7 Unit in Entomology
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For all of you fans of Survivor and Lost, this unit is for you. Unit seven, getting together and staying
alive.
[2]
In this unit you will need to learn to name and describe five ways insects get together to mate. Using an
example, describe each of the different ways insects protect themselves. You want to define two types
of mimicry: Mullerian and Batesian, and also define and give examples of aposematic coloring behavior.
In this unit you will watch several videos, so you will need to glean some information from these videos,
so pay close attention to your study guide, so you know what to look for as you watch the video clips.
[3]
The primary goal of any organism, including insects, is to pass on its genes through reproduction. In
order to do this, an organism must eat, grow and protect itself so that it can reproduce. Through this unit
you will learn how insects find mates and protect themselves by examining five reproductive strategies
and nine protective strategies.
[4]
First let's start with reproductive strategies. In unit six, you briefly learned how a female silk moth uses a
sex pheromone to attract her mate. This is just one of many fascinating reproductive strategies insects
employ to entice reproductive partners. We are going to discuss some specific mating strategies, but
before we get started, be sure to read the reproduction section in your text. Some of the strategies are
pretty racy. The five strategies to be discussed are looking good, smelling good, sounding sweet, lamp
light, and buying love.
[5]
Are you intrigued? Well, let's start with looking good. Specific insects can rely only on their looks to
attract a mate. Males will look for characteristics of his species and when he finds a female that matches
his ideal, he mates with her, or vice versa. The more a potential mate looks like the visual ideal of the
species, its chances for reproduction is much greater. At times, the one seeking the potential mate may
mistakenly mate with the wrong species or a bottle as seen in the video clip about jewel beetles. Take a
few minutes now to watch this video. (This has to be the all time class favorite of all the video clips that
we watch.) This clip is entitled Jewel Beetles and Their Beer Mates.
[6]
(Video Jewel Beetles and Their Beer Mates)
Some insects use vision to find a mate. Theyre attracted to simple good looks. A female jewel beetle
flaunts her golden dimpled wing casing, which is irresistible to a male. Shes also big, which to him is
beautiful. But in recent years, males have discovered other temptations. For a male jewel beetle, its
the simple things that are exciting. If its orange and dimpled, its just what hes looking for. Its
enormous. This must be a super femalea jewel beetle goddess! Only certain beer bottles have just
the right color and texture. All the males in the area are besotted. The real females are ignored. With
so many males on the bottle, the beetles were in danger of extinction, until the breweries began
redesigning their bottles, hoping the males would spend some time with their females.
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Well, perhaps one of the most efficient ways to attract a mate is by smell. That would have gotten those
jewel beetles out of a lot of trouble, wouldn't it? Anyway, pheromones are efficient to produce and can
travel great distances. Insects rely heavily on sexual pheromones to get together and reproduce.
Usually, the chemicals are produced from glands located in the abdomen and released from the
abdomen. As you learned in unit six, the female silk moth secretes a sex pheromone that is detected by
a male moth. He detects the chemicals with his antenna and flies upwind in a zigzag pattern until he
finds her. This is illustrated in the diagram above. Take a minute now and once the video clip titled Silk
Moth Seeks Mate.
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(Video Silk Moth Seeks Mate)
An abandoned warehouse is a huge, complicated labyrinth, so how can a lone silk moth hope to find a
mate here? Silk moths have been introduced from the rainforests of Asia, a vast place, and supremely
complex. There, silk moths developed a system that overcame their daunting surroundings. She
advertises herself with scent, a pheromone released from a gland on her abdomen. The scent consists
of long molecules with a particular shape. These are only recognized by this kind of moth. They fit like
a key in a lock into special receptors on the male moths antennae. His antennae are so sensitive, it only
takes a half-dozen molecules to excite him. He could detect her a mile away. By flying upwind and
tacking across the waves of scent, he moves in her general direction. As he gets closer, the scent grows
stronger and the waves turn into a flood. She could be anywhere here; it seems like shes everywhere.
