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Entomology
BIO 3333
INTERNAL ANATOMY
Both the Lubber grasshopper and the cockroach are frequently used
to introduce internal anatomy of insects and you’ll get a chance to
look at both during toady's lab. The cockroaches are back, you might
remember them from BIO2121? and yes they are alive! That will
require some special care in the way we handle them to be sure that
we do not have any escapees. That’s obviously not a problem with
the preserved grasshoppers. We’ll use the smaller dissecting dishes
which means there should no problem getting you dissected specimen under the microscope to take a closer look. We’ll also have
some fibre optic illuminators available at the end of some bench so
that you can see the fine internal detail a little better.
For both the live and preserved specimens observing the internal
anatomy is easier if specimens are submerged under either water
(preserved specimens) or saline (live specimens). Although at first dissecting under liquid may seem awkward it means that internal tissues
remain suspended in the bathing medium rather than collapsing into
a single mass. As a consequence it is easier to distinguish the different
organs. This is particularly true for the live cockroach and the heart
and digestive tract will continue to beat and move if you use saline to
cover the animal.
COCKROACH DISSECTION
The roaches will be knocked out using carbon dioxide and will
remain quiet for a few minutes. During that short period of time be
sure to completely remove the legs and wings and pin your animal
into the dissecting tray with its dorsal side up and pins at the tip of
the abdomen and through the sides of the pronotum. As you pin the
tip of the abdominal in place gently extend the abdomen to expose
intersegmental membranes. As you remove the wings and legs you
may have noticed a clear liquid that oozes from the wound. This is
haemolymph or blood. Why is it not red? Now that its in place take a
look with the dissecting scope and see if you can see pulsating dorsal
blood vessel, or heart, through the abdomenal cuticle. In what direction is the haemolymph moving?
Starting with the second to last tergum of the abdomen, remove each
tergite individually by cutting along the lateral edges from posterior
towards the anterior. Be careful when you make the cut you don’t
want it to be too deep, that may damage underlying organs, especially the heart. Take your time with this step and as you remove the
tergite gently tease back any connecting tissues as you remove it. At
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© JON G. HOUSEMAN
BIO 3333
Entomology
this point the underlying structures appear white or transparent and
because you used saline, and were careful, the heart should remain
in place and continue to beat. It’s the clearer streak running down
the centre of the dorsal surface. Two other structures should be visible in your dissection: considerable amount of white fluffy material
which is the fat body and numerous glistening tubes that branch and
branch throughout the body. These are the tracheal trunks and tracheal branches
Figure 1 Circulatory system in a cockroach. ©
BIODIDAC
Aorta
Segmented vessel
Dorsal diaphragm
Heart
Alary muscle
Segmented vesel
Circulatory system
The circulatory system of an insect is an open system consisting of a
heart contained in the dorsal pericardial cavity and the haemocoel
which is a series of cavities or sinuses which surround all the internal
organs.
Trace the heart backwards to the ninth abdominal segment where it
ends blindly. At the anterior end the heart extends into the thorax
where it becomes a short aorta which opens near the brain. How do
you tell the difference between the part that’s the heart and the part that’s
aorta? If you look closely at your the dissection you will see that the
heart is composed of a series of repeating diamond shapes (If the dorsal diaphragm remained attached to the tergites as you removed them
look there). This is the result of the segmentally arranged alary (or aliform) muscles that are embedded in the wall of the dorsal diaphragm
which forms the bottom (ventral) surface of the pericardial cavity.
These muscles change the shape of the pericardial cavity, and conjunction with the heart muscles open and close the ostia.
Examine the wings of a restrained cockroach that is on display and
see if you can see the blood moving through the wing veins. The
opening in the wing through which the blood moves are also exten-
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© JON G. HOUSEMAN
Entomology
BIO 3333
sions of the haemocoel and you shouldn’t confuse them with veins as
the term is used in vertebrate biology.
