Life Cycle Experiment

Author: Jennifer Hernandez

Overview: [This is the second in a series of three lessons concerning life cycles] The objectives of this lesson are to introduce the students to the use of controlled experiments in science and to the biology of the hawkmoth (hornworm). The students have been introduced to life cycles with a detailed presentation of the insect life cycle (What is a Life Cycle?). This lesson requires the students to participate in the set-up and initial data collection for a controlled experiment that examines the effects of abiotic conditions on the development of hornworm larvae. Each class will be assigned a specific treatment. The class will be divided into eight groups and each group will receive a hornworm larva. Each class will test four larvae under optimal conditions and four larvae under the treatment conditions. The tasks involved in this lesson are: discuss the biology of the hornworm, conduct initial measurements of the larvae, and place the vials under treatment conditions.

Lesson Concepts:

  • Students will gain experience in creating and implementing a controlled experiment.
  • Students will learn how to collect data as part of the experimental process.

Grade span: 7th grade

Materials:

  • Hornworm larvae
  • Vials
  • Heating pads
  • Lights
  • Food supply (tobacco, chard)
  • Vial labels
  • Scales
  • Rulers
  • Data sheet (pdf)
  • Thermometers
  • Soil (not needed until larvae ready for pupation)
  • Flight cage (not needed until end of pupation)
  • Ziploc bags
  • Hand lens
  • Petri dishes

Time: 45 minutes for initial set-up; 20 minutes for subsequent measurements; entire experiment lasts 50-60 days

Grouping: Groups of four students (original lesson designed for three classrooms but can be adapted for a single class)

Vocabulary: larva, hornworm, prolegs, thorax, abdominal segments

Procedure:

  1. (5 minutes) Review key concepts from previous lesson (What is a Life Cycle?) and place them in the context of the current experiment. Key concepts include:
       • What did we observe in the previous lesson? [Insects are exothermic and sensitive to temperature.]
       • What other factors might be important for an insect to develop? [(prompt) What does a plant need to grow?]
  2. (5 minutes) Break into groups.
  3. (5 minutes) Hand out vials with the treatment already assigned and written on the label and a supply bag (contains petri dish, hand lens, ruler, scale, small bag for weighing organism, and data sheet). Students are instructed not to open, shake, or roll the vial.
  4. (10 minutes) Discuss handling procedures for the larvae. The important points to discuss:
       • The larva is fragile and should not be squeezed, this will damage the internal organs.
       • Introduction to the parts of the larva can be done on the board (see illustration on the data sheet).
       • Basic introduction to the biology of the hornworm — what it will be eating, where it lives, what it eats in its natural environment.
       • Demonstrate handling procedures (removal of larva from the vial and from your hand). The legs and prolegs are capable of gripping so it can be difficult to remove the larva from your finger. To remove it from a surface, move slowly starting at the head and gently provide another surface for the larva to grasp, it will release each leg one at a time.
  5. (5 minutes) Students remove their larva from the vial and examine it with their hand lens. After examining the larva they can leave it in the petri dish.
  6. (10 minutes) Students follow each step of the measurements with the Fellow.
       1) Measure the length in cm and record.
       2) Measure the weight of the Ziploc bag in grams and record.
       3) Place larvae in bag, weigh and record.
       4) Subtract weight 1 from 2 and record the weight of the larvae.
  7. (5 minutes) Place the larvae in their treatment set-up.

Assigned Treatments
Optimal: 28-30°C, 24 hr light, prepared food
Temperature: ambient temperature, 24 hr light, prepared food
Light: 28-30°C, ambient light, prepared food
Food: 28-30°C, 24 hr light, tomato, tobacco, or chard

  Class 1 Class 2 Class 3
Period 1 Light Temperature
Period 2 Food Temperature
Period 4 Temperature Light
Period 6 Temperature
Period 7 Light
Period 8 Food Light

Equipment needed for each classroom
Class 3: 16 larvae total, 2 lights, 3 thermometers, 1 heating pad
Class 1: 48 larvae total (24 under optimal conditions), 5 lights, 7 thermometers, 2 heating pads
Class 2: 16 larvae total, 2 lights, 3 thermometers, 1 heating pad

8 larvae can be placed under each light. We can combine classes under one light since the vials are color coded and labeled.

Facts about hornworms
   • They occur in the southern United States and are most common in Florida.
   • The larvae eat tomato, tobacco, and potato. They eat the leaves, flowers, and fruit.
   • The adult feeds on nectar and usually lives for one week.
   • They are considered an agricultural pest.

Timeline
Day 1: Set up the experiment. Most of the larva should be in the fifth (last) instar.
Day 7: Discuss the set-up of the experiment (question/hypotheses). Conduct measurements (larvae will have gained weight). Some larvae will be ready for pupation.
Day 15: Hopefully all will be pupating.
Day 35: Students measure the pupae and examine changes. The data can be plotted on posters and compared.
Day 50: Approximate beginning of emergence. May be able to delay this by decreasing the temperature.

 
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