Order Coleoptera
(Beetles)
Coleoptera comprise the largest order of insects and with
approximately 5,000 aquatic members, ranks as one of the major
groups of
freshwater anthropods. Moreover, beetles occupy a broad spectrum of aquatic
habitats, including such systems as cold, rapid mountain streams, brackish,
stagnant waters of estuaries and salt marshes, and the intertidal zone of rocky
seashores. Beetles are important in some aquatic food chains, and several
species are consumed by fish and water-fowl. Most aquatic beetles are substrate
dwellers.
Beetles are
holometabolous insects, with a life cycle consisting basically of an egg, larva,
pupa, and adult or imago; there are some cases, however, where the egg or pupal
stage has been eliminated. Truly viviparous species, which have eliminated the
egg stage, are rare among the Coleoptera.
The size of
beetle eggs and the number of eggs produced by a single female vary
considerably. Larvae of Coleoptera, unlike those of some other orders, have no
one common name that can be applied to all forms, and various terms like grub,
white grub, wire-worm, mealworm, rootworm, glow worm, round-headed borer,
flat-headed borer, timber borer, water penny, etc. have been used for individual
larval types. The number of larval instars in Coleoptera varies from 1 to almost
30, but is normally 3 to 5.
Order Collembola
(Springtails)
Most species of
Collembola live in soil or soil related habitats, and a few are found in semi
aquatic situations. They commonly dwell in organic soils, leaf litter, mosses,
under fallen logs, beneath stones, under loose bark, in dung, nests, burrows and
caves. All aquatic species are small, being rarely larger than 3mm. Their size
and hydrophobic (water repelling) integument keep them afloat on the surface
film.
The Collembola
are generally detritus feeders capable of consuming a wide variety of dead plant
material and microflora; however, most aquatic species show a strong preference for
particular foods such as diatoms, unicellular algae and plankton.
Eggs are laid
singly or in clumps, in crevices or pockets of the substrate. Collembolans
generally pass through five or six instars before they are sexually mature.
Instars one through four are usually considered to be the immature stages. There
is no metamorphosis (ametabolous) and molting continues after the adult stage is
reached.
Collembolans
display a number of unusual features including a great divergence in total instar number (2-50+) and the absence of a definitive adult form in many
species. The Collembolans are apterygote (wingless) insects characterized by the
presence of a collophore or ventral tube (respiratory, adhesive, osmoregulatory
organ) on the center of the first abdominal segment. Primitive forms are
equipped on the 4th abdominal segment with a peculiar bifurcate
jumping organ or furcula, which is held in place by the tenaculum when the
animal is at rest.
Order Diptera (True
flies)
Diptera or true
flies are one of the most diverse insect orders. Diptera are diverse in not only
number of species, but with respect to morphological and ecological
characteristics. Aquatic Dipteran larvae are found in streams and rivers of all
sizes and rates of flow and depth, in lakes, ponds, stagnant pools, puddles,
freshwater swamps and marshes and marine inter-tidal zones; in short, almost any
place where water is present for at least a few weeks. The water may be fresh
and clean or polluted, or brackish, acidic or alkaline, and clear or turbid. The
majority of aquatic larvae are burrowing detritivores and carnivores in bottom
sediments, or herbivores in aquatic plant stems, leaves or roots.
Diptera larvae
are extremely variable morphologically. Variation is so great that no individual
or concise combination of characters exists by which all Diptera larvae can be
distinguished from all other endopterygote insect larvae. The only character
common to all Diptera larvae, but also found in representatives of several other
orders, is the absence of jointed thoracic legs. This feature, coupled with the
fact that the majority of free-living Diptera larvae are rather slender with
active directional movement, serves to distinguish most of them.
The taxonomic
and ecological diversity of Diptera is reflected in the wide range of larval
feeding habits, which encompass nearly every trophic group. Their trophic
diversity and numerical abundance make Diptera an important component in
aquatic ecosystems, both as primary consumers and as food resource for other
invertebrates, fish, amphibians, reptiles, birds and mammals. Aquatic Diptera
can serve other significant roles in the fields of water quality, biomonitoring,
conservation biology and in scientific research on the structure and function of
aquatic ecosystems, structural and ecological adaptations to aquatic
environments, and patterns and mechanisms of evolutionary divergence.
