Download COMPARATIVE ANATOMY OF TWO ARTHROPODS

COMPARATIVE ANATOMY OF TWO ARTHROPODS
BACKGROUND
The phylum Arthropoda includes at least 80% of all known animal species. More than a million species of arthropods
occupy more kinds of habitats; consume more varieties of foods than any other group of organisms. The sorting and
identification of this bewildering array of animals has occupied generations of biologists.
In this laboratory activity, you will compare the external anatomy of members of 2 classes of arthropods, class Crustacea
and class Insecta. You will perform studies similar to those employed by taxonomists as they attempt to classify and
identify animals.
OBJECTIVES
In this activity you will:
1.) Observe the characteristics of crayfish and grasshoppers that identify them as members of the phylum Arthropod.
2.) Observe differences between crayfish and grasshoppers that place them in separate classes.
MATERIALS
preserved crayfish
preserved grasshopper
dissecting tray
forceps
dissecting needle (probe)
hand lens
dissecting scissors
PROCEDURE
Part I: Characteristics of Crayfish and Grasshoppers
1.) Place the crayfish and the grasshopper side by side in the dissecting tray, dorsal sides up and anterior ends away from
you. Note that each animal has a head at its anterior end where sense organs are located.
2.) Observe that the bodies are streamlined. Imagine a line running down the middle of the dorsal side of each body,
dividing it into right and left sides. Answer questions 1 and 2 in the data section.
3.) Turn both animals ventral sides up. Examine the ventral side to see how the bodies are organized. Answer question 3
in the data section.
Arthropods are named for their jointed legs. “Arthro” comes from the Greek word for joint, and “pod” comes from the
Greek word for foot. The paired, jointed legs are called appendages, as are the other paired structures that are attached
to the body segments.
4.) Turn the crayfish over and lift it up by the middle of its body. Notice how the pairs of legs hang downward.
Observe the joints in the legs.
5.) Turn the grasshopper over and lift it by its body and not how its legs are attached. Observe the joints in the
legs. Answer questions 4 and 5 in the data section.
Arthropods have an exoskeleton that covers the body like armor. The exoskeleton is composed of chitin, a tough, flexible
substance. Only the arthropods have this kind of exoskeleton. It is considered to be one of the reasons for their
remarkable success.
6.) Examine the outer covering of each animal. With the handle of your dissecting needle (probe), test the hardness and
flexibility of the exoskeleton.
7.) Press gently on the dorsal and ventral sides of each animal in several places, proceeding from anterior to posterior.
Answer questions 6 and 7 in the data section.
8.) Examine the legs of each animal. Feel the claws of the crayfish by holding them between your thumb and forefinger.
Look a t the form and arrangement of the claws. Answer questions 8 and 9 in the data section.
9.) Look at the grasshopper claws with the hand lens. Feel them with your forefinger. Then examine the wings of the
grasshopper. They are also made of chitin. Feel the wings and test their strength. Answer questions 10 and 11 in the
data section.
10.) Find other pairs of jointed appendages on the bodies of these two animals. On the heads, find antennae.
11.) Proceeding posteriorly on both dorsal and ventral sides, note how the pairs of appendages differ from
each other. Each pair is highly specialized for a specific use in the animal’s environment.
Part II: Comparison of a crustacean and an insect.
The crayfish, classified in the Class Crustacea, is an example an arthropod adapted to living in water. Crabs, lobsters, and
shrimp are also crustaceans. The grasshopper, a member of the Class Insecta, is in many ways typical of the land-dwelling
insects.
12.) Compare the exoskeletons of the crayfish and the grasshopper. With the handle of your probe tap on the
exoskeleton of each animal. The difference in the sounds tells you that one exoskeleton is harder, or more crusty,
than the other. Both exoskeletons contain chitin, but one also contains calcium salts, which make the exoskeleton
hard. Animal parts hardened by salts such as calcium carbonate and calcium phosphate are said to be calcified.
Answer question 12 in the data section.
In general, the arthropod body is divided into three distinguishable regions—the head, thorax and abdomen. (Thorax is
the Greek word for “chest”) In the crayfish, the head and thorax are fused together, forming what is called the
cephalothorax. The covering of the cephalothorax, called the carapace, covers the first 13 segments of the crayfish. The
abdomen of the crayfish is clearly segmented.
13.) Determine the number of segments that make up the crayfish abdomen. The last segment is the telson, the single,
flat structure at the center of the fan shaped posterior end. Answer questions 13 and 14 in the data section.
The grasshopper body has three regions. The head is composed of 6 fused segments and it attached to the thorax. The
thorax consists of three segments. The abdomen, posterior to the thorax, is made up of ten complete segments plus one
incomplete segment.
14.) Examine the segments of the thorax and abdomen on the ventral surface of the grasshopper. With a hand lens,
examine the ventral side of the head. You will see paired appendages, each highly specialized. Answer question 15
in the data section.