Hes overwhelmed, confused. He knows hes very close, but where is she? They do finally find each
other, but scent is most effective over long distances. At short ranges, it can be confusing.
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To study how the insects do this, scientists have dissected off an antenna and looked at it structures and
looked how it worked. They do this by forming an electroantennogram. Scientists have found that an
isolated male silkworm moth's antenna produced an electrical signal when exposed to a chemical called
bombykol, but did not respond to any other chemical. An electroantennogram, also known as an EAG, is
used to test such signals. An EAG involves blowing a whiff of any material to be tested in a gentle
stream of air over an isolated antenna. Any electrical signal the antenna produces is then recorded by a
computer. This technique is not difficult to perform. The EAG has become standard practice today for
anyone doing insect signaling research. You can see an EAG in the diagram below. Notice the signal
must be amplified, and the read out, looks similar to the one you see on the oscilloscope.
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So, now we know how insects look good and smell good, but how do they sound? Well, some insects
use smell and sound to attract a mate, like the polka dot moth. Other insects just use sound to entice
reproductive partners. Some even employ ultrasound, sent through plant stems, to attract mates. Male
cicadas, crickets and grasshoppers are known for their ability to produce sound. Most of us have heard
their buzz on a warm summer night. Each species has a distinct song. Crickets do not rub their legs
together (as most people think, mainly from the cartoons) to produce their characteristic songs but
instead they rub their forewings together. Since I mentioned how the polka dot moth uses smell and
sound to attract a mate, let's take a few minutes to watch 2 video clips, the first, Polka Dot Moth Mating
Call and the second, Leafhopper Call. Notice how the different calls are very specific for each species.
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(Video Polka Dot Moth Mating Call)
The scent is most effective over long distances. At short ranges, it can be confusing. Thats why this
polka dot moth from the south has developed a distinct short-range system, as well. A male moth hones
in on a female scent. She avoids any last minute confusion by switching to a finer signal, a beam of
sound. Not that humans would recognize it as sound. The moths music is pitched several octaves
above human hearing. Moths hear an ultrasound. She gives him precise directions with a medley of
ultrasonic clicks. As he closes in, he clicks back. The problem with broadcasting sound, however, is
that some predators can also tune in. These brightly colored moths are very poisonous, so they dont
care who else hears their broadcast.
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(Video Leafhopper call)
Leafhoppers are not poisonous, so when they use sound, it has to be as private as a phone call. They
transmit vibrations through their legs to the plant stems, and the sound is received by touch. In other
words, leafhoppers hear with their feet. The males move from plant to plant, calling and listening for
females of their own kind. Theyre not distracted by sounds in the human world, as humans are
completely unaware that their garden plants are alive with secret music.
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Cicadas produce sound by vibrating a drum-like structure on their abdomen called a tymbal. As the male
plays his song, the female picks up the sound with a smaller organ called a tympanum. This organ has a
membrane stretched tightly across an air-filled chamber, similar to how a membrane is stretched across a
drum. When sound waves strike the tympanum membrane, it vibrates just as when you strike a drum
with a stick. The vibrations trigger a nerve, sending the message to the brain and cause the female to
respond to the male's call. Many kinds of insects have these tympanal organs. They can be located in
a variety of locations on the insects body, including the tibia, the thorax, and the abdomen.
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You'll have another assignment later in this unit, but for now I have a thought question for you. What
might be the advantages and disadvantages in using sound to attract a mate? What are the advantages
and disadvantages in using smell? Take a minute to jot down some of your ideas, then go on with the
lesson. We will come back to these questions at the end of the unit.
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Now let's take a look at lamp light. Fireflies are another common insect you may have encountered on
warm summer nights as they flash and light up the dim twilight. Fireflies use their lamps to attract mates.
