Download Unit 9 in Entomology [1] We`ve learned what insects are, how they

Unit 9 in Entomology
[1]
We've learned what insects are, how they reproduce, how they digest their food, how they move around,
some of their behaviors, and now we'll learn how insects adapt to their environment. Unit nine,
adaptation to habitats.
[2]
In this unit we’ll discuss some ecology and related terms, we’ll build a simple food web using only insects
for the primary, secondary and tertiary levels, we’ll describe ways insects have adapted to the soil and
aquatic environments and discuss the advantages of biological monitoring and the specific indications of
poor water quality using insects as indicators.
[3]
Are you ready for an adventure? We're going to take a journey to some specialized habitats where
insects have become extremely successful. In unit three, you learned some of the adaptations, such as
gills for breathing underwater, and in lab you’ve learned several different leg adaptations for digging in
soil. Now we’re going to explore some of the specific habitats where these adaptations, and others, are
useful. Before we begin the lecture, please read your textbook readings on aquatic insects and also
read the chapter on soil insects.
[4]
As in other biological systems, there are always terms that need to be defined. Let's begin with ecology.
Insect ecology is the branch of entomology that focuses on the interrelationships between insects and
their environment. For an ecologist the concept of an environment includes all of the living things as well
as the nonliving things, so it includes the biotic system as well as the abiotic system, and all of the
components interact within a framework called a natural community. A community is all of the organisms
living in a particular area and includes populations of different species of plants and animals, and many
times these are represented as a food chain. And each link of the food chain represents a trophic level
encompassing either producers, which are generally plants, or consumers, which are usually animals. A
habitat is the locality or site and the type of the environment that an organism lives in: a pond, a field, a
stump, an oak tree, these are all examples of habitats. A niche is the ecological role a species plays
within a community such as an insect that feeds on the roots of grasses or one that eats aphids on the
leaves. It’s what an organism does for a living within a habitat. A population is a group or an individual
organism that belongs to the same species and lives in a particular geographic location. And lastly, an
ecosystem. An ecosystem is the combination of the community of organisms (remember, that's like the
food chain) in an area and the abiotic factors such as the air, the water, the soil, or any other minerals or
things that are nonliving.
[5]
One way to tell about the health of an environment is through biological monitoring. Insects can be an
important tool in biological monitoring, especially in aquatic habitats. This is done by sampling the
insects in a lake or stream and measuring the number of individual insects and, most importantly, the
number of species. The results are then compared with those from samples taken in other lakes or
different portions of the stream or lake area. Some insects are very hardy and will be found in almost
any quality of water samples. Others, however, are much more sensitive and are found only in
unpolluted water. A healthy lake or stream will have a wide variety of species, including those that are
sensitive to pollution, while a polluted stream will have only a hardy species and they may not even be
present in large numbers. When taking a sample, if you only find a few species, or just the pollution
tolerant species, it means there's probably some sort of pollution in the water, either chemical or thermic.
[6]
Traditionally, water quality has been tested in the laboratory by measuring the chemicals found in water.
This method is popular because it is quick and easy to do. One problem with these kinds of tests,
though, is that the sample may be taken on a day when the pollution source is not releasing chemicals,
so no chemical would show up in the test. Another is difficultly knowing which chemicals to test for and
what effect the interaction of the different chemicals present may have on organisms. Biological
monitoring has an advantage over chemical testing because you can collect the sample at any time and
still get a reliable water quality reading. This is because the insects are constantly sampling the water
over long periods of time. If you take a sample on a day when the factory or pollution source is not
releasing chemicals, the insect you'll find in the water will not be different than the insects you would find
on a day when the factory was not releasing chemicals. The insects are there 24 hours a day, seven
days a week, so they're constantly testing the water. When taking water samples for water quality, you
usually take a sample above the pollution source and then one below the pollution source. You then
compare the results. If you have a lot of sensitive species at sample site one and then very few at
sample site two, then the source of the pollution may be a factory in between. If you find the same
number of species at both collection sites, then you would assume the factory or company is not
contributing significant amounts of pollution to that area.
