Download BIOL 202 LAB 11 Arthropoda

BIOL 202
LAB 11
Arthropoda
The phylum Arthropoda is by far the largest in the animal kingdom, containing an
estimated 10 million species! New species of arthropods are literally being discovered
every day, adding to the nearly one million which have already been described. As their
numbers suggest, they are perhaps the most successful group of animals ever to occupy
the planet. They predate dinosaurs (by several hundred million years!) and most surely
will be feeding upon the last vertebrate corpse as it slowly decays. Part of their
unsurpassed success is due to the fact that they were the first animals to inhabit land.
Between 440 and 410 million years ago, arthropods gradually moved into a previously
unexploited habitat that simultaneously was being populated by vascular plants. They
were also the first animal group to evolve the ability to fly and therefore could make use
of the 3-dimensional landscape that was devoid of any other competitors. This resulted
in an adaptive radiation of arthropods throughout the landscape. These facts, coupled
with their small size, made them ideal vectors of pollen and explain the close associations
we see today between many flowering plants and insects. Throughout their long
evolutionary history, arthropods have spread through every terrestrial, aquatic and aerial
habitat imaginable and have had a profound impact on the evolution of numerous other
species.
Despite their diversity, all arthropods share many characteristics in common. Perhaps the
most universal traits of arthropods are the presence of a segmented body and jointed
appendages, the latter trait from which the phylum name is derived. Arthropods also
possess a hard, chitinous exoskeleton that is secreted by the epidermis. As the body
grows inside, the old exoskeleton is periodically shed through a process called molting in
which the soft, new exoskeleton must be secreted and fixed in place before the old shell
can be shed. This causes severe logistic problems (as you may imagine) that arthropods
circumvent by folding the soft, new exoskeleton upon itself as it is being produced. After
the old shell is shed and the body is free of its former constraints, the new, larger shell
expands to its final size and hardens in place. In an animal encased in such a rigid suit of
armor, the coelom can play no major role in locomotion and is thus a greatly reduced
coelem. The main body cavity is instead a hemocoel, comprising part of the open
circulatory system characteristic of this group.
Today, three classes of arthropods (Arachnida, Insecta and Malacostraca) include well in
excess of 95% of all arthropod species. Providing a complete survey of the arthropod
phylum would be an impossible task, however most adult arthropods show only minor
deviations from the “standard” body plan. Thus the crayfish and grasshopper are
presented as representative body styles of the typical aquatic and terrestrial arthropod.
Crayfish Anatomy
External Anatomy
The crayfish is a member of the Class Malacostraca (formerly Crustacea) which includes
lobsters, shrimps and crabs. The crayfish is one of the few freshwater members of this
group and serves as an excellent model for studying the magnificent adaptations that
aquatic arthropods have developed. Two defining features of malacostracans are their
biramous (Y-shaped) appendages and their two sets of antennae.
1.
Obtain a preseved specimen of the freshwater crayfish.
2.
The body is divided into two main regions: the cephalothorax and the abdomen.
Notice the segmented nature of these regions and the many modified appendages
present in each of these areas (Fig. 12.1).
3.
Examine the external anatomy of the crayfish and identify the structures listed in
Table 12.1.)
Table 12.1 • External Anatomy of a Crayfish
Structure
Function
Chelipeds
Large pinchers used for grasping food and for defense
Walking legs (periopods)
Locomotion (walking on land and crawling across stream bottom)
Swimmerets (pleopods)
Modified caudal appendages for swimming
Copulatory swimmerets (male)
Larger, club-shaped swimmerets used by male to stimulate female during
copulation and fertilization
Uropod and telson (tail)
Broad, fan-shaped region of body used for rapid movement and for directional
control during leisurely locomotion
Antennae
Longer, paired appendages on head modified for chemosensory and tactile
reception
Antennules
Shorter, paired appendages on head modified for chemosensory and tactile
reception
Compound eyes
Small, dark sense organs for detecting light and forming visual images
Rostrum
Pointed region between eyes demarcating cranial end of body
Maxillipeds
Three sets of paired appendages located on ventral surface near mouth used to
manipulate food
Mandibles
Hard, chitinous mouthparts used to manipulate food into mouth
Mouth
Opening to digestive tract located on ventral surface of body
Anus
Terminal point of digestive system located on ventral surface of body
Internal Anatomy
1.
Using the pointed end of your dissecting scissors, make two incisions along the
dorso-lateral margins of the crayfish from the caudal end of the cephalothorax to the
rostrum, angling the incisions medially so they meet at the tip of the rostrum.
2.
Gently peel away the dorsal portion of the carapace, being careful not to pull away
any of the internal organs that are attached to the underside of the shell.
3.
Next, remove one of the remaining lateral sides of the carapace to reveal the
feathery, branched gills.
4.
Notice that the gills are actually external. They reside between two pieces of
exoskeleton—the outer lateral side of the carapace and a thinner, inner chitinous
membrane.
5.
Notice also that the gills are attached to the walking legs at their ventral juncture.
6.
Continue the dorso-lateral incisions made earlier, this time directing them caudally
through the abdomen toward the telson.
7.
Carefully remove the portion of the exoskeleton covering the dorsal surface of the
abdomen to expose the musculature of the tail and the blood vessels and digestive
organs located within this region.
8.
Work from the dorsal surface downward (ventrally), identifying the organs and
structures described in Table 12.2 and depicted in the illustrations.
9.
After differentiating between the cardiac chamber and the pyloric chamber of the
stomach, remove them from the crayfish and open the cardiac chamber to reveal the
gastric mill. The crayfish uses these chitinous teeth to mechanically grind its food
into smaller pieces for digestive enzymes secreted by the digestive glands to act
upon as the food moves into the pyloric chamber of the stomach.