Their light organ is located in the last few abdominal segments and each species has its own flashing
pattern. Remember fireflies are a beetle, so keep that in mind to think about the structures. Male
fireflies fly above grassy areas or bushes where female fireflies wait. A male of species A may flick
three short flashes and one long, and the female of species A will flick a single flash back. In contrast,
a male of species B could flick two short flashes and two longs flashes, and thus, would not attract
females of species A. A unique behavior that has evolved with these light organs, females of Photuris
species lure males of different species by mimicking their female's flashing pattern. When the misled
male lands, the fraudulent female attacks him and then eats him. This bizarre behavior was discovered
by a firefly expert, Dr. Jim Lloyd, a professor here in the Department of Entomology. You can read his
journal article Aggressive Mimicry in Photuris: Firefly Femme Fatales which will give you a personal
account of his discovery.
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So what does a guy do when he wants to win a girl over, he buys her flowers or chocolates, or perhaps
even builds up a small estate? Male insects do something similar for their potential mates. A male fruit
fly, for example, will stake a claim to a walnut as his estate, so a female will use his walnut to lay her
eggs. Stag beetles will fight off other males to stake out his estate, so a female will mate with him.
Some males even spin silk in their mandibles to wrap up gifts for their sweetheart. Watch the following
two videos to see how insects buy love.
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(Video Stag Beetles Battle for Mating Rights)
Real males in the insect world do resort to combat, and evolve nasty weapons just for fighting duels.
Outside a temple in Thailand, deerhorn stag beetles clash head-on for the honor of claiming a female.
But the fight isnt just a brawl. It is a ritual, every bit as formal as anything inside the temple. There are
rules to follow. The first to dislodge his opponent is the winner. His prize is the female and she gets to
mate with the champion. Females never have time for losers.
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(Video Fruit Fly Mating)
A male walnut fly also has a simple goal in life: he has to lay claim to a blemish on the surface of a
walnut. Its not easy. Other males will challenge him. Every male knows that a female fly needs to lay
her eggs inside the hard shell, but she can only pierce the casing through a soft blemish. The male that
controls the blemish will be guaranteed a visit from a female sooner or later. Theres no room for a rival.
These two males could just fight it out, but the bigger one would win, so why not just take each others
measurements, decide whos bigger, who would lose, and avoid the battle altogether? The dispute is
settled and no one gets hurt.
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Now let's move from reproductive strategies to protective strategies. As you have seen in the five
reproductive strategies discussed, insects have evolved fascinating behavioral patterns in order to mate.
They have also evolved incredible strategies to protect themselves, especially in developing marvelous
camouflage, powerful stinging weapons, and the ability to spray poisonous chemicals. These three
strategies, along with six others, will be discussed in more detail. The lecture slides will highlight each
strategy, but don't forget to read the textbook where you'll find some great examples of these strategies.
The nine protective strategies are: timing, radar detection, hiding, speed, armor plating, armed and
dangerous, chemical warfare, imposters, and migration. Notice the picture on the right. Those arent
thorns, those are actually small hemimetabolous insects related to cicadas.
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Timing. Insects have developed biological rhythms that have proven very successful for the continuation
of their kind. Some have adapted to molting in the early morning when humidity is high, emerging in the
cooler months when predators are not as active or emerging at the same time so chances for mating
increases or simply going into diapause to escape harsh temperatures. Diapause is a time when there is
little or no growth such as in the egg stage, larval stage, or pupal stage. In diapausing adults, it is a
period of no reproduction. It is similar to hibernation. As the days shorten and temperatures cool in the
autumn months, this triggers a response in the insects brain. Developmental or reproductive hormones
are no longer released depending on the stage the insect is in. In diapausing pupae, ecdysone is not
produced, and thus, the pupae cannot emerge until the hormone level surges to stimulate molting. In
diapausing adults, the corpora allata inactive is and reproduction is no longer stimulated by the juvenile
hormone. When spring comes and the days begin to lengthen, the brain responds and stimulates the
secretion of the suppressed hormones. The pupae will then molt as ecdysone is released, and
reproduction will begin in the diapausing adults as the juvenile hormone levels return to their normal
levels.