[7]
Let's take a look at the diagram. Here, we have sample site one, where we’ll collect insect samples,
sample site two and a southerly river flow. So when sampling, there are several common indicators to
consider. Mayflies indicate an increase in particulate matter. That means that there's some form of
solid material being released into the water. Blood worms, which are Chironomids, increase when
oxygen levels drop. There are also chemical tests that can tell you the biological oxygen demand of a
water sample. The insects, however, are there all of the time and will give you an accurate
measurement of whether there is adequate oxygen. Chironomids are usually found at the bottom near
the sediment layers, so you would not only want to take surface samples and samples of flying insects
but you would also want to take a sediment sample, which can be obtained by taking a sample with an
Ekman dredge. Besides chemical pollution there can also be thermal pollution. One indicator of this are
stoneflies. The Plecopterans decline as water temperature increases. So if there are no stoneflies in an
area where there should be, where you would expect them to be, then maybe there's some sort of
pollution by hot water being dumped into the lake or stream. Many different insect species will actually
disappear with pesticide runoff. The pesticides getting into the stream will kill the insects and so there
will be none there for you to sample. And if there's eutrophication, there'll be an abundance of only a few
species. Eutrophication as an extra supply of organic matter within a system, so if things like wastewater
from a sewage treatment plant is being dumped into a lake or stream and there's much more organic
matter, then you would expect to collect many specimens from only a few species.
[8]
Let's take a look at aquatic habitats. Well, there are different kinds of aquatic habitats and with each
habitat comes different insect fauna and different functional feeding groups. The aquatic habitats we’ll
discuss include lentic, which is standing water like a pond or a lake, lotic which is running water like a
stream or river, marine, which is intertidal and littoral as saltwater intrusion, and also temporary bodies of
water, like puddles or crab holes or tree holes. Important factors when considering aquatic habitats
include the water speed, the water temperature, and any dissolved solvents like salt or organic matter.
[9]
You know that some insects are herbivores, some are carnivores, and some are decomposers. Well, we
can further divide those feeding groups into functional feeding groups. And there are some specific
functional feeding groups for the aquatic habitat. And these include the shredders, which feed on living
or decomposing plant tissues, and this can include wood which they can chew, they can mine or they can
gouge. An example of this is the water boatman. There are collectors which feed on fine particulate
matter by filtering particles from aqueous suspension, so these would be things like black fly larvae or
even caddis flies. There are scrapers, which feed on attached algae and diatoms by grazing solid
surfaces so if a leaf or piece of wood falls into the water, to the aquatic habitat, it will actually graze, just
like a cow in a pasture, will graze the things that are growing on whatever fell into the water. There are
also piercers which feed on cell and tissue fluids from vascular plants of larger algae by piercing the cell
wall and sucking out the content. There are predators which feed on living animal tissue and these can
include Helgrammites, Megaloptera, and they're only found in clean water, so it is usually quickly moving
water. There are parasites which feed on living animal tissue as external or internal parasites on any
stage of another organism. And lastly, there are scavengers which feed on dead plant or animal
materials, and this can be called detritus. And one example of this as a water strider.
[10]
Take a close look at the drawing of the stream in front of you. This is a cross section, so you looking
down the deepest part of the stream with a shoreline on each side. Below the stream are enlarged
pictures of insects with lines indicating where they would be found in the stream. Think about where in
the stream these insects are located. Is the water moving fast or slow? Is it deep? Is it shallow, rocky,
muddy? Think about the functional feeding groups. What are these insects eating? On the next few
slides we’ll discuss the location of these insects in this kind of habitat. You may want to refer back to this
diagram from time to time.
[11]
Okay, let’s start with the mosquito larvae that you saw on the left-hand side of the diagram. Notice that it
is found in the quiet part of the stream and it is near the surface of the water. The mosquito larvae
represents a number of insects that hang from the surface tension of the water as though they were
hanging from the ceiling. They do this so they can remain near the surface and breathe through their
snorkel like tail that has a spiracle opening at the end. They hang from the surface using a special ring
of hairs that surround the spiracle. These hairs extend out over the surface of the water like grappling
hooks. The hairs, spiracle opening and trachea are all hydrofuge structures. That means that they are
water repellent and will allow the insect to hang there. As these insects are not fixed at the surface, they
can move their muscles and fall onto the bottom to avoid predation, but eventually they'll float back up to
the surface so they can breathe.
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Now let's move on to the damselfly larvae. This insect represents predacious aquatic insects that
ambush their prey as they swim by. It also represents insects that have developed gills to help them
breathe underwater. They use their gills for cutaneous gas exchange. Next is the water strider. The
water strider represents those insects that live on the water’s surface, supported by the surface tension.
These insects also have hydrofuge structures similar to the mosquito larva. In this case, instead of hairs,
it is their long legs and tarsi. Water striders in the family Gerridae are also scavengers of dead and dying
insects that fall on the water’s surface. Water boatmen represent those insects that are simply good
swimmers. They graze on aquatic plants near the bottom of the stream or pond. The water boatman
also represents those insects that carry with them air from the surface, as a film or bubble called a
plastron on the outside of the body or between the wings and abdomen. As oxygen is sucked out of the
bubble of air, the ratio of nitrogen in the bubble increases, causing the bubble to absorb oxygen from the
water. This gas exchange increases the amount of time the insect can stay underwater. It's like having
its own scuba tank. We've covered about half of the insects in the diagram, so you may wish to go back
to the diagram now and see where these insects fit into the aquatic system. One arthropod we didn't
cover was the springtail. Remember the Collembola is a springtail that we discussed in unit three.