As a
holometabolous insect, or one that undergoes complete metamorphosis, the
Dipteran life cycle includes a series of distinct stages or instars. A typical
life cycle consists of a brief egg stage (usually a few days or weeks, but
sometimes much longer), 3-4 larval instars, a pupal stage of varying length and
an adult stage that lasts from less than 2 hours to several weeks or even
months.
Order
Ephemeroptera (Mayflies)
All mayflies
are aquatic as larvae. Mayflies occur in an extremely wide variety of standing
and running water habitats, the greatest diversity being found in
rocky-bottomed, second and third-order, headwater streams.
The eggs of
mayflies are usually deposited at the water surface, a few at a time or all in
one or two clusters. In some species, the female crawls beneath the water and
lays rows of eggs on the substrate. A few drop clusters of eggs from the air.
Eggs of most mayflies have sticky coverings, frequently with specialized
anchoring devices.
The larvae
undergo numerous molts. Length of larval life varies with temperature and is
usually 3 to 6 months. Most mayfly larvae are collectors or scrapers and feed on
a variety of detritus and algae, and some macrophyte and animal material. A few
are omnivores. A few species are true carnivores and are found frequently in
large rivers.
The subimago (a
winged but usually sexually immature stage) in the life cycle of mayflies is
unique among living insect orders. The Ephemeroptera has two winged instars, the
subimago and the adult (or imago). The length of the life of the subimago instar
tends to be corelated with the length of life of the adult.
Adults of most
species live two hours to three days, but some live less than 90 minutes. The
females of some genera may live for weeks, especially genera that hold the eggs
until they are ready to hatch. Most species have three caudal filaments (a
terminal filament and two cerci). In some species only the cerci are well
developed, the terminal filament being represented by a short rudiment or
absent. The terminal filament when present may vary in length and thickness
relative to the cerci. The length of the caudal filaments varies from shorter
than the body to two or three times the length of the body.
Although
mayflies are rarely pests in the traditional sense, the adults of certain
species become a nuisance when they are attracted to lights in large numbers.
Mayfly larvae are generally regarded as being of positive economic
importance. They play a significant role in the transfer of nutrients to fish
and by emergence into the terrestrial segment of the environment and thus are of
considerable interest and importance to fly fishermen and aquatic biologists.
Order
Hemiptera (True Bugs)
The Hemiptera
is composed of those insects usually referred to as true bugs. True bugs can be
found in a wide variety of aquatic and terrestrial habitats. Many species are of considerable direct importance to
humans as pets of agricultural crops and health. Fifteen of the 17 major
families of Heteroptera associated with the aquatic habitat are represented in
the North American insect fauna.
The aquatic and
semi aquatic Heteroptera are remarkable for their diversity of form, reflecting
adaptations to wide variety of niches. They occupy many varied habitats,
including saline ponds, high mountain lakes, hot springs, and large rivers.
Basically they are predators; many species seem to be relatively resistant to
predation, which is often attributed to the possession of characteristic
Heteropteran scent glands.
Although there
are exceptions, most aquatic Heteroptera lay their eggs in the spring, develop
during the warmer months, over-winter as adults, and repeat the cycle. Eggs are
of various forms and are laid in a wide variety of places, either glued to a
substrate or inserted in the earth or plants. Larvae of true bugs are minute,
ca. 1 mm, to very large, 25 mm. Most Hemipterans are either uni- or bivoltine
with 5 instars.
Order Lepidoptera (Butterflies
and Moths)
The Lepidoptera contains well over 100,000 species and more than 100 families
worldwide. Larvae of Lepidoptera possess a distinct head, chewing mouthparts,
three thoracic legs and ten abdominal segments with crochet-bearing prolegs on
segments 3, 4, 5, 6 and 10.
Caterpillars are perhaps the most
commonly encountered immature insects since they are primarily phytophagous and
many are exposed feeders. Most are terrestrial but larvae of a few groups are
found in aquatic habitats, where they feed on detritus or in stems and roots of
macrophytes. Exposed feeders are heavily
parasitized are preyed upon, as a result many unusual adaptations and habits
have evolved like cryptic coloration.