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As an insect enters diapause, it becomes relatively inactive but not immobile. Its metabolism slows
down, so the need for food is very low, and there's no need for the insect to be moving about searching
for food. It seeks a protective covering such as a bear searches for a cave to hibernate; there it will wait
out the winter. This does cause problems for some people, things like ladybugs aggregate in the winter
time and they might find a nice niche to hold out the cold months in a window sill in your house.
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Radar detection and hiding. Nocturnal predators of insects have to rely on other senses besides sight to
detect their prey. They have to rely on chemical and/or audio cues. Bats target their prey with
echolocation by sending out high frequency sounds which bounce off victims and reflect back to the bat
ears. Even though bats have this terrific mechanism to catch prey, some insects have developed ways
to get around it. Praying mantids can detect the radar, and drop out of sight before a bat can catch it.
Camouflage is a terrific way to avoid predators. What the predators don't see, they can't catch. Insects
have developed remarkable concealing techniques such as seen in a geometrid moth below. The wavy
lines and blotchy patterns conceal it when up against a tree. Treehoppers seem to be a part of a tree
stem or can look like a thorn on a branch. Walkingsticks, as their name suggests, look like twigs or
branches. To see how the praying mantis escapes the bat, watch the video Praying Mantis Maverick
Maneuver.
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(Video Praying Mantis Maverick Maneuver)
In its miniature universe, a praying mantis is a fearsome predator, but it still needs its own defenses.
Every insect is just a convenient mouthful of protein to bigger animals. When it takes to the air, the
predator becomes the prey. This mantis has been targeted by the sonar of a bat. But with its single
ear, the mantis can hear the bats pulses of ultrasound. When the bat gets closer, the mantis
deliberately stalls and then tumbles out of the air to safety. And to the bat, its target has just
disappeared.
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Speed is an obvious strategy for survival. If you are too fast, then your predator can't get you. Tiger
beetles are an example of this; they are adept runners. Grasshoppers jump quickly and high before a
bird can snatch them. Flies are great evaders when trying to kill them with a fly swatter. Cockroaches,
well, they start running before they even think about it. It's great for them, but an annoyance for us who
would like to get rid of them.
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Sometimes insects use their abilities just to survive harsh environmental conditions. Because of their
tough exoskeleton, insects often can survive massive blows, such as being run over by a Land Rover.
Other mechanical defenses may be slippery scales or cuticle, or autotomy, which is the ability to lose an
expendable limb. Take a minute now and watch the video Beetles Rock Hard.
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(Video Beetles Rock Hard)
Less athletic beetles rely on being tough. That was the human equivalent of being run over by a
steamroller.
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I think we have all been the victim of the insect defense, a sting. Well, bees and wasps give terrible
stings when attacked. The ovipositor of a female is modified and fused to form the stiff piercing organ
that stings. Glands near the ovipositor release toxic venom into the wound made by the weapon.
Honey bees have tiny hooks at the tips and once the stinger has been pierced into the victim, the barbs
prevent the stinger from being removed, thus, as the bee struggles to get away, the stinger is left behind
pumping more and more venom in the prey. The only consolation is that as the bee flies away, it leaves
part of its internal organs, so they will go away and die. Keep in mind, this is only true of honey bees.
As if it weren't irritating enough just discussing stings, let's take a minute and watch a video entitled Bee
Sting. It really will make your skin crawl.
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(Video Bee Sting)
Now, the sting of the beequick in its action, persistent in its pain. Once discarded, remarkably it
continues to deliver its poison. This is but one weapon within a battery, lined up against the world.
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The military werent the first to come up with weapons of mass destruction. Insects have always used
chemical warfare. Using chemical warfare is a powerful protective strategy. Insects can either taste
bad or use chemicals to thwart predators. An example of this is the bombardier beetle. The bombardier
beetle sprays a noxious chemical from his abdominal tip that irritates toads, ants, and even large spiders.
