The Aquatic Environment Chapter 10.4 Entomology http://entomologa.ru/outline/index.htm

1. THE AQUATIC
ENVIRONMENT
Chapter 10.4

2. TWO DIFFERENT AQUATIC
PHYSICAL ENVIRONMENTS
1. Lotic (flowing)
2. Lentic (standing)

3. LOTIC ADAPTATIONS

4. LOTIC ADAPTATIONS
In lotic systems, the velocity of flowing water influences:
1. substrate type (surface or material on or from which an organism
lives, grows, or obtains its nourishment), with boulders deposited
in fast-flow and fine sediments in slow-flow areas;
2. transport of particles, either as a food source for filter- feeders or,
during peak flows, as scouring agents;
3. maintenance of high levels of dissolved oxygen.

5. A stream or river contains heterogeneous micro-habitats, with riffles
(shallower, stony, fast-flowing sections) interspersed with deeper
natural pools.
The Santa Cruz River riffles

6. Areas of erosion of the banks alternate with areas where sediments
are deposited, and there may be areas of unstable, shifting sandy
substrates. The banks may have trees (a vegetated riparian zone-he
interface between land and a river or stream) or be unstable, with
mobile deposits that change with every flood. Typically, where there
is riparian vegetation, there will be local accumulations
of drifted allochthonous (external to the stream) material such as leaf
packs and wood.

7. In parts of the world where extensive pristine, forested catchments
remain, the courses of streams often are periodically blocked
by naturally fallen trees. Where the stream is open to light, and
nutrient levels allow, autochthonous (produced within the stream)
growth of plants and macroalgae (macrophytes) will occur. Aquatic
flowering plants may be abundant, especially in chalk streams.

8. Characteristic insect faunas inhabit these various substrates, many
with particular morphological modifications (change in form or
structure) . Thus, those that live in strong currents
(rheophilic species) tend to be dorsoventrally (back) flattened
(Fig. 10.5), sometimes with laterally projecting legs. This is not
strictly an adaptation to strong currents, as such modification
is found in many aquatic insects. Nevertheless, the shape and
behavior minimizes or avoids exposure by allowing the insect
to remain within a boundary layer of still water close to the surface
of the substrate. However, the fine-scale hydraulic flow of natural
waters is much more complex than once believed, and the
relationship between body shape, streamlining, and current velocity
is not simple.

9. The cases constructed by many rheophilic caddisflies assist
in streamlining or otherwise modifying the effects of flow. The variety
of shapes of the cases (Fig. 10.6) must act as ballast against
displacement. Several aquatic larvae have suckers (Fig. 10.4) that
allow the insect to stick to quite smooth exposed surfaces, such
as rock-faces on waterfalls and cascades.

10. Silk is widely produced, allowing maintenance of position in fast flow.
Black-fly larvae (Simuliidae) (see the vignette to this chapter) attach
their posterior claws to a silken pad that they spin on a rock surface.
Others, including hydropsychid caddisflies (Fig. 10.7) and many
chironomid midges, use silk in constructing retreats. Some spin
silken mesh nets to trap food brought into proximity by the stream
flow.

11. Many lotic insects are smaller than their counterparts in standing
waters. Their size, together with flexible body design, allows them
to live amongst the cracks and crevices of boulders, stones, and
pebbles in the bed (benthos) of the stream, or even in unstable,
sandy substrates. Another means of avoiding the current is to live
in accumulations of leaves (leaf packs) or to mine in immersed
wood — substrates that are used by many beetles and specialist
dipterans, such as crane-fly larvae (Diptera: Tipulidae).

12. Two behavioral strategies are more evident in running waters than
elsewhere. The first is the strategic use of the current
to allow drift from an unsuitable location, with the possibility
of finding a more suitable patch. Predatory aquatic insects frequently
drift to locate aggregations of prey. Many other insects, such
as stoneflies and mayflies, notably Baetis (Ephemeroptera: Baetidae),
may show a diurnal periodic pattern of drift. “Catastrophic” drift
is a behavioral response to physical disturbance, such as pollution
or severe flow episodes. An alternative response, of burrowing deep
into the substrate (the hyporheic zone), is a second particularly lotic
behavior. In the hyporheic zone, the vagaries of flow regime,
temperature, and perhaps predation can be avoided, although food
and oxygen availability may be diminished.

13. LENTIC ADAPTATIONS

14. LENTIC ADAPTATIONS
Still Water
The lentic water surface is used by many more species
[the neustic community (walks on water due to high surface tension)
of semi-aquatic insects] than the lotic surface, because the physical
properties of surface tension in standing water that can support
an insect are disrupted in turbulent flowing water.
Water strider

15. Between the water surface and the benthos, planktonic organisms live
in a zone divisible into an upper limnetic zone (i.e. penetrated
by light) and a deeper profundal zone. The most abundant planktonic
insects belong to Chaoborus(Diptera: Chaoboridae); these “phantom
midges” undergo diurnal vertical migration, and their predation
on Daphnia is discussed in section 13.4. Other insects such as diving
beetles (Dytiscidae) and many hemipterans, such as Corixidae, dive
and swim actively through this zone in search of prey.

16. In the littoral zone, in which light reaches the benthos and
macrophytes can grow, insect diversity is at its maximum. Many
differentiated microhabitats are available and physico-chemical
factors are less restricting than in the dark, cold, and perhaps anoxic
conditions of the deeper waters.