When you look at the living things in a pond or stream you will see invertebrates at different stages of development. Typically the first life stage is an egg which hatches into a larva. The larva of most insects grows slowly into the adult form, at which stage it is ready to reproduce. After reproduction, in most cases, the adults die. This sequence of developmental change through an animal's life is called its life cycle. Some animals spend their whole life cycle in the water such as water snails and leeches; others, particularly the animals that can fly like mayflies and dragonflies, spend their adult stage out of the water.
We will now look at the stages of development in more detail.
The eggs of freshwater animals vary in their size and how they are grouped together. For example, leeches lay their eggs in cocoons, each one containing 5 to 15 eggs. Other animals lay their eggs in various patterns surrounded by jelly, for example the eggs of the phantom midge are arranged in round flat clusters on the water surface. Other non-biting midges anchor their eggs to water plants. The great diving beetle lays its sausage-shape eggs individually in slits made in the stems of water plants.
Once the egg hatches, the animal is now called a larvae. Most of your sample collected from a pond or stream will be made up of larvae, for example, mayfly and stonefly larvae.
Insects and crustaceans (for example, the freshwater shrimp) have their skeletons on the outside of the body. In order to grow they need to change their skeleton: the old one splits and is shead, revealing a new one underneath. This process is called moulting. The number of moults varies; mayflies can have between 20 and 30 moults. The stage between each moult is called an instar.
Empty skeletons can often be found when you take a sample from a pond or river though they can be confused with dead animals. These can sometimes be used to identify the animal they came from such as those from dragonflies and non-biting midges.
Some animals such as the freshwater shrimp and water snails do not have a larval stage and the eggs hatch into smaller versions of the adult.
Many larvae look similar to the adults while some have to change again, forming a pupa, before becoming adults.
For some animals the change from larvae to adult is too much to go through in one go. These animals undergo another developmental stage called the pupa. The most familiar example of a pupa is in the life cycle of the butterfly where the adult can be seen emerging out of its chrysalis. Caddisflies (distant relatives of butterflies and moths) also have a pupal stage. At the end of the pupal stage the skin breaks and the adult emerges.
The adult stage of the animal's life is short compared to its larval stage: the main role of the adult is to find a mate and reproduce. For example, mayflies live as adults for 1 - 2 days only compared to their larval stage that lasts several months.
Adult pond and stream animals lay their eggs in different ways. Some mayflies crawl beneath the water surface to lay their eggs, whilst others lay them on the surface of the water. Many adult freshwater animals die soon after mating and egg laying.
Once the eggs have been laid the cycle begins again.