The sense organs of the head are noticeable on both animals. The antennae are jointed, paired appendages. Each pair of
antennae is attached to a segment.
15.) Look closely at the bases of the antennae on each animal and count the pairs of antennae on each. The anterior, Yshaped antennae of crustaceans are known as antennules. Answer questions 16 and 17 in the data section.
The large eyes on the head of a grasshopper are compound eyes, each made up of more than 2000 units, called facets.
Each facet receives light from part of the surroundings and forms an image. The many separate images reach the brain by
means of nerves. The “picture” received by the brain consists of as many small units as there are facets in the eye. This
kind of vision is called mosaic vision, and is unique in arthropods. Animals with compound eyes are very sensitive to
movements in their surroundings. As an object moves, its image moves from one facet of the compound eye to the next.
16.) Look at the large eyes on the head of the grasshopper. Note the curved surface of each eye, and the amount that
the eye extends outward from the head. Also examine the eye with the hand lens. Be sure to have good light on the
specimen. Answer question 18 in the data section.
17.) With the hand lens, examine the crayfish eyes. Look at the attachments of the eyes, also, observe where they are
mounted on the head. Answer questions 19 and 20 in the data section.
Some Arthropods have simple eyes, called ocelli (ocellus, singular), in addition to the compound eyes. Such
eyes are light sensors only and do not form images.
18.) Pick up the grasshopper and look straight at its face. Using the hand lens, look for 3 simple eyes. One
ocellus is in the center of the face between the antennae. The other 2 ocelli are above the bases of the
antennae, close to the compound eyes. Answer questions 21 and 22 in the data section.
Crayfish, shrimp, lobsters and crabs are classified as decapods (Order Decapoda). All have five pairs of walking
legs, with the first pair often modified as pincers or chelipeds. The legs are not all alike and are adapted for
different functions. All of the pairs of legs are attached to the last five segments of the cephlothorax.
19.) Look at the pairs of legs on the crayfish. The first pair, the chelipeds, are larger and heavier than the other
legs. They are adapted for catching food and for offense and defense. The next four pair are the walking
legs. Answer question 23 in the data section.
The thorax of the grasshopper has been called its “locomotor box”. Two pair of walking legs are attached to the
first 2 segments of the thorax. One pair of jumping legs is attached to the 3 rd (last) segment. Wings are
attached to the dorsal side of the thorax.
20.) Examine the legs and wings of the grasshopper. Notice the differences between the pairs of legs and the
pairs of wings. Answer questions 24 - 26 in the data section.
Almost every segment of the abdomen of crustaceans bears a pair of appendages on the ventral side. Insect, however,
have no abdominal appendages except on the last segment of the female.
21.) Look at the ventral side of the abdominal segments of the crayfish. The small, jointed appendages are called
swimmerets.
In male crayfish, the first pair of swimmerets are modified into pipe-like copulatory organs. In the fall, the male deposits
sperm through these organs into the body of the female. The female crayfish has paired openings between the last pair
of walking legs where the sperm are received. The sperm remain in a pouch, the seminal receptacle, all winter. In the
spring, 300 to 600 eggs are shed through the openings of the internal oviducts, located at the bases of the third pair of
walking legs. As the eggs are released, the stored sperm are released simultaneously, resulting in fertilization. The mass
of fertilized eggs becomes attached to the female’s swimmerets. The eggs hatch in about 5 to 6 weeks, and the larvae
remain attached to the mother for several more weeks.
22.) Determine the sex of your crayfish specimen. Look at the modified first pair of swimmerets to see if the specimen is
a male or a female. Answer question 27 in the data section.
The sex of a grasshopper can be determined from its external structure. In the female, the abdomen ends in a fourpointed structure called an ovipositor. The ovipositor is used to dig a hole in which the fertilized eggs are deposited. In
the male, the abdomen ends in a single tip without the ovipositor. Answer question 28 in the data section.
The abdomen of the crayfish is adapted for fast movement in water. Muscles extend the length of the abdomen, and the
last two segments bear broad flipper, forming a fantail. When the abdomen muscles contract, the fantail pushes strongly
against the water, sending the crayfish backwards at great speed. The last segment is the central telson. The segment to
which the telson is attached bears a pair of broad, flat uropods. The uropods and the telson form the fantail.
Telson
Uropod
The respiratory oranges of the crayfish are the gills, where the exchange of carbon dioxide and oxygen takes place. Gills
are adaptations for life in water.
23.) Place the crayfish dorsal side up. Lift the right edge of the carapace slightly, just above the walking legs. Notice the
thickness.
24.) With scissors, carefully cut off the right side of the carapace. See the figure below (dotted line). Be sure to cut only
the carapace. Answer question 29 in the data section.
