Swimming and Other Movements, Stream Insaects

The physical world of an insect living in water or air is ruled by the fluid velocity relative to its body. Thus, swimming fast in still JL_ water or slowly against a current of flowing water can be physically similar for an insect. However, flow in streams is physically harsh and often constraining the movements of stream insects. Imagine a mountain stream with its typical current velocity of 50 cms_1 and a 1-cm-long mayfly larva swimming upstream. To move relative to the stream bottom, this larva must propel itself at more than 50 body lengths s_1, which is a difficult accomplishment. Therefore, stream insects rarely swim and live predominantly at or in the surface layer of the stream bottom, where friction between the moving water and the solid bottom material generates extremely complex flow patterns. As a consequence, the aquatic stages of stream insects move in a physical world that is much more complicated than that of lake insects.

LIFE IN FLOW NEAR THE STREAM BOTTOM

Streams offer a mosaic of flow conditions, including areas with (almost) still water. Where a stream runs over a rough (e.g., stony) bottom, flow can be laminar or turbulent (or transitional between these two). The difference between the laminar and the turbulent flow regime is easily understood by watching the smoke released by a cigarette in a calm place. Close to the tip, the ascending smoke travels in an orderly manner on a narrow path as a laminar flow, until suddenly the flow becomes turbulent and travels whirling on a wider path. The physical formulas for these regimes are quite different; thus, these two flow regimes constitute different physical worlds.
The Reynolds number Re indicates the flow regime for a particular flow situation through Re = l X U/u, where ) is a length dimension (e.g., the body length of an insect), U is the relative velocity to the insect body, and u is the kinematic viscosity of the water. Low Re indicates laminar and high Re indicates turbulent regime. Therefore, in general, small and slow (relative to the flow) stream insects (having a low ) X U) experience laminar flow in which viscous forces matter, whereas large and fast ones experience turbulent flow, where inertial forces predominate.
However, near the bottom of a stream, things are a bit more complicated. When water runs over a stone, the water in contact with the stone surface does not slip relative to the surface (the no-slip condition), and a velocity gradient develops above the surface (e.g., as in Fig. 1C). Near to the surface, the flow can be laminar (“laminar sublayer”). If the stone is embedded among other stones of similar size (Fig. 1A) and the water is shallow relative to the stone size, turbulent eddies may extend to the stone surface and disrupt the formation of the laminar sublayer. As a consequence, velocity varies tremendously over short periods near the stone surface (Fig. 1B), as well as around insects living at that surface. To maneuver in that type of flow, a 1-cm-long insect must deal with velocity changes from 5 to more than 40 body lengths s_1, and this within a fraction of a second. The body vibrations of black flies or elongated caddisflies staying at the surface of a stone in a shallow stream nicely illustrate that these insects have a very “shaky” life.
In natural streams, isolated larger bottom elements that rise far into the water column (Fig. 1A) are often very abundant. Above the surfaces of these elements, the no-slip condition produces a steep velocity gradient (Fig. 1C). Flow is generally laminar nearest to the surface (low standard deviation of the velocity in the laminar sublayer: Fig. 1C), whereas it is turbulent in the remaining part of the steep velocity gradient (elevated standard deviation in Fig. 1C). At a given location, the geometry (e.g., the thickness of the laminar sublayer) of this steep gradient changes with changing flow. That is, discharge variation in a stream causes temporal variability of the physical conditions at that location. At a given flow, the geometry of this gradient varies spatially along the slightly curved bottom element. The changes of the isovel (line of equal velocity) pattern along the curvature (Fig. 1D) demonstrate considerable spatial variability of the physical conditions over the distance of a few millimeters. As a consequence of these temporal and spatial patterns, a relatively sessile black fly larva must deal with temporal physical variation caused by changing stream discharge, whereas a relatively mobile mayfly larva must deal with spatial physical variation when moving at the bottom surface.
Whether sessile or mobile, stream insects living in steep velocity gradients can modify the flow patterns to a considerable extent (cf. Fig. 1D-G ) . Frequently, these stream insects are so large (e.g., in Fig. 1F and 1G) that their lower part is in the laminar flow, their intermediate part is in the layer of elevated turbulence, and their upper part extends beyond the steep velocity gradient. These insects live with different body parts in three different physical “worlds,” and the relative importance of these different worlds experienced by their bodies changes when the insects move. Thus, the flow conditions