Some insects use a more subtle chemical warfare. As they are feeding on foliage, they incorporate
poisonous plant chemicals into their body tissue; this makes the insects unpalatable to others. For an
example of chemical warfare watch the video Bombardier Beetle.
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(Video Bombardier Beetle)
Of all the insects that are chemically protected, there is no doubt which takes the prize: the bombardier
beetle, its weaponry a salvo of artillery. Its a high-tech beetle without parallel in the animal kingdom.
No insect has been so much sheer fun to work with as the bombardier beetle. Just watching how it fends
off an attack is spectacular. You can actually see it fire and hear the shot. Its long been known that
these beetles spray poisonous quinonesthey fire them from their abdominal tip with a crack and a puff
of smoke. Bombardiers have a well-stocked armory. If under constant attack, they can fire more than
twenty times and theyre amazingly quick at it. Their escape is immediate, leaving the spider with a
mouthful of quinone. It wipes its body on the ground. Then it frantically cleans its mouthparts. Toads,
too, hunt at night. They gulp their food, striking and swallowing at one fell swoop. The bombardiers
fast retort stops the toad in mid-action. The crack, the puff of smoke and the instant action all point to a
very sophisticated form of defense. What is it that the toad feels on its tongue? Is it just an irritation,
some sort of chemical soreness? Eisner put the question to the test. I popped the bombardier beetle
in my mouth and could sympathize with the toads. There was an acrid taste and also an immediate
peppery burning sensation. And the beetle seemed to be such a phenomenal marksman. Even small
ants were always hit.
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Another defense is aposematic coloring. Usually when an insect is poisonous, they have aposematic
coloration or behavior to warn predators of their distastefulness. The word aposematic means being
conspicuous and serving to warn. The Utetheisa moth, picture a few slides before, has a brightly colored
pattern making it very conspicuous to predators. Remember it eats poisonous legumes so therefore is
distasteful to birds. If a naive scrub Jay eats an Utetheisa moth for the first time, he quickly finds out how
unpalatable the moth is, as his stomach is upset and he vomits his dinner. It is difficult for him to forget
the bright pink, black and white pattern. The next time he sees this moth, he will avoid eating it,
remembering his unpleasant experience. A good example of aposematic coloration are wasps and bees,
because of their unmistakable yellow and black markings. Once an unsuspecting predator is stung by
one of these, he won't forget what they look like, because they instantly recognize the pattern. The next
time he sees anything that resembles the pattern, he will quickly get out of harm's way, even if the insect
is not harmful. The insect to the right it's actually a wasp that looks like an ant and they do have a
powerful sting. For an example take a look at the video Quinine: The secret of My Success.
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(Video Quinine: The Secret of My Success)
If an insect has evolved a way of being insensitive to a plants poisons, then why not use these poisons in
its own defense against others? Eisner and his colleagues have made a detailed study of one moth that
lives on a poisonous legume, and the research turned out to have an unexpected twist. The moth is
called Utetheisa. We first noticed that the caterpillars of Utetheisa competed for the seeds of the plant.
Thats where the poisons are concentrated and theyre very potent. Chemically, theyre a type of
alkaloid, bitter and highly toxic to us. The caterpillar chews its way into the pod and eats the whole seed.