Respiratory system
You will notice large silvery tubes which run along each side of the
animal. These are the tracheal trunks which connect to the outside
through the spiracular openings and their silvery appearance is
because they are filled with air. The trachea are supported internally
by a spiral ring of cuticle, the taenidia, which can be observed with
the dissecting microscope. Try and trace the tracheal system. You will
notice that it extends throughout the whole body and that the tubes
become smaller and smaller. What is the difference between a tracheal
tube and a trachiole? Carefully remove a piece of the tracheal trunk
and place it under a wet mount coverslip so that you can see the
detail a little better. We also have prepared slides of the connection
between the trachea and a spiracular opening that you can also take
a look at.
Figure 2 Internal anatomy of the female cockroach. © BIODIDAC
Salivary gland
Salivary reservoir
Crop (foregut)
Thoracic ganglia
Gizzard
Abdominal ganglia
Nerver cord
Digestive caeca
Ovary (composed
of 8 ovarioles)
Intestine
(midgut)
Rectum
Malpighian tubules
Collaterial gland
Hindgut
Fat body
The large amounts of white fluffy material that hide just about everything is the fat body. This organ is more than a simple store of fats as
its name suggests but is involved in protein synthesis, detoxification
and a wide range of metabolic activities essential to an insect. The
closest analogy in the vertebrate system is the liver. (Why do we use
the term analogy rather than homology?) To observe the other organs
you will have to gently remove this tissue. Pick it out a bit at a time
and wipe off your forcepts using a piece of paper towel. You might
want to do this while you’re looking through the dissecting microscope so that you don’t pull anything out by mistake - remember
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BIO 3333
Entomology
we’re only removing the fat body and trachea that are embedded in
it.
Digestive system
Once you start removing the fat body you should start to see the
coiled tube of the digestive system that lies underneath it. Cut forward through the thorax and gently remove the muscle tissue to
reveal more of the alimentary tract in this tagma. Did you notice that
there wasn’t much else other than the muscle in the tagma - Why is that?
An insect’s gut is made up of three regions: the stomodeum (foregut),
mesenteron (midgut) and proctodeum (hindgut). The foregut and
hind gut are both lined with cuticle and the midgut contents are contained in a peritrophic membrane. What is the role of the peritrophic
membrane? The mouth leads into a tubular pharynx which passes
over the tentorium and becomes the oesophagus where it passes out
of the head into the thorax. It is at this point that the alimentary tract
becomes visible in your dissection. The oesophagus changes into a
crop, which is an enlarged sac extending into the abdomen of your
specimen (It’s also filled with air from when the roaches were
knocked out). If you gently move the muscles aside in the mesothoracic area you may be able to see whitish grey salivary glands which
lie on either side of the crop. The salivary ducts empty into the buccal cavity through the base of the hypopharynx.
Figure 3 Internal anatomy of the male cockroach. © BIODIDAC
Salivary gland
Salivary reservoir
Crop
(foregut)
Thoracic ganglia
Nerve cord
Gizzard
Digestive caeca
Intestine
(midgut)
Abdominal ganglia
Mushroom bodies
Malpighian tubules
Ejaculatory duct
Hindgut
Gently unwind the coiled portion of the alimentary canal and position it to one side so that all its parts are visible. Where the crop narrows abruptly it becomes the gizzard, or proventriculus which is a
short hardened region. The proventriculus marks the junction
between the foregut and midgut. The most anterior portion of the
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Entomology
BIO 3333
midgut is also marked by the presence of eight blindly ending tubular
outgrowths called the gastric caeca. These outpockets of the midgut
provide regions for specialised digestive events. Since the midgut is
the only part of the gut that is not lined with cuticle is the principle
site of enzyme secretion and nutrient absorption. Why would these
processes not occur where there is a cuticular lining?
Excretory system
The most posterior end of the midgut is marked by the point where
the Malpighian tubules join the alimentary canal. These fine tubes
that extend throughout the body are involved in the excretory processes and their position suspended in the haemolymph allows them
to filter and purify the haemolymph that surrounds them. Remove the
tubules and place them on a microscope slide and examine their
appearance. We also have some prepared slides for you to look at.
The Malpighian tubules mark the beginning of the hind gut which
starts with the large intestine or colon. The rectum, which is short
and is identifiable by longitudinal striations, is located at the most
posterior end of the hindgut. It opens to the exterior through the
anus.
