Cycles of life

Many of the animals found living in the water are insects going through the juvenile stage of their life cycle. Depending on the insect group concerned, we call this stage a nymph or a larva. The adult insect lays eggs in or near the water, and once these have hatched the juvenile will feed and live as an aquatic animal until it is time to leave the water as an adult. This stage may last just a few months or, as in the case of some dragonflies, a couple of years.

A nymph shows many of its adult features, e.g. same number of legs and tails, and visible wing buds on the back. When it reaches adulthood it will simply crawl out of the water, sit on a plant stem, shed its outer skin and "emerge". You can often find the discarded "casts" around the edge of ponds and rivers in the spring. Mayflies, damselflies and dragonflies all produce nymphs, and their life cycles are said to involve incomplete or partial metamorphosis.

A larva looks very different from the adult form, and has to undergo an extra stage between juvenile and adult . We call this stage the pupa; within the pupal case, or cocoon, the insect becomes completely transformed. Caddis flies, black-flies and crane-flies all produce larvae, and their life cycles are said to involve complete or full metamorphosis.

Amphibians - frogs, toads and newts - also go through a metamorphosis, allowing adults and young to live in different habitats. The familiar passage from egg (spawn) through larva (tadpole) to adult in frogs and toads is repeated in a very similar way in newts.

Aims: To explore the principles and details of life cycles and metamorphosis.

Activities

1. Freshwater aquaria

Pond creatures can be maintained in the classroom in a tank of water. Place your tank in a warm and light place but not in permanent direct sunlight. Introduce some pond water and pond sediment to start off your community of pond life. Tap water used to top up the aquarium should be left for two days to allow its chlorine to be released. Aquatic plants should be included, as well as some emergent plants or sticks to allow creatures to crawl out of the water, and an island with a slope for beaching if tadpoles are to be kept through metamorphosis. Be careful not to include too many predators. Once set up, the aquarium should be self maintaining. Remember to use a cover which prevents escapes.

2. Whose eggs are these?

Set up small freshwater aquaria in which to observe all stages of the life cycles of single species. Exclude other species whose eggs etc. might be confused with those of the species under study, but make sure that the organisms on which they feed are present. Suitable subjects for study might include insect larvae, snails etc. You will also be able to identify any eggs found in the pond by hatching them under such conditions.

3. Tadpole tank

Set up and maintain an aquarium. Use only a small amount of spawn, as tadpoles will die if overcrowded. An air pump (if available) is a useful precaution against deoxygenation, but make sure it does not stir up the water too much. Drop a few rabbit food pellets into the water each day during the tadpoles' early (vegetarian) stages; feed a little flaked fish food daily from the time they grow hind legs. Other creatures should take up any surplus, as long as it is not too excessive, so you should not need to change the water. Newt tadpoles can be kept in the same conditions, but will find their food (small creatures) within the community of pond life in the aquarium. As well as monitoring growth and development, you will be able to observe life processes such as feeding, breathing and locomotion; each of these varies with age.

Pond Pack Introduction Why study ponds? Pond-dipping Safety for teachers Safety for kids Animal variety Moving and breathing Cycles of life Webs of life Plant life Maths English Geography History Art and design Make a pond Plant identification Animal identification Activity sheets Bug dial Where to go Acknowledgements EDUCATION NATURE SEARCH . . . . . . . . .