LABORATORY EXERCISE #5

KINGDOM ANIMALIA - PART III

 

THE ARTHROPODS

 

Introduction

 

Contrary to the opinion of most humans, self-centered as we are, it is probably the arthropods, not the mammals that represent the pinnacle of evolutionary advancement.  Arthropods occupy nearly every conceivable habitat on the earth and exceed all other animal and plant phyla combined in both number of species and number of individuals.

 

Arthropods are of great importance to human society.  Animals such as crabs, shrimp and lobsters are important food items, while many insects play a critical role in the reproduction of valuable plant species.  On the negative side, arthropods may be vectors of deadly diseases and one group, the insects, are humankind's chief competitors for food, fiber and other important commodities.

 

Arthropods are characterized by a more-or-less hard exterior skeleton (exoskeleton) made of a material called chitin.  Your hair and fingernails are composed of a similar material.  To allow movement, this exoskeleton is jointed, much like a suit of medieval armor.  To allow growth, this covering must be cast off from time to time and replaced by a larger exoskeleton.  This process is termed molting and is indulged in by all arthropods.

 

Like the annelids from which they probably evolved, the arthropods are segmented.  Unlike their ancestors, however, the arthropod body has several segments fused and otherwise specialized to form two or three body regions.  Animals such as the insects have three body regions; an anterior head, a medial thorax and a posterior abdomen.  Crustaceans, however, have only two; a cephalothorax, which represents the fused head and thorax, and the posterior abdomen.

Arthropod Diversity

 

As mentioned above, there is a great deal of diversity within the phylum Arthropoda.  Following are the major groups or classes:

 

1. The trilobites are an ancient group of organisms, all of which have been extinct for nearly 250 million years.  They dominated the oceans of the world during the 350 million years of their existence, however, and thus merit our consideration.  Trilobites get their name for the three lobes of their bodies, one medial and two lateral.  One can readily identify the three body regions of the trilobite, these being the anterior cephalon, a medial thorax and the posterior pygidium.  There was considerable variety in shape, habitat, and way of living among the trilobites.

 

ACTIVITIES

 

a.  Examine the trilobite specimens provided and see if you can identify the three lobes and the three body regions.

 

 

 

2. The insects are the most diverse group of arthropods.  They first appeared on earth around 400 million years ago and the basic design hasn't changed much.  As mentioned earlier, the insects have three body regions.  Each of the three thoracic segments has a pair of jointed legs.  Unique among the arthropods are the wings, which appear on the last two thoracic segments of most insects.  The head of the insect is characterized by the presence of paired mouthparts, a pair of compound eyes and a single pair of antennae.  Respiration among the insects is carried out by a system of air tubes or tracheae.  These open to the exterior through valved openings termed spiracles.  Some insects undergo a radical change in form (metamorphosis) during their development.  The forms are called larva, pupa and adult.  An example is the butterfly, an adult form that began life as a caterpillar (larva), then formed a cocoon in which the pupa reorganized itself into the adult.  There are more species of insect than all other animals combined.

 

 

ACTIVITIES

 

b.  Select a specimen of the large lubber grasshopper provided and compare it to Figure 103a and c in the Photo Atlas of Biology.  Be able to identify the following parts:  Head, thorax, abdomen, antennae, femur, compound eyes and spiracles.

 

c. Using the 4X objective on your microscope, examine the slide of the hog louse, an ectoparasite of swine.  Locate the three body regions, the antennae, and count the legs.

 

 

 

3. The crustaceans are to the water what the insects are to the land.  These organisms have only two body regions; a cephalothorax covered with a hard carapace, and the abdomen.  The number of legs varies among the Crustacea, but they usually have a stalked pair of compound eyes and two pairs of antennae.  Since they are water-dwellers, they respire with gills.  Many of our favorite "shellfish" are crustaceans, but there are many others also.  An important category of crustacean is the micro-crustacean or planktonic variety.  These serve as important food sources for a variety of organisms, from small fish to whales.  Another interesting crustacean is the barnacle.  These secrete hard shells, which form crusts on virtually anything in the ocean.

 

ACTIVITIES

 

d.  Select a crayfish, rinse it well in water and place it in a dissecting pan.  Using illustrations 101a and b in Perry and Morton as a guide, be able to identify all external parts labeled.

 

e.   Examine the slide of the common microcrustacean Daphnia under the 10X objective of your microscope.  Figure 102c shows this creature.

 

f.  Examine the other crustacean specimens provided and be able to identify them by common name, class, phylum and kingdom.  These will include crabs, shrimps, barnacles, and pill bugs.

 

 

 

4. The arachnids are a misunderstood and much-maligned group of organisms.  Largely due to the fact that a small number of their species constitute a threat to the well being of humans, these animals are often indiscriminately killed on sight.  This is unfortunate, for the arachnids are some of our chief allies in the ongoing battle against pestiferous insects.  These animals are characterized by having two body regions, a cephalothorax and an abdomen.  They have four pairs of walking legs and at least one pair of simple eyes.  They lack antennae, but may have special body parts associated with poison glands, which they use in getting food (they are all predators) and in defense.  Included in this taxonomic class are the spiders, harvestmen, scorpions, mites and ticks.

 

ACTIVITIES

 

g.  Examine the large spider (Tarantula) in the plastic block.  Count the number of walking legs.  Identify the two body regions.  Turn the spider over and note the prominent appendages just anterior to the first pair of walking legs.  These are the pedipalps.  In male spiders these are modified for use in transferring sperm to the female.  Anterior to the pedipalps are the "jaws" with which the spider injects venom into its prey.  Note the spinnerets projecting posteriorly from the abdomen.  These organs allow the spider to spin silk threads, which may be used in many ways, depending on the spider’s mode of life.  Compare the tarantula with the more-common garden spider, an illustration of which may be seen in Photo Atlas Figure 104c.

 

h.  Find the plastimount containing the scorpion.  How many pairs of walking legs do you count?  Note that the pedipalps are modified into "pincers".  Perhaps the most distinctive feature of the scorpion is the stinger at the tip of its abdomen.  This stinger is raised up over the back and plunged into the prey being held by the "pincers".

 

 

 

5. The multi-legged arthropods fall into two classes, the centipedes and the millipedes.  The centipedes (sometimes called "hundred leggers") are predaceous organisms, which occupy a variety of habitats.  All have an elongate, somewhat flattened body with one pair of legs on each segment.  On the ventral side of the first thoracic segment is a poison claw with which the centipede kills its prey.  You may have seen very active, long-legged centipedes scurrying about your basement looking for insects.  You may also have encountered sluggish, short-legged specimens living under rocks or in rotting leaves.  The millipedes ("thousand leggers") are docile creatures, which live on dead and decaying materials in the moist woodlands.  With the exception of the three thoracic segments, which have one pair of legs, each segment of the millipede bears two pairs of stubby legs.  These animals are round in cross-section and are often brightly colored.  As a defense mechanism, many millipedes exude a foul smelling secretion, or a highly caustic material, which can burn the skin if they are handled.

 

 

ACTIVITIES

 

k. Examine the specimens of multi-legged arthropods provided.  Can you tell which are centipedes (Figure 104b) and which are millipedes (104a)?  Locate the three body regions of each.  How many legs per abdominal segment?  Do they appear round or somewhat flattened in cross-section?  When you have identified the centipedes, look for the poison claw on the ventral surface.