Reproductive system
Be sure that you take a look at the both the male and female reproductive systems - you’re responsible for both! How do you identify the
difference externally?
In females two ovaries lie in the fat body on each side of the abdomen and each is composed of eight ovarioles with each of these producing eggs. The eggs pass down the oviduct and are enclosed within
the ootheca, which is a special egg case. The ootheca is then dropped
onto the substrate as a white soft structure that immediately hardens
and darkens as sclerotization occurs. This forms a strong protective
covering for the developing insects inside. If you have an intact
ootheca please dispose of it in the 70% ethanol solution that has
been provided. This is essential to insure that the building does not
become infested with cockroaches. Find the lateral oviducts and
although you may not see it you should be aware that they empty
into a common oviduct or median oviduct that opens to the exterior
via the ovipositor. The reason that you may have trouble seeing the
common oviduct is that in this region are large accessory glands
which have the appearance of wet spaghetti. They are involved in
producing the ootheca which contains the eggs. Located at the junction of the oviducts is a small brown spermatheca which stores sperm
received during mating . As the eggs pass this point in the reproductive tract they are fertilised.
In the male the testes are difficult to see since they are translucent
and dispersed within the mass of the fat body. Each testes is composed of a number of follicles and each of these produces sperm.
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Entomology
Sperm travel down the vas deferens, to the common medial ejaculatory duct and are subsequently passed to the female during mating .
The two large prominent glands with finger like projections are
located at the posterior end of the abdomen are the male accessory
glands or mushroom bodies. The male accessory glands are involved
in producing the seminal fluids that are passed with the sperm to the
female and for producing the spermatophore. If you gently push aside
the mushroom bodies you will also see a longer finger-like gland, the
conglobate gland which also opens into the ejaculatory duct.
Nervous system
The nervous system is visible once the alimentary system has been
pushed to one side. The nerve cord appears as a white line running
down the ventral midline of the animal. Remove the saline from your
dissecting dish and flood the body cavity of roach with some 70%
alcohol. This fixes the tissue and as a consequence the nerve cord
will appear whiter, but rather brittle, so be careful as you touch or
remove excess tissue that surrounds it. The nerve cord in insects is a
paired structure and a single ganglia is located in each segment. The
paired nature of the nerve cord may, however be obscured by excessive fatty tissue. You will notice that the last abdominal ganglion
appears larger then the other ganglia located within this body tagma.
This is the consequence of a fusion of the last few ganglia and this
composite ganglia is involved in the reproductive functions.
Clear away muscle tissue in the thorax and trace the nerve cord forward and locate the enlarged ganglia in the three thoracic segments.
Are they all the same size? Why might they have different sizes? In the
thorax apodemes, which are internal inflections of the cuticle, are
present and are involved in the muscle insertions. Carefully cut away
the wall on the side of the head to expose the large mandibular muscles. Carefully pick away the muscle tissue and you should be able to
locate the brain, supraesophagial ganglia which is connected, via circumoesophageal nerve cords to the underlying suboesopgagial ganglia.
Prepared slides of sections through the compound eye are also available for you take a look at.
GRASSHOPPER DISSECTION.
Secure the specimen in the dissection tray, and orient it as you did
with roach in the first part of the lab. You will be removing the
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Entomology
BIO 3333
tergites just like you did with the cockroach dissection but the diagram that you have is a sagital section.
Proventriculus
Aorta
Crop
Figure 4 Internal anatomy of the female grasshopper. © BIODIDAC
Alary muscle
Haemocoel
Heart
Ovariole
Ostia
Brain
Oviduct
Rectum
Anus
Vagina
Pharynx
Mouth
Subesophagial
ganglia
Salivary
glands
Midgut
Digestive caeca
Seminal vesicle
Malpighian tubules
Ventral nerve cord
Circulatory system
It won’t be as easy to see the circulatory system in the grasshopper.