When the larvae are crowded on the plant, there may be winners and losers in this competition. Those
best able to break into the pod may end up with the most poisonous alkaloid in their bodies, and as an
adult, the moth retains them. Unlike many other moths that remain hidden by day, Utetheisa risks
exposure. The poisonous alkaloids are its defense, the coloration its warning. Red, white, and yellow
make it extremely conspicuous. Bright and attractive, it lives in a world full of many dangers. Utetheisa
has problems with spiders. If it flies into an orb-weaving spider, it might be eaten. It may have problems
with bats, and of course it may have problems with birds. Birds are among the major enemies of
Utetheisa. They feed on many insects, and these scrub jays, in particular, are aggressive toward insects
that come within sight. Eisner tested the moths palatability. He offered the scrub jays edible and
inedible insects. First, a tasty moth. Its eaten with relish. Now the jay is given a choice: edible insects
in the palm and an Utetheisa between thumb and forefinger. All but Utetheisa are taken. The warningly
colored moth is undoubtedly seen, but ignored. It will survive another day. The gourmet jay remains
selective and only accepts the best. Another edible moth is grabbed and eaten piecemeal. Pulling the
wings outthey often do that. Wings are just scales and paper, essentially. Its like taking the wrapper
off a candy bar. The head. Thorax and body, that was the good part. Yeah, you liked that, didnt you?
Utetheisa continues to be ignored, whether offered by itself or in combination with edible insects. It
survives an occasional peck, but is obviously well-protected.
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Now we move on to another defenses strategy, imposters. We will first cover Batesian mimicry. Rather
than just having the conspicuous coloring to warn off predators, some insects display conspicuous
behavior. They may purposefully fly slowly and deliberately so that their aposematic colors are well
seen. Some firefly larvae will flash their lamp organ when a predator approaches to warn them. Firefly
larvae are also distasteful to many organisms. So why not just mimic a wasp's bright yellow and black
pattern? Using another insect's aposematic coloring to your advantage makes sense. That way a
predator would think you are a wasp and leave you alone. Mimicking an organism that is harmful, but
youre not yourself harmful, is called Batesian mimicry. Look at the insect on the left. At first glance you
would think it is a wasp, but it's actually a moth, a harmless moth, mimicking a wasp. Were you fooled?
Be careful for things like this, whenever you take lab quizzes; many times your lab teacher will throw one
of these in the fool you on the exam.
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Another type of mimicry is Mullerian mimicry. Mullerian mimicry is a little different in that both species
mimic each other and they are both bad to eat. The monarch and the viceroy butterflies are malaria
mimics of each other. They look alike but they are both harmful to birds because as larvae the
caterpillars feed on milkweed and milkweed contains toxic chemicals that induce vomiting in birds and
cattle. In the past, it was thought monarchs and viceroys were Batesian mimics of each other, but this is
not so. It was not originally known that viceroys were as unpalatable to birds as monarchs.
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The last defensive strategy we'll cover is migration. Just as birds migrate to escape the cold northern
winters, insects have developed intricate migration patterns as well. It's hard to imagine such tiny
organisms traveling extensive distances. Monarchs are famous for their 1000 mile journey to Mexico
and southern California each year. At each wintering site, you can find millions of butterflies just on one
tree. Not only do insects travel south to escape harsh winter conditions, they have also been known to
migrate to find more favorable food conditions. Locusts have been recorded to migrate from Western
Africa to the West Indies of North America. This is an estimated 3100 miles and it took six days to fly
that distance. Scientists speculate that the locusts probably took advantage of the wind currents across
the Atlantic to glide some of the distance.
[36]
Remember the thought question from before? Well, now it is time for the actual assignment. So in a
journal entry, answer the following question: What are the advantages and the disadvantages of using the
following methods of getting together? Make sure to cover looking good, smelling good, and sounding
sweet. Feel free to collaborate with each other for ideas. You can either email each other, use the
telephone, or actually talk to other students, but you are responsible for your own essay, and it must be in
your own words, you are not allowed to submit the same thoughts. So insect rules of dating Look
Good! Smell Good! And Sound Sweet!
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Review Quiz
[38]
Well, that completes this unit. In review, insects have evolved various strategies to mate and protect
themselves. An organisms main purpose or goal in its life is to reproduce and continue on its species.
Insects use at least 14 strategies to accomplish this. First they have to reproduce and they also have to
survive. You have learned a little bit about each one. Make sure to review these strategies before the
exam.
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References