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  Entomology Department > Insect Zoo > Lesson Plans

Life Cycles

Grade Level

PreK-4

Time

One hour for discussion and set-up, another one to six weeks for observations and recording data.

Materials

  • One butter container (or something similar) with a lid for each child or group of children
  • Oatmeal (enough for approximately 1/2 inch deep per container)
  • Mealworms (purchased at any pet store)
  • Magnifying glasses
  • Charts or calendars for recording growth

Objectives

1. Students will identify the life stages of four different Iowa insects.

2. Students will care for and observe the life cycle of one species.

Discussion: Types of Insect Life Cycles

There are three different types of insect life cycles. Some insects go through complete metamorphosis (change). This is where the larva (immature insect) is very different from the adult, and a distinct pupa (ex: cocoon) is formed. The stages of complete metamorphosis are egg, larva, pupa, and adult. The larva goes through different stages of growth as it molts (sheds) its exoskeleton.

Incomplete metamorphosis is where the immature stage (or nymph) looks like the adult. There is no pupa stage. The stages of incomplete metamorphosis are egg, immature, immature, and adult. There are as many immature stages as the insect molts its exoskeleton.

Ametabolous insects do not show metamorphosis. The immature insect looks just like the adult, only it may be missing sexual organs. The ametabolous stages are egg, larva (many), and adult.

Discussion: Specific Life Cycle of the Mealworm/Beetle

Mealworms go through complete metamorphosis, changing into beetles. They start life as an egg and then hatch into the larval stage (mealworm). Mealworms can molt (shed their exoskeleton) from nine to twenty times before settling into the pupa stage. The pupa is typically white at first. It darkens just before the beetle emerges. This whole cycle can take three to five months.

The mealworm (larval stage) is generally yellowish brown and can be up to an inch long. The adult beetles are black with hardened wings. It is extremely difficult to tell males from females.

Discussion: Specific Life Cycle of Butterflies and Moths

Butterflies and moths also go through complete metamorphosis. They start as an egg and hatch into the larval stage, known as the caterpillar. The caterpillar goes through many growth stages, molting a various number of times depending on the species. The caterpillar eventually stops eating and changes into the pupa stage. The pupa of a butterfly is called a chrysalis, while the pupa of a moth is a cocoon. The length of time for the pupa stage is dependent on light and temperature. Most are in the pupa stage at least a couple weeks, some up to seven months. The adult butterfly or moth emerges out of the pupa stage wet and groggy. It spends its first few hours drying its wings before flying.

Discussion: Specific Life Cycle of Crickets

Crickets are an excellent example of incomplete metamorphosis. The immature crickets (nymphs) appear similar to the adults, but do not have fully developed wings. As they grow, the wing pads can be observed (not all species of crickets have wings). Crickets grow in length each time they molt.

Crickets can be brown or black with long antennae. They often hold their wings flat over their body. The female cricket can be identified by the presence of a long tube-like structure on the back of its abdomen. This "ovipositor" is for laying eggs.

Discussion: Specific Life Cycle of Springtails and Silverfish

Springtails and silverfish are ametabolous. They have an egg stage, and hatch out looking exactly like the adult. The nymphs may be missing the sexual organs needed for reproduction. Some may possess these organs, but are unable to reproduce until the adult stage.

Activity 1: Caring for and Observing Mealworms

The easiest insect to observe in the classroom is the mealworm. Mealworms can be purchased at any pet store for a minimal price in containers of 25, 50, 100 or more. The smaller size mealworms tend to change into beetles more readily than the large mealworms.

Have the children create a "home" for his or her mealworm by putting a layer of dry oatmeal in the bottom of a clean, used butter (or similar) container with a lid. Be sure to poke small air holes in the lid. Mealworms do not need water.

Pass out one or two mealworms to each child or group of children. Have the children measure the mealworm at the start of the experiment, and at weekly intervals. Be sure to have the children check daily for molts. The mealworms can be handled daily as long as the children are gentle. Keep the containers in a cool, shady spot.

Be prepared for deaths. Insects lay many, many eggs because of the high risk of death of larvae. Explain this to the children in advance, and keep extra mealworms on hand to replace if needed.

Options:

1. Keep a calendar for each mealworm to record number of molts and measurements.

2. Place the emerging beetles on moist sphagnum moss and feed bread (mold is okay) to observe growth and possible mating and/or egg laying.

3. Keep track of the number of mealworms that live and die, and hypothesize possible reasons for each.

4. Have the children research mealworms before the experiment, specifically food, habitat, predators, why it is beneficial, etc.

5. Have the children keep a journal with drawings of the different stages.

Optional Activity 1: Other Classroom Insects

Many insects can often be ordered from scientific magazines or on the web. Care instructions and media (food) are included in most. Be aware, though, that not all insects are native to your area. Many must be destroyed instead of released, and this can be difficult for small children. Some examples of other classroom insects are butterflies and moths, ladybugs, flies, and crickets. Some of these require special care and instructions. Be certain to research any classroom pet before deciding if it is right for your students.

Optional Activity 2: Field Collections

With permission, have the children collect a small container of moist soil (near ponds or streams are best). The children can then use magnifying glasses or a classroom microscope to search for insects from that particular habitat.

Please stress the importance of returning any live catch back to its original place without damage.

Conclusion and Review

Review with the children the three different types of life cycles and examples each. Have the children compare and contrast the life cycles to each other, as well as to the human (or other animal) life cycle. Discuss the possible reasons for one mealworm successfully molting/changing and another dying.

Additional Materials

Journal Drawings Sheet (PDF)



© 2004 Iowa State University Entomology Department. Last modified 3/15/04 by John VanDyk.