[13]
The mayfly larvae illustrated in the diagram is a type that burrows in the mud at the bottom of lakes and
slow moving streams and rivers. They eat small food particles that float past in the water. The caddisfly
larvae is a case builder. It builds a home for itself out of rocks or other debris. It feeds by grazing on the
algae or other plant material on the bottom of the habitat. Other caddisfly larvae actually spin a web
between rocks on the stream bottom in which they catch small food particles. They are kind of like
fishermen, only they're collecting plant material. The stonefly is an example of an insect that is adapted
to living in a strong current by having an extremely flattened body and strong grasping legs. It feeds on
other insects or on pieces of dead leaves in the water. For an example of an insect living in an extreme
aquatic habitat, take a few minutes to watch the video titled “Brine Flies”.
[14]
(Brine Flies video)
But a place doesn’t have to be dry to be a desert. Mono Lake in southern California—so salty that few
creatures can survive here except primitive algae. And, of course, insects. This is the domain of the
brine flies. No predators, no competitors, nothing to keep the numbers in check. At the peak of the
season, there are 2,000 tons of flies around this shore. There’s nothing to eat on the shore, but there’s
plenty underwater. The brine flies submerge in a self-contained bubble of air, and feed on the algae. In
searing heat or in bitter cold, insects can survive.
[15]
Let's move from the wet habitat to the dry habitat in the soil environment. Many, if not most, insects
spend at least part of their life in or on the soil environment. Most only spend the egg, larval or pupae
stage there, with the pupal stage being the most common. Advantages of living in the soil include a
constant and moderated temperature that is cooler than the outside air in the summer and warmer than
the outside air in the winter. The air down in the soil also is very humid, which most insects prefer. The
soil also provides concealment and protection for vulnerable eggs, larvae and pupae.
[16]
Take a look at this soil profile. If you look closely you'll see some definite differences between the
insects that are deeper in the soil than those on the surface. Take a look at the appearance of the
insect: the appendages, the color, and the body shape.
[17]
The insects at the top of the illustration are those that live both above and below the soil surface, and
often inhabit any leaf litter that might be present. These insects in general have good vision, pigmented
bodies, so they have color, and they have long appendages. As you move deeper down in the soil
profile, a trend in the appearance of the insects becomes apparent. The length and size of the
appendages are reduced, pigmentation of the body is lost, the bodies are less scleritized and eyes are
reduced or are completely absent. Other less obvious adaptations include rings of short spines around
the body that help the legless insects undulate through the soil. Just as a thought question: many
insects that live their entire lives in the soil are very small. Why do you think being small would be an
advantage in the soil?
[18]
Insects are great at recycling nutrients in the soil. Below is a diagram of how a dung beetle recycles
nutrients. A dung beetle will live below a dung pad, usually a cow pie, and makes burrows in the soil.
She forms the dung into balls called dung bells she will pull down into her burrows. Within these dung
bells she will lay her eggs. As the eggs hatch, the larva will feed on the nutrient-rich dung. Because of
her dung bells, the dung beetle helps to put nutrients back into the soil and she also helps to aerate the
soil, which helps plants to grow.
[19]
Ants help the environment as well, because they often form intricate burrows beneath the earth’s surface
and these also helps to aerate the soil. Many of you may have watched ants pull all sorts of food into
their nests. Often ants will carry seeds, pieces of leaves, and other organic material into their
underground homes. In so doing, the soil is enriched with organic material and the ants are fed.
Leafcutter ants are famous for doing this. Each leafcutter ant will clip a large portion of a leaf with its
powerful jaws, carry the leafcut to the nest, where the leafcuts are used to grow a fungus that the ants
harvest for food. If you've ever visited an insect museum at a zoo, you'll probably have seen these
leafcutter ants. They'll even take larger flowers into their burrow, which makes it quite colorful. Take a
minute now and watch the video titled “Leafcutter Ants”.
[20]
(Leafcutter Ants video)
But the most famous tropical ants are not so much weeders as plunderers of plants. Leafcutter ants
remove more greenery than any other animal from South American forests, and they devastate human
crops. They use powerful jaws to slice the thickest leaves. Each segment is the equivalent of a
500-pound weight, but ants have tremendous strength. Their labors continue day and night in a complex
process involving many ants of all sizes doing different jobs with great efficiency. The scale of the
operation is vast. Returning to their nest, leafcutters run the ant equivalent of a marathon at a 4 minute
mile pace all the way. In Ed Wilson’s eyes, theirs is the most impressive of all ant societies. “This is the
surface of the nest of just one colony of leafcutter ants. A colony this size can hold as many as two or
even three million workers, all the daughters of a single queen that they keep deep down inside the nest.
