Sample Lesson from Variety of Life
What Group of Organisms Is the Most Numerous?
Introduction
In this lesson: Students explore the diverse world of arthropods
and look for structural patterns they can use for developing or
refining their classification systems.
Specific Explorations
This lesson gives students opportunities to:
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compare the characteristics of different kinds of
arthropods (6s9)
compile data gathered through investigation in order to
record and present results, using charts, tables, labeled graphs, and scatter plots
produced by hand or with a computer (6s15)
identify various kinds of plant or animal organisms in a given plot using
commercially produced biological or classification keys (6s20)
Also: 6s4, 6s7
Assessment
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Collect and review students' completed (but still transitional) classification
systems, taking note especially of the features/characteristics they are using as
the basis for their systems. Note also the way in which they have designed their
systems (e.g., simple? simplistic? complex? sophisticated? easy-to-follow? and
so on). (6s9, 6s15)
Check to verify that students understand how a classification key (likely a
dichotomous key) works and that they can use it - i.e., follow through the steps to identify at least one organism they observe. Reproducible 5B: Identify the
Evergreen on page 45 provides a simple identification key that students can use
if you like. You could ask selected students to explain how easy or difficult the
key was to use, and what made it so. (6s20)
Getting Organized
Time required: Activate 10 minutes; Explore 25 minutes; Apply 25 minutes (not
including out-of-class use of an identification key)
Materials: arthropod samples (live or preserved if possible), copies of Reproducible 5A:
Classify Imaginary Organisms on page 44
Advance Preparation: If you would like your students to observe live or preserved
arthropods, prepare for their arrival well in advance. Decide how you will ensure the
health and safety of live organisms. If you can't, or if you have doubts, it is best not to
use live organisms. Check your district's guidelines on the use of live animals in the
classroom, and/or seek the guidance of a consultant.
Lesson Vocabulary
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arthropod: an animal that has an exoskeleton and three or more pairs of jointed
legs
exoskeleton: a hard outside covering of an animal that has no skeleton inside its
body
identification key: a set of questions, descriptions, and/or pictures that guide you
through a process of identifying an organism
Content Background
The phylum Arthropoda includes more than one million identified species. (A phylum is
the second-most-inclusive grouping of organisms in the biological classification system.
See page 57 for more information. Biologists also informally sort animals into two
general groups: vertebrates and invertebrates. These terms are also introduced on
page 57.
Arthropods are distinguished by having pairs of jointed appendages (arthropod means
"jointed foot."), a hard external skeleton - an exoskeleton - covering the whole body
(including the legs), and a segmented body. Members include lobsters, shrimp,
centipedes, millipedes, spiders, and insects. (For further details about arthropod
characteristics, see page 46.)
Biologists cannot possibly recognize each individual species. Instead they rely on
interactive identification charts called keys, which outline different features that an
organism may or may not have; as you compare descriptions with the organism you're
observing, you are led to other, more refined features that eventually narrow down to a
single species (or class, or order, depending on the nature of the key). Some keys are
designed using a question-answer format, rather than descriptions. A common type is a
dichotomous key, which uses pairs of questions or descriptions.
Activate
The Wonder of Arthropods
This lesson represents the first major test of, and likely challenge to, the classification
systems students have developed. It also won't be the last, and students should be
made aware of this, if they aren't already.
Invite students to read the first paragraph on page 18. Ask them to explain their
understanding of the statement in the second-last line - that about half of the known
species of organisms are arthropods. (This will ensure they haven't misread or
misinterpreted the sentence as "half of all animal species.") The impact of this fact is
somewhat staggering. You could ask students what they think could possibly account
for such incredible success for a group of organisms. Some students may suggest size;
certainly, being small in such a vast world such as the Earth is, has lots of advantages
for organisms as adaptable as arthropods. Students may also infer that arthropods are
somehow well suited (adapted) for living in a broad range of environments, which is
true. It isn't necessary for students to come to this realization now, however. This
question may best be posed (or re-posed) at the end of Lesson 6.
Preconceptions
Many students (like many adults) may find it hard to see that lobsters, shrimps, and
crabs have much in common with spiders and insects. The Exploration on page 18 asks
students to "look for patterns" to aid in their classifying. It is this pattern-seeking (e.g.,
despite differences in numbers, all arthropods have at least two pairs of legs; or all
arthropods have a hard, external covering) that can help them recognize similarities.
Curiosity Place
Right now, at least one billion billion (that's not a typo!) arthropods are alive on the
planet.
Explore
Preparing for the Exploration
Suggested Grouping: four
Materials per group: at least 10 arthropod samples (live, preserved, pictures, or any
combination of these)
Approximate Time: 25 minutes
Classroom Management: If you have live arthropods available, be sure to discuss
respectful and responsible handling of these living things with students. The basic
needs of these organisms must be provided for, and they should be kept confined for as
little time as possible. The organisms should be observed with the eyes only (minimal or
not touching of any kind), and should be returned to their natural environment as soon
as possible when the Exploration is over. If you have preserved samples, students
should also understand that these once-living organisms must be used by other
students in other classes; the animals should be examined carefully and respectfully,
and returned to you in the same condition in which they first obtained them.
