3.4. Insects and Their Life History

Because of the importance of food for fish it is important for you to understand a little about insects and how they live. Looking for insects is one of things you will be doing when you survey your watercourse. As food for so many fish and other animals, the presence or absence of insects are important indicators of stream health.

Many insects use the water for the early stages of their lives. Winged adults can often be seen flying above or resting on the stream vegetation and water, but it is beneath the surface of the water, among the rocks and stones, that the real abundance of insects occurs in immature forms.

The adult female of aquatic insects lays her eggs in or on the water. Then the insect life cycle follows one of the two patterns:

1. In forms with simple life cycles, eggs hatch into "nymphs". These can be somewhat like adults except they lack wings,

2. In forms with complex life-cycles, the eggs hatch into "larvae", a grub like form of life. After a period of feeding, a mature larvae develops into a pupa (a resting stage during which the adult structures, most noticeably the wing pads on the back of the thorax, develop).

Both larvae and nymphs are voracious feeders. Some clamber about the bottom of the stream browsing on plant material, some prey on other animals, and others remain fixed in one place getting their food from water by means of intricate filtering mechanisms. Growth is by molting, a shedding of the body-covering to allow for enlargement. As maturity approaches, the pupa or mature nymph ascends to the water surface or walks ashore to allow the adult to emerge. During this phase it becomes very vulnerable to foraging fish. After emergence, the adult rests for a time to allow its wings to harden before flying off. Mating on the water takes place in the adult phase, completing the cycle.

Immature stages of aquatic insects occur on or under the water, but only the true bugs (Hemiptera) and the beetles (Coleoptera) have aquatic adults. In the other major groups of aquatic insects, the adults live on land, usually remaining close to the stream. They can often be seen in mating swarms or laying eggs on the water surface. The adult stage is mainly a reproductive phase; many adults are short lived and most do not feed.

Important examples of aquatic insects with simple life-cycles are stoneflies (Plecoptera) and mayflies (Ephemeroptera). Complex life cycle types include the true flies (Diptera) and Caddis flies (Trichoptera).

Aquatic insects occur in all bodies of fresh water, from stagnant pools to the largest lakes, from the smallest trickle to the fastest rivers. A typical stream suitable for trout and salmon is roughly composed of two alternating and contrasting habitats: the riffle and the pool (described earlier). Riffles, where the water is shallow and fast, contain gravel with a lot of oxygen. This gravel offers favourable spaces for many animals, and it is here that most of the stream insects occur. Diatoms (minute green plant cells), a principal food of herbivorous larvae and nymphs, grow on the tops of stones in these rapid sections. Fast currents also carry minute particles of food to insects such as black fly larvae and the net spinning caddis, which feed by filtering mechanisms.

The three illustrations on the following pages show the life cycles of the caddis fly, mayfly, and the stonefly.