As the foragers go into the nest, they proceed on downward for as much as 15, 20 feet. That’s, in
human terms, equivalent of about a mile. In its lifespan, the colony can move as much as 40,000 pounds
of soil. That’s the ant equivalent of the Great Wall of China. That means that the leafcutters are pretty
vital elements of the rainforest’s ecosystem—they move, they aerate, they fertilize more soil than
earthworms.”
[21]
You may have heard of a food chain or a food web. A food web represents the path that energy takes as
it passes through a community of organisms. This energy is passed on when an organism is eaten by
another organism, as indicated by the arrows in the diagram below. Written to the right side of the food
web are the labels indicating different trophic levels. These trophic levels represent how many
organisms the food energy has passed through. So at the very bottom of the diagram you have your
producers, then the animals that feed on the producers are known as primary consumers. Then the
animals that feed on the primary consumers are secondary consumers and those that feed on secondary
consumers are tertiary consumers and on up and up and up to the quaternary consumers, etc.
[22]
Now let’s take a closer look at these levels. At the bottom of the food web is the producer trophic level.
These are the plants that actually make the food out of energy from the sun, soil nutrients, and carbon
dioxide. These are the food sources that will be used by all the other organisms. The next trophic level
is the primary consumer level. As the name indicates, these are the animals that consume the
producers. Included in this level are all the herbivores, which are plant eating animals, from
grasshoppers all the way to cows. On the third level are the secondary consumers which feed on the
primary consumers. Examples include a wolf eating a deer, or us, as humans, eating a steak, or a
praying mantis eating a grasshopper. If you look closely, you'll see that things start to get complicated at
this level because two of the secondary consumers are shown to feed on plants directly. This shows that
the trophic level an organism represents is solely dependent on what it eats. For example, the mouse on
the secondary consumer level is placed there because it is eating a grasshopper. However, the cream
colored arrow shows it can also act as a primary consumer when it eats plants, probably seeds.
Because many organisms eat a variety of food, food webs can be extremely complicated. Just think of
all the different types of food you eat.
[23]
So you can see how complicated a food web can actually get. I would like for you to draw a food web
similar to the one on the diagram. Begin with the producers at the bottom and then on each level above
that, I would like you to substitute all of the animals with insects. The food web you make doesn’t need
to be as extensive as the one on the diagram but should include at least eight different kinds of insects
that extend all way up to at least the tertiary level. You can either draw the insects, use clipart of insects,
or simply write the name of the insect, but be sure to be fairly specific. If you can, use the family level.
You've learned several family names throughout this unit. When describing your insect don't just say
beetle or fly. Remember, it is important to know exactly what the insect is feeding on because they can
fit on different levels depending on their diet. If you would like some feedback on this assignment, feel
free to send an e-mail, because it will be fair game for the exam.
[24]
The process of transfer of energy may actually surprise you. To the right is a figure depicting the amount
of energy that is actually converted into new biomass in each successive trophic level. The numbers at
the right indicate the number of joules of energy contained in the bodies of organisms at that particular
level. In this figure, there are 10,000 J of energy stored in the body of plants at the producer level. If the
primary consumers were to eat 10,000 J worth of plants, only 1000 J worth of heat energy would actually
be turned into grasshopper bodies. The rest would either be burned as energy or simply passed on as
excrement. The same amount of loss would occur on average at the next two levels which is why a field
can support more mice than it can snakes. This is called the general rule of tens, since on average only
10% of energy is transferred from one trophic level to the next. So you can see how little energy is
transferred to the tertiary level, that started out at the producer level. So just as a thought question: if
you were the leader of a food starved nation and had to choose to use the land for grain or for beef
production, which would you choose? Remember grain would be on the producer level and cows are
herbivores and would be on the primary consumer level.
[25]
Review Quiz
[26]
In conclusion, make sure to review the terms given in this lecture and be able to create a food web with
different trophic levels. Also, be able to identify different species by their functional feeding group. The
aquatic feeding groups were listed in the lecture, but you have to dig out the ones from the soil readings.
Just so you have an idea of some of the terms, they include saprophage, xylophages, coprophage,
necrophage, mycophage, etc. There is quite a bit of textbook reading for this unit, but it is really
interesting. When else you get to read about corpse-eating? Pretty intriguing, huh? This concludes
unit nine.