In this lesson, students will consider the possibility of gigantic ants evolving. To answer this question, students first perform a comparative dissection of three types of arthropods. Next, they perform a webquest in which they determine what factors affect the maximum size of an exoskeleton. Finally, students consider all they have learned as they write a response that explains if gigantic ants or any other arthropod could evolve on land. Here is an overview of what they will learn today.
Have students watch a clip from the movie Them.
Then have them consider the following questions:
(Note: I have students watch the clip from 0:00-1:36. Here is a sample student response. )
To familiarize themselves with the structures found arthropods, students should complete a dissection of a crayfish, bee, and grasshopper. During the dissection they will describe the similarities and differences between the three specimens in order to determine if gigantic arthropods could ever occur on Earth.
Students may use the attached handout to help them with the dissection.
For this dissection, students will need the following equipment:
For bee dissection
Using the directions on the Comparative Dissection of Arthropods handout and a dissecting stereoscope, students will examine the external anatomy of the bee and make sketches of important structures. Next, students will mount the specimen in wax on the Petri dish and cover it with isopropyl alcohol to protect the delicate internal structures of the bee during the dissection. Students will then dissect the abdomen of the bee and identify the heart, tracheal sacs, and parts of the digestive system. Then students will turn their attention to the dissection of the thorax and examine the flight muscles and thoracic glands. Finally, students will remove the head of the bee and glue it to a microscope slide with a drop of superglue. Using a compound microscope, student will identify important structures on the bee's head. When students have completed this portion of the dissection, they should place the disposable Petri dishes in the chemical disposal bucket. They should place the microscope slides in the broken glassware bucket.
For crayfish dissection
Using the directions on the Comparative Dissection of Arthropods handout, students will first identify structure on the external anatomy of the crayfish. They will pay particular attention to the appendages that help the crayfish eat, move, and swim. Then they will remove the carapace and examine the internal structures of the crayfish. Students will make sketches of the different organ systems as they dissect. Finally, students will determine if they have a male or a female crayfish. Students should dispose of their specimen in the chemical disposal bucket. The crayfish appendage handout and the crayfish mouth appendage handout should be clean with disinfecting wipes and placed at the supply station. The rest of the equipment should be used in the dissection of the grasshopper.
For grasshopper dissection
Using the Comparative Dissection of Arthropods handout, students will only look at external structures in this dissection. Once they obtain a grasshopper and rinse off the preservative with water, they should identify the three regions (head, thorax, and abdomen) on the insect. Then they will compare the structures on the grasshopper's head with the crayfish. Next, they will compare the appendages on the body of the grasshopper with the bee and the crayfish. Finally, they will determine the sex of their grasshopper. The grasshopper should be disposed in the chemical disposal bucket. The appendage worksheet should be cleaned with a disinfecting wipe and returned to the supply station. The rest of the equipment should be washed with soap and water and allowed to air dry.
Using the attached handout, students will complete The Arthropod Story which highlights the defining characteristics of this phylum. Students will first explore what are arthropods and where they are found throughout the world. Next, students will look at the ecological niches that arthropods fill. Armed with that knowledge, students learn about the defining characteristics of arthropods and compare those characteristics to the organisms that they just dissected. Then, students will travel back in time to the Cambrian period and examine several living things that were part of the Cambrian explosion. In particular, they will look at organisms that were found preserved in the Burgess Shale. Using the criteria that taxonomists have determine to define an arthropods, students will evaluate if the fossils belong in the Phylum Arthropoda or if they are some other type of animal. Then students will determine the location of that fossilized organisms on the phylogenetic tree of animals. Finally, students will explore what physical characteristics have made arthropods one of the most successful phyla on the planet and what evolutionary constraints have lead to their relatively small size.
Using the attached powerpoint summarize the main ideas discovered in the dissection and interactive. As a class explore the defining characteristics of the phylum Arthropoda which are bilateral symmetry, a segmented body, a tough exoskeleton, jointed appendages, and many appendages. Then, discuss the evolutionary constraints that this type of body plan has on the size of the living thing.
Finally, have students revisit their journal entry from the beginning of the lesson. They should consider the following questions and revise their written response based on their new knowledge of arthropods.