The eggs of Lepidoptera are
diverse in structure, abundance and the ways they are laid. Caterpillars spin
silk through a conspicuous labial spinneret. Species in many different taxa use
silk in diverse ways – for webbing together leaves or other materials to
constructing large tents or webs or silken tube shelters. Many caterpillars spin
cocoons when they pupate. Others pupate in the ground or litter.
Lepidopteran adults are
characterized by the presence of overlapping scales on the two pairs of wings,
body and legs. They usually possess a sucking proboscis (coiled).
Order Megaloptera
(Dobsonflies and Alderflies)
This group of
holometabolous aquatic insects contains some of the largest and most spectacular
species. The aquatic larvae are predaceous and inhabit both lotic and lentic
environments in tropical and temperate climates; however, all eggs, pupae, and
adults are terrestrial. Large numbers of adults are seldom seen in nature
because they are short-lived, secretive, and many species are nocturnal. They
can be found in spring seeps, streams, rivers, lakes, ponds, swamps, and in
temporarily dry streambeds. Small animals, mostly other insect larvae, are used
as food.
Larvae pass
through many instars and take 1-5 years to complete development. Last-stage
larvae are usually most abundant in late winter or early spring. Larvae of
Megaloptera are slightly flattened; possess distinct head with conspicuous
mouthparts, a 3-segmented thorax with 3 pairs of segmented legs, and a distinct
10-segmented abdomen. Identification of larval Megaloptera is based primarily on
the number of abdominal filaments, the presence or absence of ventral abdominal
gill tufts, and the appearance and location of the eighth abdominal spiracles.
Pupation occurs in cells in the soil adjacent to a body of water, in logs in or
near the water, or in dry streambeds.
Order
Neuroptera (Spongillaflies)
The Neuroptera
comprise a relatively well-defined, presumably monophyletic, but diverse group
of holometabolous insects. Large numbers of adults are seldom seen in nature
because they are short-lived, secretive, and many species are nocturnal.
In many, but
not all families, the eggs are laid singly or in clusters at the end of silken
threads; in other families the eggs are deposited onto soil or attached to a
substrate.
Neuropteran larvae
are characterized by the presence of suctorial mouthparts consisting of sickle-shaped, grooved mandibles and maxillae (laciniae)
that form feeding tubes. Most larvae are
terrestrial, and prey on
soft-bodied arthropods. However, one group, the sisyrids are aquatic and larvae
are usually found associated with freshwater sponges (Spongilla). They occur on the surface
or in the cavities of the host, and pierce the sponge cells and suck the fluids
with their elongated mouthparts. Larvae pass through three instars, and some
species have several generations each year. Just before pupation, the larvae
leave the water, climb onto shoreline plants or other objects, and spin a
2-layered silk cocoon in which to pupate. Adult spongillaflies appear to be primarily nocturnal.
Order Odonata
(Dragonflies and Damselflies)
Adult Odonata,
known as dragonflies and damselflies, fly consciously near almost any body of
fresh water on sunny, warm days. The scientific name, Odonata, is derived from
the Greek “Odon”, meaning tooth, and apparently refers to the sharp teeth on the
mouthparts
Odonata are
aquatic, with wings developed externally in larvae. Odonata immatures undergo
hemimetabolous (incomplete) metamorphosis. Wing structure is extremely important
in Odonate maneuverability, as much of their time is spent in aerial pursuit of
food, habitat or mates, and in escape from predators: some species oviposit when
flying.
Mating takes
place in flight or when perched, over water, or near water, or some distance
from the water, and at various times of the day. The number of eggs laid by a
female in one egg-laying bout varies from a few hundred to a few thousand.
Odonata have an incomplete metamorphosis with the immature stage (nymph, naiad,
larvae) going through 10-15 instars. Wing pads appear after the sixth or seventh
molt, becoming swollen late in the final instar, a sign of imminent emergence.
