occasions of showers. In such occasions, the intestinal content of the blackflies' larvae, generally humus-coloured, may become red, if the water contains mud of that colour. The number of organisms filling their intestine, without choice, with any kind of debris, mud or earth only in part profitable, is enormous among invertebrates and includes also almost all larvae of batrachians, part of which belong to the rhyacophilous fauna and some may fix themselves to stones by means of the mouth or by adhesive organs.
Inquiring now about the processes of respiration, these must be adapted to water, even in species that, in their adult life, breathe ambient air through tracheae. In forms that live fixed in torrential waters, there is a development of soft and branched appendices corresponding to blood gills. In blepharocerids, there is one pair in each larval segment; in the simuliids, there is only a very large and branched appendix which may be withdrawn into the anal orifice. In the nymphs, forming a transitional stage towards the perfect insect, breathing organs are varied, but different from those of the larvae and adults. Breathing at the water surface is nonexistant in larvae.
Now arises the question: what are the advantages and disadvantages of life in torrential waters and the ensuing special adaptations? Water, under these conditions, is cooler, purer and more aerated, besides being more constant, and organisms accustomed and adaptated to those conditions barely stand their absence. They thrive perfectly well in very cold water, but do not suffer higher temperatures presented by still water under hot climates and seasons and the action of sunrays. They also avoid, more easily, complete dessication, to which they canot resist. Purity may be contested in times of floods, but when waters become turbid, this is not the result of contamination or putrefaction. Aeration is due to mechanical mixture with the air, which is more easily dissolved into cooler water. If we keep rhyacophilous larvae just wet, or under a thin cold layer of water, they will resist for a longer time than under some centimetres of still water. The mechanical pressure they suffer in torrential water is hard to appreciate, but seems to be favourable. However, the aeration and agitation produced by air bubbles rapidly succeeding each other in water of artificial cultures may substitute, at least for some less demanding species, the use of a continuous flow of water.
Life in torrential waters seems to me to protect the blepharocerids against internal parasites and external enemies. In simuliids larvae of Mermis and microsporidians in a low proportion of individuals have been observed. Worst enemies are certain trichopterous larvae, which invade their colonies.
Summing it up, life in torrential waters only became possible through special adaptations limited to very few groups. These, however, have already produced a regular number of species.
Considering that the fauna of very turbulent waters is mostly formed by aquatic larvae that must transform themselves into air-breathing winged adults, it is worthwhile to ask how the passage from one environment to the other is made. In Trichoptera and Diptera, sexual union is hard to observe, but undoubtedly takes place out of water. Existing observations, albeit very defficient, indicate several modes of larval penetration into the aquatic environment. Leptids of the genus Atherix [now family Athericidae] lay their eggs on leaves of riparious plants and the larvae let themselves fall into the water, where they sink. I have found many postures on leaves of Hedychium coronarium, most abundant on the margins of running waters. Certain tabanids perhaps behave in an analogous way. In torrents without vegetation, eggs are laid over dry or only wet stones in the middle or beside the water and not near its surface; during the first flood, however small, they become soaked. This occurs, most probably, with certain simuliids, with blepharocerids and with Maruina. It was observed, in a European simuliid, that the female may lay the eggs below the water, descending through the stem of a plant, but this is certainly not the general rule. Adults of Psephenus have been found by me gathered in groups, below the water, in backwaters of torrents, in such a way that they can copulate and lay their eggs below water.
The ecdysis of winged rhyacophilous with fixed pupae is made, as a rule, when these are exposed to the air, by occasion of a drought. Waiting for it, already adult or some months-old simuliids and blepharocerids with hardened teguments can wait as nymphs and pupae for months, which explains why in certain seasons they are not seen free-living, whereas in other occasions they appear in huge numbers. Ecdysis is rapidly processed, sometimes, but only exceptionally; simuliids may emerge still below the water, which does not wet them. They do not show a teneral state, observed in so many other insects soon after ecdysis, but can fly away immediately after it.
As the intensity of the current decreases, the fauna of creeks and rivers becomes richer in species which are similar to those of standing water. The need of special dispositions to resist the strength of the torrent disappears and the fauna only shows the general adaptations to life in freshwater.