Science of Water : Concepts and Applications

Water Ecology 187
UNITS OF ORGANIZATION
Macroinvertebrates, like all other organisms, are classifi ed and named. Macroinvertebrates are
classifi ed and named using a taxonomic hierarchy. The taxonomic hierarchy for the caddisfl y (a
macroinvertebrate insect commonly found in streams) is shown below.
Kingdom: Animalia (animals)
Phylum: Arthropoda (“jointed legs”)
Class: Insecta (insect)
Order: Trichoptera (caddisfl y)
Family: Hydropsychidae (net-spinning caddis)
Genus species: Hydropsyche morosa
INSECT MACROINVERTEBRATES
As mentioned, the macroinvertebrates are the best-studied and most diverse animals in streams;
therefore, we devote our discussion to the various macroinvertebrate groups.
While it is true that noninsect macroinvertebrates, such as Oligochaeta (worms), Hirudinea
(leeches), and Acari (water mites), are frequently encountered groups in lotic environments, the
insects are among the most conspicuous inhabitants of streams. In most cases, it is the larval stages
of these insects that are aquatic, whereas the adults are terrestrial. Typically, the larval stage is
extended, while the adult lifespan is short.
The most important insects groups in streams are Ephemeroptera (mayfl ies), Plecoptera (stonefl
ies), Trichoptera (caddisfl ies), Diptera (true fl ies), Coleoptera (beetles), Hemiptera (bugs), Megaloptera
(alderfl ies and dobsonfl ies), and Odonata (dragonfl ies and damselfl ies). The identifi cation of these
different orders is usually easy and there are many keys and specialized references (Merritt and Cummins,
1996) available to help with the identifi cation up to species. In contrast, specialist taxonomists
can often diagnose only some genera and species, particularly in Diptera.
As mentioned, insect macroinvertebrates are ubiquitous in streams and are often represented by
many species. Although the numbers refer to aquatic species, a majority is to be found in streams.
Lotic insects are found among many different orders and brief accounts of their biology are presented
in the following sections.
(1) MAYFLIES (ORDER: EPHEMEROPTERA)
Streams and rivers are generally inhabited by many species of mayfl ies (common names: mays,
mayfl y, upwings, duns, dippers, and spinners) and, in fact, most species are restricted to streams.
For the experienced freshwater ecologist who looks upon a mayfl y nymph, recognition is obtained
through trained observation: abdomen with leaf-like or feather-like gills, legs with a single tarsal
claw, generally (but not always) with three cerci (three “tails,” two cerci, and between them
usually a terminal fi lament; see Figure 6.19). The experienced ecologist knows that mayfl ies are
hemimetabolous insects (i.e., where larvae or nymphs resemble wingless adults) that go through
many postembryonic molts, often in the range between 20 and 30. For some species, body length
increases about 15% for each instar (i.e., time between each molt). During instars the nymph is very
vulnerable to its principal animal, bird fi sh, amphibian, and insect predators such as diving beetles,
frogs, salamanders, swifts, phoebes, and dragon fl y nymphs and adults.
Mayfl y nymphs are mainly grazers or collector-gatherers feeding on algae and fi ne detritus,
although a few genera are predatory. Some members fi lter particles from the water using hairfringed
legs or maxillary palps. Shredders are rare among mayfl ies. In general, mayfl y nymphs
tend to live mostly in unpolluted streams, where with densities of up to 10,000/m 2 , they contribute
substantially to secondary producers.