Animals - Freshwater Ecology: Concepts and Environmental Applications - page 170

Flies and midges (Diptera) constitute a large group, to which about
40% of all aquatic insects belong (Hilsenhoff, 1991). This group is domi-
nated by the family Chironomidae (Fig. 9.9E), which includes about one-
third of all species of aquatic Diptera. The Diptera also includes many
aquatic larvae with adults that are nuisance species for humans, including
the mosquitoes, black flies, biting midges, horseflies, and deerflies. Mos-
quitoes are very important vectors of disease in many parts of the world.
The group also includes the “blood worms,” which are chironomid midge
larvae that have hemoglobin to allow them to survive in low-oxygen sedi-
ments. The blood worms provide a food source for benthic predators in
lakes and for waterfowl in shallow wetlands. The Diptera also contains the
phantom midges that are zooplankton predators found in many lakes.
Diptera are found in a wide variety of aquatic habitats and exhibit a
tremendous array of adaptations to them. For example, Eristalis (Syrphi-
dae) larvae have elongated respiratory siphons that allow them to breathe
atmospheric O 2 and thrive in anaerobic waters. Shorefly larvae (Ephydri-
dae) can be abundant in extreme environments, such as hot springs and
saline lakes.
Subphylum Crustacea
About 4000 species of crustaceans have been described from freshwa-
ters of the world (Covich and Thorp, 1991). The subphylum Crustacea in-
cludes some species that are central in aquatic food webs and ecosystem
structure: cladocerans and copepods, which are the key primary consumers
in many lakes, and decapods (crayfish and others), which are important om-
nivorous consumers in benthic food webs of numerous rivers, lakes, and
ponds. The Crustacea are characterized by respiration through gills or across
the body surface, a hard chitinous exoskeleton, two pairs of antennae, and
most body segments with paired and jointed appendages. The crustacean
taxonomic groups considered here are the Ostracoda, Copepoda, Bran-
chiopoda, Decapoda, Mysidaceae, Isopoda, Amphipoda, and Bathynellacea.
The Ostracoda (seed shrimp; Fig. 9.11C) are benthic species that are
covered by a carapace often composed of chitin and calcium carbonate.
The carapace is particularly well preserved in sediments; paleolimnologists
make use of the group as indicators of ancient environments and these fos-
sils are the oldest known microfauna (Delorme, 1991). Ostracods (some-
times spelled ostracodes) are found in most aquatic habitats, including tem-
porary pools (even in very small pools in the bracts of bromeliads), ponds,
lakes, rivers, and groundwaters. Species can be found in hot springs and
hypersaline habitats. Ostracods mostly eat detritus and algae; a few are
scavengers or predaceous species. They can reach tremendous numbers in
some benthic habitats and may serve as an important food source for
predators that specialize on benthic microfauna.
The Copepoda is a microcrustacean group that includes an important
component of zooplankton in lakes, but most species are found associated
with the benthos of streams, wetlands, and groundwaters. About 500
freshwater species have been described worldwide (Williamson, 1991).
They are characterized by a cylindrical body (Figs. 9.11H and 9.11I ) 0.2-2
mm long with numerous segmented legs on the head and abdomen. They
have conspicuous first antennae and a single, simple, anterior eye. The