Take the microscope and look closely at the dorsal surface before you
remove the tergites and you may be able to see the lighter line running down the centre of that surface - that’s the heart. If you cut
carefully up the sides of the abdomen, starting at the tip of the abdomen you should be able to lift off all of the abdominal tergites in one
piece. In the preserved specimen you will have lifted off the dorsal
diaphragm at the same time and you should be able to see that, and
the alary muscles, involved in pumping blood through the animal
Respiratory system
The preservative that is used to prepare the grasshoppers has spread
throughout the tracheal system and the silvery tubes that we say in
the cockroach are now muck darker because they are no longer filled
with air. Locate the spiracles along the sides of the abdomen and follow the tracheal trunks as they branch and become smaller.
Fat Body
Just like in the cockroach the fat body is found all through the
abdominal cavity. The preservative has given it a much more crumbly
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BIO 3333
Entomology
texture and its much easier to remove it to expose the underlying
organ systems
Reproductive systems
The main parts of the female and male reproductive system are found
on top of the gut. Be sure that you see each!
In the female ovarian tubules are arranged linearly down the length
of the oviduct and depending on the reproductive state of your animal you should be able to see eggs maturing inside the oviducts. In
the cockroach the ovarioles were all joined together at a common
point. In the hopper the oviduct extends down the sides of gut and
join underneath it to form the unpaired vagina which open to the
outside. Later in the dissection when the alimentary tract has been
removed you may be able to see the sac-like seminal receptacle
where sperm is stored after the female has mated with male. Grasshoppers such as this don’t have the elaborate accessory gland system that
we saw in the roach. Why?
The male system is much simpler and tubular testes lie on top of the
digestive tract and joined underneath it to form the sperm duct of vas
defrens. It may be hard to see the testes in the grasshopper because
the preservative makes them have the same appearance as the fat
body. You’ll need to look closely using the dissecting scope to see the
difference.
Digestive system
The digestive system of the grasshopper looks very different from that
of the roach and one of those differences is that it is not coiled inside
the abdomen. Gently remove the reproductive system to expose the
underlying gut. The junction of the midgut and hindgut is marked by
large digestive caeca that extend both towards the front and back of
the animal. Like the cockroach the digestive caeca create a specialised environment for the breakdown if ingested food. To the front of
the animal is the oesophagus and ultimately the pharynx that is
embedded in the head. The valve at the junction between the foregut
and hind gut is a hardened proventriculus that is involved in grinding
up the food before it passes into the midgut.
As in all insects, the junction between the midgut and hind gut is
marked by the location where the Maliphigian tubules empty into the
alimentary tract. Behind that is the hindgut and if you dissect far
enough back you’ll see the larger diameter rectum.
Nervous system
Remove the remaining tissue and you should be able to see the
underlying nerve cord. Be sure to remove the muscles in the thorax as
to see the ganglia there. Are they the same size as the abdominal gan-
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BIO 3333
glia? What about the size of the very last terminal ganglia compared
to the others?
PREPARED SLIDES
We have a number of prepared slides that better illustrate some of
the internal organ systems
Tracheal System
Prepared slides in insect trachea and their attachment to a thoracic
spiracle are available for you to take a look at. This is a complex spiracle and two rather large tracheal trunks and a variety of other small
tubes extend from the spiracular opening . Look at the slide under
higher power can you see the taenidia?
Malpighian tubules
Prepared slides of Malpighian tubules are available. The tubule itself
is only a cell thick and the hollow lumen is continuous with the digestive system.
Compound and simple eyes
In the radial section of the compound eye you should be able see
both compound eyes and ocelli. In the compound eye distinguish the
difference between the dioptric apparatus and the underlying photo
receptive part of the eye as they form each of the ommatidia. Behind
it you can see the optic lobe of the brain were the nerve ends from
the sensory cells collect and integrate the information that they have
detected.
In the simple ocellus the organization is different and photoreceptive
cells are bunched underneath the lens rather then forming ommatidia. How does this effect the way that this photorecptor is believed to
function?
Flight Muscles - Johnson’s organ
Microscope slides of the male mosquitoes are availabe. The thorax is
transparent enought that you can see the indirect flight musculature.
One of the muscle group runs dorsoventrally and the other longitudinally. What is the effect on wing movement when either of these contracts?
Compare the male and female antennae. The knob-like base of the
male antenna is a it’s Johnson’s organ which it uses to detect the
buzzing of the females wings.
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© JON G. HOUSEMAN