FIGURE 1 Flow patterns near the stream bottom and near three fully grown bottom-dwelling stream insects. (A) Profile of a stream bottom. (B) Short-time velocity variation measured 0.2 cm above the surface of a natural stone that was embedded among other stones of similar size [cf. (A)] in a natural stream riffle. (C)-(G) Flow patterns measured above a curved bottom in a laboratory flume, simulating natural conditions that prevail above bottom elements (large stones, rocks, wooden logs) rising distinctly above neighboring bottom elements [cf. (A)]. (C) Mean and standard deviation of the velocity at different height above the highest point of the curved bottom; the column indicates the height of the layer having a steep velocity gradient. (D) Profile of the curved bottom and isovels [lines of equal velocities, drawn for velocity steps of 4cms~1 for the same flow as in (C)]; the column indicates the height of the layer having a steep velocity gradient above the highest point of the curved bottom [cf. C]. (E)-(G) Changes of the isovel pattern shown in (D) by the cases of two caddisflies and a dorsoventrally flattened mayfly  (B) redrawn and simplified after Hart, D. D., and Finelli, C. M. (1999). Physical-biological coupling in streams: The pervasive effects of flow on benthic organisms. Annu. Rev. Ecol. Syst. 30, 363-395, with permission from Annual Review of Ecology and Systematics Vol. 30 (1999) by Annual Reviews (G) redrawn and simplified after Statzner, B., and Holm, T. F. (1982).
near larger bottom elements are also physically harsh for stream insects because they variably affect factors of physiological relevance for the insects (e.g., facility of respiration, drag, or lift force).

MOVEMENTS IN THE STREAM ENVIRONMENT

The surface of the stream bottom provides essential resources (e.g., food, oxygen-rich water). To exploit these resources, insects have developed many different strategies that relate to movements at and in the stream bottom, or in the free-flowing water column.

Movements at and in the Stream Bottom

A strategy frequently used to deal with the physical harshness at the stream bottom is temporal avoidance. Particularly, mayfly and stonefly larvae move vertically among the interstices of the stream bottom, to the top of the bottom surface and back into deeper bottom layers that offer shelter from the surface flow. Such movement produces daily and/or seasonal patterns in the presence of these larvae at the bottom surface. However, the dissolved oxygen concentration of the water interferes with the use of interstitial flow shelters. If the oxygen in the interstices drops below a species-specific critical value, the physiological means of regulatory oxygen consumption (gill movements or undulatory body movements) become insufficient to meet respiratory needs. Thus, larvae leave the interstices and crawl to current-exposed locations at the front faces or tops of stones, where oxygen concentrations are usually higher and the elevated flow increases the renewal rate of oxygen at respiratory surfaces.
Another means of staying or moving in the flow near the bottom surface is safe fixation. Often long, curved tarsal claws that enable a good attachment to the bottom surface are found in stream insects (e.g., adult riffle beetles). In addition, hooks or claws situated near the end of the abdomen may be used for bottom attachment. Stream-dwelling larvae of caddisflies, moths, and midges use silk for such attachment. Within the true midges and the caddisflies, many species use silk to build tubelike larval retreats that are fixed at the bottom surface. Caseless polycentropodid and hydropsychid caddis larvae crawling over coarse bottom material glue a silken safety thread in a zigzag line on their path. Other caddisfly larvae that build cases temporarily secure these cases with silk. Finally, black fly larvae spin tiny silk carpets, fix them at the bottom surface, and then attach their posterior abdominal hooks to these carpets.
The stream insects that colonize the most extreme physical conditions are the larvae of the net-winged midges. They can live at rock surfaces where velocities exceed 2 ms_1, fixing themselves to the rock with six ventral suckers. The larvae can precisely adapt their locomotion to a given situation because they move by releasing one or more suckers at a time, having at least two suckers firmly attached to the rock at any moment. This highly specialized locomotory system enables the larvae to move straight upflow at a relative velocity (to the rock surface) of —0.03 body length I_1, which seems to be a rather poor performance. However, the larvae achieve this movement against water velocities of 2 ms_1. Therefore, their physically relevant speed corresponds to —300 body lengths s_1, which is an extraordinary performance (to achieve equivalent performance, a human would have to swim 100 m in less than 0.2 s).
Claws, silk, suckers, and other means not detailed here enable stream insects to maneuver actively at or in the stream bottom. Daily movements tend to carry the insects upstream because their body (even of dead nymphs and exuviae) is oriented upstream. Thereby, the larvae of mayflies, stoneflies, and caseless caddisflies may move upstream (mostly at night) several meters per day. Other movements at or in the bottom have seasonal patterns. The nymphs of some mayfly or stonefly species crawl bankward prior to emergence, and old larvae of some limnephilid caddisflies even move toward land in early summer, either to feed on semiaquatic plants or to aestivate outside the stream. In streams that freeze down to the bottom, true midges, dance flies, and some caddisflies can overwinter in the frozen habitat, whereas all other insects actively move away when facing an advancing freezing front. Among the mayflies, the nymphs of many species crawl bankward when the water level of the streams rises during springtime. If the adjacent floodplain of a stream becomes inundated, the nymphs may continue their movements toward the floodplain, where most nymphal growth and development can take place. Other mayfly species not only move toward the stream banks but continue to move upstream near the shoreline at a speed of about 100 mday-1. By following the shoreline of the main stream that bends toward the tributaries, the nymphs move into the tributaries and finally reach marshy areas drained by the tributaries. The nymphs complete their development in these marshy areas.
Active movements of stream insects are also caused by occasionally occurring, extreme hydrological events—floods and droughts. During floods, insects may move toward deeper bottom layers to find flow shelter, but evidence for such behavior is equivocal.
A giant water bug living in desert streams has a successful response to flooding. During periods of heavy rainfall that often precede flash floods in these streams, adult and juvenile water bugs move toward the banks, leave the stream, and crawl up to about 20 m over land toward sheltered areas (from where they return to the stream after 24h). Some studies report that stream insects burrow deeper into the humid sediments to avoid effects of drying, whereas other studies do not confirm such movements. With sinking water level in a stream, insects may also crawl away over land (adult beetles), or they crawl up- or downstream to places where water remains. The speed of insects during such movements can be rather high. For example, the larvae of a cased limnephilid caddisfly crawl downstream at about 12 mh_1.
Finally, the aerial females of a few species of stream insects crawl down on solid objects through the water surface and fix their eggs on the submerged surface of the object, or they oviposit in an aquatic host insect. The first behavior has been primarily observed in mayflies, caddisflies, and black flies; the second in parasitic wasps.