10.
Use a dissecting microscope to scan the dorsal aspect of the digestive glands for the
small reproductive structures. Ovaries will be easier to spot than the extremely
small testes, but careful examination of this region along the median plane of the
crayfish should reveal the reproductive organs. The size of the testes and ovaries
and the presence of eggs within the ovaries depends upon the time of year that the
specimens were collected, since reproduction in this group is seasonal.
Table 12.2 • Internal Anatomy of a Crayfish
Structure
Function
Gills
Respiration
Esophagus
Passageway between moth and cardiac portion of the stomach
Cardiac chamber of stomach
Thick-walled, cranial portion of the stomach containing gastric mill—chitinous
teeth which grind food into smaller pieces
Pyloric chamber of the stomach
Thin-walled chamber where chemical digestion of food occurs
Digestive glands
Accessory digestive organs that secrete enzymes into the pyloric stomach to
facilitate chemical breakdown of the food
Intestine
Long tube passing through the “tail” region in which nutrients are absorbed into
the bloodstream for delivery to the body tissues
Heart
Specialized, muscularized chamber containing ostia (holes) to allow passive
uptake of blood which is delivered to the body tissues through arteries, but veins
do not exist in this open system
Dorsal artery
Longitudinal blood vessel that distributes blood to the dorsal aspect of the body
Ventral artery
Longitudinal blood vessel that distributes blood to the ventral aspect of the body
Testes (male)
Site of sperm production (may be difficult to locate on specimen)
Ovaries (female)
Site of egg production
Green glands
Paired excretory organs found along the ventral margin of the head region; they
release waste out of the crayfish through small pores in the ventral body wall
Brain
Small, radiate structure lying dorsal to the green glands; houses the majority of
neural ganglia in the crayfish
Circumesophageal connection
(of ventral nerve cord)
Branches of the ventral nerve cord that bifurcate at the base of the brain and wrap
around the esophagus before merging along the ventral surface of the crayfish
just caudal to the esophagus
Ventral nerve cord
Long, white “cord” located along the ventral surface of the body; contains large
swellings of ganglia that handle the majority of coordination without intervention
by the brain
Grasshopper Anatomy
The grasshopper serves as a representative example of the class Insecta, a subgroup of
arthropods with six, uniramous (unbranched) walking appendages, a single set of
antennae and three distinct body regions: head, thorax and abdomen. During the
evolution of arthropods, departure from the ancestral aquatic lifestyle favored the
development of characteristics that permitted successful adaptations to the many
ecological hurdles associated with terrestrial living. Among them, (1) stronger, more
efficient support systems and walking appendages, (2) waxy cuticles built to withstand
the osmotic stresses of “dry” air, yet permeable enough for aerial gas exchange, (3) the
ability to fertilize eggs internally to prevent desiccation of “naked” gametes, (4)
specialized excretory and digestive structures designed to conserve water and (5)
appendages modified into wings to take advantage of the previously unexploited aerial
habitat, were hallmarks in the evolution of insects.
External Anatomy
1.
Obtain a preserved specimen of Romalea, the common grasshopper.
2.
The body is divided into three main regions: the head, the thorax and the
abdomen. Notice the segmented nature of these regions and the many modified
appendages present in each of these areas.
3.
Determining the sex of your grasshopper can be accomplished by examining the
caudal portion of the abdomen. In addition to an anus, females possess an
ovipositor ventral to the anus. This opening is bordered by two pairs of chitinous
“teeth” that thrust into the soil and flex outward, creating a chamber in which the
female deposits her eggs. Males lack an ovipositor.
4.
Examine the external anatomy of the grasshopper and identify the structures listed
in Table 12.3.
Table 12.3 • External Anatomy of the Grasshopper
Structure
Function
Spiracles
External openings in abdomen that allow air flow into and out of tracheae
Compound eyes
Paired, complex, image-forming photoreceptors composed of numerous
ommatidia which create a fairly coarse-grained picture of their visual field
Orcellus (pl.=ocelli)
Simple photoreceptor consisting of a small cup backed by light-absorbing
pigments; not capable of image formation
Antennae
Thin, paired appendages on head modified for chemosensory and tactile
reception
Mouthparts
Multiple sets of hard, chitinous appendages used to chew food and manipulate
food into mouth
Wings
Two sets of paired appendages modified for flight
Arthropod Slides
1. Squash bug wings (ZK 5-26)
2. Honey bee mouth parts (ZK 8-14)
3. Honey bee sting and poison sac (ZK 8-18)
4. Honey bee legs (ZK 8-17)
5. Honey bee antenna (ZK 8-15)
6. Honey bee wing (ZK 8-16)
7. Cricket foreleg w/tympanum (ZK 3-27)
8. Butterfly wing (ZK 5-56)
9. Grasshopper head (ZK 3-14)
10. Horsefly wing + haltere (ZK 6-26)
11. Housefly head, leg, and wing (ZK 6-16, ZK 6-17, ZK 6-14)
12. Housefly mouthparts (ZK 6-24)
13. Grasshopper mouthparts (ZK 3-141)
14. Insect antennae (ZK 2-15)
15. Insect trachea (ZK 2-184)
16. Insect legs (ZK 2-17)
17. Insect cornea (ZK 2-181)
18. Insect wings (ZK 2-16)
19. Insect compound eye (ZK 2-182)
20. Insect wings (ZK 2-16)
21. Insect spiracle (ZK 2-183)
22. Mosquito slides: (ZK 6-35)
23. Crayfish gill (ZK 1-417)