25.) With thumb and forefinger, grasp the entire joint of the most posterior walking leg where it joins the body. Be sure
you have a firm hold. Carefully twist and remove the leg with its attached gill. Be sure not to destroy the rest of the
body.
26.) Place the leg and gill in a Petri dish. Move the leg in the water so that the featherlike branches of the gill spread out
Note the large surface area provided by the fine branching. Answer question 30 in the data section.
The respiratory structures of the grasshopper are adapted for life on land. Insects are true breathers, forcing air into and
out of their bodies by means of muscles. On each side of almost every abdominal segment is an opening called a spiracle,
where air enters and leaves the insect. Each spiracle opens into a breathing tube, a tracheae. The respiratory gases are
not carried in the blood. They diffuse through the walls of the tracheae, which are supported by rings of chitin. There are
also spiracles and tracheae in the thorax. Spiracles and tracheae are characteristic of insects.
27.) Examine the lower sides of the abdomen and thorax. Answer question 31 in the data section.
NAME ________________________________
COMPARATIVE ANATOMY OF TWO ARTHROPODS
DATA SECTION
1.) What kind of symmetry is characteristic of the crayfish and grasshopper? ____________________________________
2.) Are any parts of the bodies divided into sections, or segments, similar to the segments found in an earthworm? _____
3.) Seen from the ventral sides, do the bodies appear to be segmented? _______________________________________
4.) Are the legs attached to the dorsal or to the ventral sides of the animals? ____________________________________
5.) How are the leg attachments related to the segments? ___________________________________________________
6.) Which parts are harder than others? __________________________________________________________________
7.) Which parts are more flexible than others? _____________________________________________________________
8.) Is the exoskeleton on the legs firm enough to make a good pushing or walking leg? ____________________________
9.) Do the claws of the crayfish seem like good pinchers? Explain. _____________________________________________
_________________________________________________________________________________________________
10.) How do you think chitin makes the claws suitable for clinging to grass and leaves? ____________________________
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11.) What properties of chitin might make it a good wing material? ____________________________________________
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12.) Which animal has the calcified exoskeleton? For what purpose? __________________________________________
_________________________________________________________________________________________________
13.) How many segments make up the crayfish abdomen? ___________________________________________________
14.) How many segments make up the entire crayfish body? _________________________________________________
15.) How many segments make up the grasshopper body? ___________________________________________________
16.) How many pairs of antennae does the crayfish have? ___________________________________________________
17.) How many pairs of antennae does the grasshopper have? _______________________________________________
18.) Considering the structure and placement of the compound eyes, can the grasshopper see in all directions? ________
19.) Are the crayfish eyes compound? Explain. ____________________________________________________________
20.) Can the crayfish see in all directions without turning around? How? _______________________________________
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21.) How do all the ocelli compare in size to the compound eyes? _____________________________________________
22.) Does the crayfish have ocelli? ______________________________________________________________________
23.) How many pairs of legs does the crayfish have? ________________________________________________________
24.) How many pairs of wings are attached to the grasshopper’s thorax? _______________________________________
25.) Where are they attached? _________________________________________________________________________
26.) In all, how many pairs of locomotor appendages are attached to the 3 segments of the thorax of an insect? _______
_________________________________________________________________________________________________
27.) What is the sex of your crayfish? How do you know? ___________________________________________________
_________________________________________________________________________________________________
28.) What is the sex of your grasshopper? How do you know? ________________________________________________
_________________________________________________________________________________________________
29.) How many sets of gills can you see? _________________________________________________________________
30.) What purpose does the gill serve? Why is it important to have the surface area? _____________________________
_________________________________________________________________________________________________
31.) How many spiracles can you see? How many are in the abdominal segments and how may are the in the thorax?
_________________________________________________________________________________________________
ANALYSIS QUESTIONS
1.) For what anatomical characteristic are the arthropods named? ____________________________________________
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2.) Give 3 characteristics of arthropod bodies. _____________________________________________________________
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3.) Give 3 characteristics of chitin that make it a good material for the exoskeletons and wings of arthropods.
_________________________________________________________________________________________________
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4.) What 3 body regions are characteristic of insects? _______________________________________________________
5.) What 2 body regions does a crayfish have? _____________________________________________________________
6.) Give the meaning of the statement: “The crustacean exoskeleton is calcified.” _______________________________
_________________________________________________________________________________________________
7.) Considering your own body, name any parts that are calcified? ____________________________________________
8.) It has been said,, “If humans could jump the way a grasshopper can, they could jump over barns.” What
characteristics of the grasshopper body help to accomplish such feats of jumping? ____________________________
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9.) What can crayfish do well that a grasshopper cannot do at all? _____________________________________________
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10.) Discuss the vision of grasshoppers and crayfish. Confine your remarks to items you read about in the lab. How is this
an advantage? __________________________________________________________________________________
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