Movements in the Free-Flowing Water Column

Drifting downstream with the flow is the typical movement of stream insects in the water column. Physically, a passively drifting insect barely moves because the water velocity relative to its body is almost zero. However, relative to the stream bottom, it travels at the speed of the flow.
The drift of stream insects is perhaps one of the most frequently studied topics in stream ecology. Given that both the diversity of stream insects and the diversity of running water conditions across the continents are very high, there is evidence for almost any conceivable drift response. For example, the following factors may increase, decrease, or have no effect on the natural drift of stream insects: sunlight or moonlight; current velocity; stream discharge; type of the bottom substrate; turbidity, oxygen concentration, ion concentration, or temperature of the water; organic matter; food; predators; microbial pathogens; larger parasites; molting process; benthic density; and age or behavior of the drifting species. Thus, it is difficult to identify clear patterns in the drift of stream insects.
Typically, stream insects drift at night. This drift may be caused by accidental dislodgement of the insects through the current or by deliberate, active entries into the drift. Active entries occur when a habitat patch is overcrowded and resources (e.g., food, space, preferred flow conditions) are lacking. Approximately 10-30% of the insect population of a stream reach may drift during one night, and they may travel between 2 and 20 m during one drift movement. However, caddisfly larvae that build heavy cases from gravel typically have distinctly lower drift rates and shorter drift distances. Insects that produce silk may also have shorter drift distances. Hydropsychid and polycentropodid caddisfly larvae, and black fly larvae, can fix a silk thread to the stream bottom, actively enter into the drift and, prolonging the thread by spinning, rope themselves downstream over several centimeters until they resettle at the bottom. Polycentropodid caddis larvae that have lost contact with the bottom often spin “anchor” threads of 30 cm in length. When such a thread adheres to the stream bottom, the larva is able to return to the bottom by climbing down along the thread. Another means of affecting drift distance is used by drifting larvae of mayflies, stoneflies, and caseless caddisflies. They modify their body posture, thereby either decreasing or increasing their sinking speed, and thus the distance traveled in the water column.
The intensity of everyday drift events usually varies across seasons, either because of seasonal changes in factors affecting the insect drift or because of the seasonal occurrence of a particular developmental stage that drifts more than other stages. A typical example for the first cause is the seasonal change of temperature in temperate streams and the related effects on the insect drift. A typical example for the second cause is found in hydropsychid caddisflies that may have newly hatched first instars that drift at much higher rates than later instars.
Occasional natural events such as floods and droughts, or unnatural ones such as pesticide or pollution spills, also increase drift. During such events and in contrast to the everyday drift, insects also drift during the daytime, and the distances traveled are longer. When discharge increases, and thus near-bottom velocity, many stream insects are dislodged. Among the sessile forms that fix themselves firmly to the stream bottom, those sitting on submerged wood or leaf litter may travel extremely far downstream on dislodged litter pieces that drift for long distances. In contrast, when discharge and thus near-bottom velocity decreases during a drought, many insects release their grasp on the stream bottom and swim upward until they are caught and transported by the free flow in the water column.
Another cause for massive upward swimming of insects from the bottom is relatively low oxygen concentration in the water, a condition typical of streams with organic pollution. Drifting insects may swim, either to avoid sinking so that they stay longer in the water column or to return faster to the stream bottom. Finally, some mayfly larvae swim over short distances when they encounter predators or aggressive con-specifics. These larvae are such good swimmers that they can travel against the current just above the stream bottom, where the velocity is reduced. The larvae of most insect groups swim by body undulations.
An exceptional type of movement among the stream insects is the diving of aerial females to reach submerged oviposition sites. Female black flies can dive through thin water layers flowing over rocks to get foothold at the rock surface, where they affix the eggs. Similarly, female caddisflies dive vertically and swim to oviposition sites below inclined submerged stones.