Dragonfly
larvae are generalized carnivores, feeding on any aquatic animal of an
appropriate size that they can capture. The most unique characteristic is the
peculiar labium or lower lip. It is folded upon itself at midlength and turned
backward beneath the front legs which alone separates the larvae of Odonata from
closely related aquatic orders. The labium is ejected to grasp the prey. The
labium, when extended is approximately as long as the fore legs, but at rest may
mask the face up to the eyes. It is designed for grasping and holding prey. The
sharp hooks and spines of the palpal lobes, situated at the front of the labium,
clutch the prey: the maxillae and mandibles cut and manipulate for ingestion.
Larvae usually
conceal themselves by either burrowing in substrate, sprawling amongst fine
sediment and detritus, or climbing on vascular plants. Predators of larvae of
Odonata include aquatic birds, fish and large predaceous insects. Larvae of
Zygoptera (damselflies) are more slender than those of Anisoptera (dragonflies).
Zygoptera have 3 external tracheal gills: a pair of lateral gills and a median
gill. These are usually laterally flattened and used for swimming as well as
respiration.
In adults, the
wings of Anisoptera are held horizontally when at rest, whereas in most
Zygoptera they are held together above the body. Predators of adult Odonata
include many species of birds, amphibians, bats, spiders, wasps, other
dragonflies, and other insects.
Mosquitoes
comprise an important part of the diet of adults and larvae of many Odonates,
but other insects are preyed upon also, many of which are considered pests.
Odonata are thus regarded as beneficial predators.
Order Plecoptera
(Stoneflies)
Immature
stoneflies are usually found in clear, cold unpolluted rivers, streams, and
lakes with high levels of dissolved oxygen, although a few species are able to
survive in habitats that warm up or dry up during part of the year.
The combination
of two long cerci and two tarsal claws distinguishes stonefly larvae from those
of other aquatic orders. Larvae are herbivores, detritivores or carnivores,
feeding on aquatic plants, algae or detritus or on other insects and small
animals. The larvae of herbivorous families are rounded or robust, whereas those
of carnivorous families are dorso-ventrally flattened. Larvae go through 12-24
instars, depending on species, sex, size and habitat conditions.
Food ingestion
by larvae may vary depending on species, developmental stage or time of day.
Some species are, for example, shredders or predators throughout development;
however others may change their feeding habit as development proceeds.
Adults live about 1-4 weeks,
although winter species often have greater longevity.
Order Trichoptera
(Caddisflies)
The Trichoptera
or caddisflies, one of the largest groups of aquatic insects, are closely
related to the Lepidoptera. They are holometabolous.
Trichopteran
larvae are found in most types of fresh water habitats. Larval Trichoptera
consume decaying organic materials of plant and animal origin, assimilating
mainly the associated fungi and bacteria, and thereby contributing to the
reduction of large organic pieces such as leaves into small feacal particles
that are themselves colonized by fungi and consumed by other aquatic larvae
including Trichoptera. They also consume algae of all types; they scrape diatoms
from rock substrates. Some larvae are mainly predaceous. Generally, larval
Trichoptera show little selectivity of food, but they are highly and diversely
specialized for food acquisition.
Caddisfly
larvae are best known for the remarkable nets, retreats and portable cases they
construct. Silk emitted through an opening at the tip of the labium is used
either by itself or to fasten together rock fragments and pieces of plant
materials.
Eggs are
deposited in water in most families. Although most species of North American
caddisflies are univoltine, some require two years for development and others
less than a year. Larvae of most species have 5 instars, after which they fasten
the case with silk to a solid substrate, sealing off the ends. The actual pupal
stage lasts for two to three weeks, although in some groups it is preceded by a
prepupal phase of up to several weeks duration where the larva is in diapause.
Adults are mostly quiescent during the day, but fly actively during evening and
night hours. Some are known to feed on plant nectar. Most adult caddisflies live
less than a month.
Some
Trichoptera are predaceous on other insects and small invertebrates. Thus,
Trichopteran larvae are involved in the transfer of energy and nutrients from
the degradation process through to higher trophic levels. Because Trichoptera
are one of the most diverse and abundant groups of organisms in all types of
freshwater habitats, their contribution to these fundamental ecological roles is
extremely important.
