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)