Students of Water Ecology at the New Jersey School of
Conservation spend their class wading in the tributaries of the Big Flatbrook,
outfitted in rubber boots, turning over river-worn rocks to look for the
benthic macroinvertebrates that cling to the bottom of these stones. A benthic macroinvertebrate is an organism
that has no backbone, is visible to the naked eye, and lives on the bottom
layer of a body of water, often in the sediment. Most of the benthic macroinvertebrates that
students find are actually the larvae of aquatic insects, those insects that
live a portion of their life cycle in the water. In order to become an adult, these aquatic
insects undergo a metamorphosis and leave the water once their wings have dried.
Perhaps our least favorite aquatic insect is the mosquito,
of which there are 2,500 different species worldwide and 150 species in the United States! The mosquito, of the order Diptera, goes
through four distinct life stages, three of which are spent in the water. Adult mosquitoes lay eggs one at a time on
the surface of the water or on damp soil that will become flooded by
water. Some species make egg rafts,
where the eggs stick together and float on the water. Within about 48 hours, these eggs will hatch
into larvae. The larvae, also known as
“wigglers” or “wrigglers,” live in the water for one to two weeks. They use a tube called a siphon to breathe
oxygen at the surface of the water.
Eating micro-organisms and organic matter such as algae, the larvae go
through four molts, during which they shed their skin and grow larger. After the fourth molt, they become pupae and
begin the final metamorphosis to adult mosquitoes. The pupae, who do not feed, remain in the
water for one to four more days, depending on the species of mosquito and the
temperature of the water. Lighter than
water, the pupa floats at the surface and uses two breathing tubes called
trumpets to receive oxygen. Pupae are
known as “tumblers” because when disturbed, they dive in a jerking, tumbling
motion before floating back to the surface.
Within a pupal shell, the adult mosquito develops over a period of about
two days. Finally, the pupal skin will
split and the adult mosquito emerges from the skin. At this point, the adult will rest on the
surface of the water, allowing its new body to dry and harden and its wings to
spread.
Mosquito Larva
The adult male mosquito only lives for about a week, while
the adult female mosquito lives for about a month. However, temperature, humidity, and time of
year also affect the lifespan of this insect pest. If you find yourself sitting outside on a
summer evening, slapping away at the mosquitoes biting your ankles, you are
actually targeting only the females of the species! Female mosquitoes depend on the protein in
blood to produce their eggs. The males,
in contrast, only feed on flower nectar and other plant juices. Humans aren’t the only ones that these ladies
bite, however; cows, chickens, deer, rabbits, snakes, frogs, and many other
types of animals also make up a tasty dinner menu. If you’re worried about the mosquitoes being
a problem in your yard this summer, make sure you eliminate any standing water;
mosquitoes aren’t picky, so they will lay their eggs in any kind of water! Keep in mind that mosquitoes make up an
important part of the food chain, providing sustenance for many creatures,
including bats, frogs, and fish.
Preferring cleaner water than the mosquito to lay their
eggs, dragonflies and damselflies are aquatic insects that belong to the order
Odonata, which means “toothed ones.” They
are called this because each dragonfly and damselfly has a prehensile labium, or
extendable jaw, beneath the head; this jaw can extend faster than most prey can
react. This adaptation makes the bite of
the Odonata fatal to its prey, which includes mayflies, caddisflies, gnats,
ants, termites, and other small insects.
All six legs of the Odonata are located close to the head; rather than
using their legs for walking, the legs are most often used to catch prey or to
perch.
Dragonfly Larva
It is easy to tell the difference between dragonflies and
damselflies. Dragonflies have stout
bodies and when resting, they extend their wings to each side. The damselfly, in contrast, has a very
slender body, and when resting, holds its wings together above the body. Dragonflies and damselflies are additionally
characterized by their eyes, which are very large in proportion to their
heads. Their eyes are so large that over
80% of brain power is devoted exclusively to analyzing visual information! Odonata species have two pairs of long wings
that are made strong and flexible by a strong crossvein and many small
criss-crossing veins. With these wings,
dragonflies, who can fly faster than damselflies, ambulate forward at about 100
body-lengths per second, backward at about 3 body-lengths per second, or hover
in the air for about a minute. While
most species have wingspans that are about 5 to 8 centimeters wide, 325
million-year-old fossils show that ancestors of dragonflies had wingspans up to
30 inches wide! With exceptional sight
and agile flight, dragonflies and damselflies may escape from their predators, which
include birds, lizards, frogs, spiders, fish, water bugs, and even larger
dragonflies.
