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LESSON 1 - INVESTIGATING DIFFERENT TYPES OF METAMORPHOSIS BY GROUPING ORGANISMS

OBJECTIVE

· Identifying arthropods.
· Grouping invertebrates and vertebrates.

VOCABULARY:

·        arthropod
·         insect
·         metamorphosis

MATERIALS:

· Set of different groups of insects

· Set of plastic insects

· Life Cycle of Mosquito models

· Insect worksheet

BACKGROUND:

Arthropods are bilaterally symmetrical and have jointed body segments with a pair of appendages attached to each body segment. The body is covered with a cuticle (thickened substance) which comes off ("molts") when the arthropod gets larger. There is no internal skeleton. Many arthropods go through metamorphosis or physical changes. So a butterfly is the adult insect but the caterpillar is one of the early stages and looks more like a "worm" than an insect. Many arthropods like mosquitoes spend most of their life in an aquatic form before it metamorphoses into a flying insect.

Identification of arthropods is not easy, because there are so many "creepy crawlies" out there.

Insects develop from egg to adult in a process called metamorphosis. Different groups undergo metamorphosis in different stages. Some arthropods have a different phase that looks totally different than the adult. This is called complete metamorphosis. which may be generally classified as either gradual or complete. Gradual or metamorphosis has three stages – egg, nymph, and adult. Nymphs generally look much like their adult stage except for being smaller and lacking wings, if the species has winged adults. Common examples include stink bugs, grasshoppers, and cockroaches.

About 75% of all insect species go through the four stages of complete metamorphosis - egg, larva, pupa, and adult. The larva is a specialized feeding stage that looks very different from the adult.

I. Simple Metamorphosis
Insects with simple metamorphosis have three life stages: egg, nymph and adult. Adults and nymphs of these insects usually feed on the same foods. These insects may have wings, which if present, develop externally. Young are known as nymphs and there is no resting stage (pupa).

Note: Some naming conventions provide sub-categories for simple metamorphosis as follows:
1) insects with no metamorphosis—or no apparent metamorphosis (such as Springtails, Order: Collembola);
2) insects with incomplete metamorphosis (Egg > nymph > adult) (such as Dragonflies, Order: Odonata); and
3) insects with gradual metamorphosis (such as True Bugs, Order: Hemiptera).

II. Complete Metamorphosis (Egg > Larva (more precisely: larval instars) > pupa > adult )
Insects with complete metamorphosis have four (4) life stages: egg, larva, pupa and adult. Larvae are very different in form from the adult. The active immature stages are known generally as larvae.

These insects also have a resting stage known as a pupa. The pupal stage is a transition stage, when the larva transforms into the adult. Wings, if present, develop internally within a pupa. The pupa molts to the adult form.

Insects with complete metamorphosis include lacewings (Order Neuroptera), beetles (Order Coleoptera), butterflies and moths (Order Lepidoptera), true flies (Order Diptera) and wasps and bees (Order Hymenoptera). In some insect orders, larvae are referred to by other names, such as maggots (flies), caterpillars (butterflies and moths) or grubs (beetles). Larvae of insects with complete metamorphosis usually have chewing mouthparts; many are pests of various crops.

III. No Metamorphosis (Egg-Adult) no change in appearance. Example would be a silverfish.

PROCEDURE:

1. Today we will be talking about the life cycle of insects. The majority go through metamorphosis though not all do. The word metamorphosis means to change shape.

2. There are two kinds of metamorphosis: complete metamorphosis or incomplete/simple metamorphosis. (No metamorphosis means hatched insect just gets bigger over time.)

Play video on complete/incomplete (simple) metamorphosis.

3. Put the specimens from an entomologist on the table (as a display). They were collected in the 1980’s so please tell the students the importance of not touching them. This is a typical way in which an entomologist collects his specimens and identifies them. Have them try and determine which one is which, and try to have them guess what type of metamorphoses they go through. Remember it is difficult to determine the different stages, so the more they go over it the more they will be familiar with the changes. Important part is for them to realize that they look very different. Look at following site for charts on the type of metamorphoses (https://www.uky.edu/Ag/Entomology/ythfacts/4h/unit2/metavari.htm).

4. Sorting Animals and Arthropods. The Phylum Animalia is composed of vertebrates (with backbone) and invertebrates (without backbone). At each table put a set of organisms and set of “Bug Worksheets.” Ask the students to separate them into invertebrates and vertebrates. After you check each of the tables, ask students to put all the vertebrates back into the bag. ANSWERS: snake (reptile), shark (fish), bat (mammal), frog (amphibian), and penguin (bird).

5. Sorting Arthropods. Tell students that the models that are left are all Arthropod or animals that have an exoskeleton. An exoskeleton is when the “skeleton” is on the outside. Vertebrates have their skeletons inside. Having an exoskeleton poses a problem for Arthropods. Ask students what that problem is….. after they guess, tell them the problem is when they grow they will not fit into their exoskeleton so they have to lose it (molt) in order to grow bigger.

Of the models left, ask students if they can sort them into more groups. The placemat might help them interact with the models. Give them a few moments to see if they can sort them into arachnids (8 legs), insects (6 legs) and millipedes (many legs). You might want to see if the students can identify some of the insects. Answers are below (please note we do not have models for each group).

Order Odonta (Dragonfly) - they go through incomplete metamorphoses, with the eggs laid on the surface of the water and then they change to a nymph that looks nothing like a dragonfly,. They emerge from the water and then become a land insect.

Order Hymenoptera – Ants, bees, wasps - complete

Order Lepidotera – Moths and Butterflies go through complete – so egg to larva to pupa to adult

Order Diptera - Mosquitoes and Flies - complete metamorphosis (many have aquatic stage, like mosquito)

Order Orthoptera – Grasshoppers - gradual

6. Look at the stages of one insect of the Order Diptera, the Mosquito (using the models). Mosquito has full metamorphoses and has an aquatic stage before they fly on land. They are considered a "bad" insect in that they transmitt diseases through their need for blood. Only the females require blood. They need the proteins to lay eggs.

Read the following and see if students at each table can determine which of the models is the correct one. Read the information below and have each table select the correct stage.

(Mass of Eggs) A female mosquito needs a water source in which to lay her eggs. Stagnant water, such as ponds and marshes, make ideal nesting grounds. She will lay her eggs one at a time, laying several hundred in total. Some species lay their eggs in connected rafts, which others’ eggs stay separated in the water.

(White with tuffs of hairs) Larva. In just a couple of days mosquito eggs will hatch into larvae. This is an active, growing stage in which the larvae must eat often and shed their skin as they grow. The larvae stay in the water in which the eggs were laid, eating whatever organism matter is present.

(Tan with big head). Pupa. After the larvae shed their skin several times, they enter the pupa stage. As with most insects, this is an inactive stage in which the mosquito transitions into an adult only about two days to complete the stage, after which the mosquito is an adult

Mosquito. After its pupa skin splits open, an adult emerges still in the water in which the eggs were laid. It takes a few minutes for its winds to dry before the mosquito can take flight. Only female mosquitoes feed on blood, which is needed for them to lay eggs.

Play the video showing Mosquito metamorphosis: https://www.youtube.com/watch?v=wFfO7f8Vr9c (3 min)

To get rid of or reduce mosquitos people are told to get rid of standing water around their houses. Knowing about the life cycle of the mosquito, why does getting rid of standing water help get rid of mosquitos?