Exploration
(Student Book page 18)
Discover arthropod structures.
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The questions listed in their books are intended to help students focus their
attention on specific structural details to aid their sorting and grouping ideas.
Encourage students to use their powers of observation to observe these and any
other details they notice.
The simplest method for organizing and recording their classification system is to
use the Step 1 questions as organizers. Encourage students to consider
additional options. If necessary, stimulate creative ideas by reminding students of
the various ways for recording classification systems that they saw on page 15.
Students should be able to provide detailed and well-reasoned criteria for their
classification decisions.
Troubleshooting
Although the questions provided in Step 1 of the Exploration point to characteristics that
scientists actually use for classifying arthropods, students should feel the same flexibility
and freedom to classify these organisms as they did in the previous Lesson. If
necessary, remind students that they are developing a classification system that makes
sense to them and that could aid them in identifying arthropods. You aren't looking for or
expecting "right answers" - because there aren't any.
Exploration Results
Based on the features (mainly or exclusively structural) that students observe, they will
invent a variety of classification systems for organizing arthropods.
Interpret your results.
(Student Book page 18)
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Answers will vary, but students will likely point to structural features such as legs
(shape and number), absence or presence of eyes, absence or presence of
antennae, shape and number of body parts/sections, presence or absence of a
shell. Note: If using pictures alone, it's unlikely that students will infer the
presence of an exoskeleton (by whatever name they call it). It's possible, but still
unlikely, that they could infer something about a hard or protective covering.
Given the questions provided in Step 1, students' systems will probably be more
similar than those in Lesson 4. Encourage students to reflect on any differences
they notice. This question will help students assess their classification systems.
Technically the centipede, grasshopper, and scorpion are members of the
phylum Arthropoda. The earthworm belongs to another phylum (Annelida), as
does the snail (Mollusca). Students may identify the snail as an arthropod
because of its shell. The absence of legs in both the snail and the earthworm
should exclude both of them from students' systems; however, students' criteria
may allow for the inclusion of either or both.
Communicate your results.
Some students may believe their systems don't require improvement. On the basis of
this Exploration alone, their belief may be justified. The Exploration in Lesson 6,
however, may force them to re-evaluate their beliefs. Regardless, encourage students
to look for the strengths in all the class systems, and to be supportive and encouraging
when suggesting improvements.
Helping the Struggling Student
If you plan to ask students to use the student book photos for the Exploration, some
students may have difficulty visualizing the arthropods in three dimensions. If possible,
have several live, preserved, or model specimens available for students to examine
more closely. A high school or university biology department may be willing to help.
If some students need additional practice and reinforcement for observing and using
features to classify organisms, you could give them copies of Reproducible 5: Classify
Imaginary Organisms on page 44.
Apply
Exploration Connection
(Student Book page 21)
Using identification keys
Ask students to examine the portion of the key shown in their books. Invite students to
use it to identify the animal (butterfly) in the photo. For example, ask:
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How would you answer the first question? (Yes, it has legs.)
Then which question would you go to? ("How many legs does the arthropod
have?")
What does the answer to this question tell you? (It has six legs, so go to
"Insects.")
Various kinds of identification keys are available in books, at specialty stores such as
birding stores, and on the Internet. Reproducible 5B: Identify the Evergreen on page 45
provides a key that students could use to identify coniferous (evergreen) trees. Students
may enjoy visiting the Website from which this identification key originated. (When you
go to this site, scroll down to the Identifying Trees hyperlink.) Many of the tree names
are hyperlinked to other web pages where students can learn more about these
organisms. You could also invite students to search for other identification keys on the
Internet. (Note, however, that the few sites to be found tend to be either for much
younger or much older students.)
Think!
Students should be able to identify insects and spiders as arthropods. Other
identifications will depend on the organisms on the class list, as well as the criteria that
students used to develop their classification systems.
Global Perspectives
Some arthropods (insects especially) are sources or carriers of disease (e.g., malaria,
Dutch elm disease). The stings and bites of certain arthropods, such as scorpions, can
cause paralysis or death. As well, arthropods can inflict heavy damage on the crops and
livestock that we need to supply our food.
However, the food-eating habits of some arthropods are helpful. Dragonflies, for
example, help keep "pesty" mosquito and black fly populations in balance. Many
arthropods are scavengers, helping to reduce and recycle vast amounts of dead organic
matter. Insects are also an important source of protein in the diets of many cultures.
Students could investigate and report on these and other examples of ways that we
depend on or fear arthropods. Other topics include the role of insects as pollinators;
how we use various arthropod species for food; the role of arthropods in terrestrial and
aquatic food chains; and how we use arthropod "products" (e.g., honey, silk, toxins) in
the food and pharmaceutical industries.
Integrating Science
Language Arts/Dramatic Arts:
Work with students to review journalistic reporting and interviewing techniques and
practices. Each student could then research information about an arthropod of their
choice and take turns, in pairs, interviewing each other. The students who are being
interviewed must "be" the arthropod they have researched. The interviewers must
discover, through their questions, what kind of arthropod they are interviewing.