Damselfly Larva
Like mosquitoes, dragonflies and damselflies spend a part of
their lives in the water. However, they
only go through three distinct life stages.
Dragonflies lay round eggs in the water, while damselflies lay
cylindrical, long eggs. The larvae of
the Odonata are known as “nymphs,” and most of the insects’ lives are spent in
this stage. While a mosquito molts only
four times, the Odonata may molt six to fifteen times! In order to breathe in the larval stage,
nymphs expand and contract their abdomens to move water over their gills. Depending on many factors, they may remain in
the larval stage for up to six years.
While the mosquito enters into a pupal stage, dragonflies and
damselflies will simply crawl out of the water, molt one final time, and emerge
from their old skin as winged adults.
Because they skip the pupal stage, Odonata are known as
“hemimetabolous,” or those who undergo an incomplete metamorphosis. With a longer lifespan than mosquitoes, adult
Odanata can live for up to six months under favorable conditions.
While dragonflies are more sensitive to pollution than
damselflies, both are indicators of healthy ecosystems. Many factors affect the distribution of
nymphs, including acidity of water, the amount and type of aquatic vegetation,
temperature, and the type of environment.
Cool streams, rivers, ponds, marshy areas, and still clear water are
favorites of various species. Did you
know that the New Jersey School of Conservation is located in the Odonata
capital of the country? Sussex County
has more species of Odonata than any other county in the country, tallying in
at 145 species! Due to its varied
topography and geology, poorly drained swamps and marshes, and rich boreal
habitat with plenty of clean rivers, lakes, and streams, dragonflies and
damselflies love to call Sussex
County home.
Even more sensitive to the environment than the Odonata are
mayflies, members of the order Ephemeroptera. Mayflies, of which there are 2,000 species,
are the only insects to go through two flying stages in their
metamorphosis. Mayflies mate in swarms during
calm weather before depositing their eggs, flying low over the water or dipping
their abdomens on the surface. Some
adults even submerge themselves and lay their eggs underwater. Many adult females die on the surface of the
water after laying their eggs. Like
mosquitoes, dragonflies, and damselflies, mayflies molt several times when in
their larval stage, during which they are also known as “nymphs.” Nymphs graze on bacteria on the river
floor. As they grow older, they form
oval-shaped gills which beat in order to regulate the flow of water and oxygen
through the body.
Mayfly Larva
During the final molt as a nymph, the
mayfly floats to the surface of the water and opens its wings to enter the
first flying stage, where it is known as a subimago. Floating on the surface of the water, waiting
for the strength to fly, this is the most vulnerable stage of the mayfly’s
life. Once it has the strength to fly,
the mayfly will find protection under a tree or in long grasses, where it will
again molt within 24 to 48 hours, entering the second flying stage as an
imago. During this final molt, the
mayfly’s tails and legs grow larger, giving them more stability in flight and
greater reproductive success, respectively.
Mayflies, in both the nymph and adult stages, can be recognized by their
three caudal filaments, or tails, at the tip of the abdomen. As adults, their large forewings are usually
kept upright, while their hind wings are reduced or nonexistent. Once it becomes an adult, the imago will mate
and then live for only hours to a day.
This is the reason that the order Ephemeroptera was given its name: ephemeros is Greek for “lasting but a
day.”
The adult mayflies may only last for a day,
but they let us know that the ecosystem is strong and healthy. In their various life stages, mayflies
provide food for animals as varied as snails, fish, frogs, birds, beetles, and
flies. Mayfly nymphs are extremely
sensitive to pollution and poor water quality, so their presence indicates a
very healthy stream. Students in Water
Ecology, collecting the macroinvertebrates they find on the stream bottom,
always come up with mayfly larva, those organisms that tell us just how clean
the Big Flatbrook is. As an indicator
species, this aquatic insect is very instructive of the healthy aquatic
environment that students wade through on sunny afternoons. Whether they find “wigglers,” “tumblers,”
“toothed ones,” or those who “last but a day,” students curious enough to get
their feet wet and flip over a few slippery river stones have a great deal to
learn from the aquatic insects found in the Big Flatbrook.
References:
Introduction to the Odonata. (n.d.). Odonata: Dragonflies
and Damselflies. Retrieved May 23, 2012, from http://www.ucmp.berkeley.edu/arthropoda/uniramia/odonatoida.html
Mayflies. (n.d.). Ephemeroptera. Retrieved May 23,
2012, from http://www.ucmp.berkeley.edu/arthropoda/uniramia/ephemeroptera.html
The dragonflies and damselflies of New Jersey. (n.d.). New Jersey Odonata. Retrieved
May 23, 2012, from http://www.njodes.com/
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