Entomology for beginners

By Bruce Tait

Most fly fishing is based upon imitating the foodstuff of our quarry. For the river angler this predominantly means imitating insects at various stages of their life cycle. To be able to do this we need to be able to identify the insects and have a basic knowledge of their life cycle.

It is not necessary however to fully understand all the groups, sub groups and classes or to be able to identify them by their Latin names. What is important is to understand the basic types and at what stage in the life cycle the fish are taking them.

The following is a brief introduction to the basic fly types, including a fly calendar to show when during the year we can expect to see them and hence when a suitable imitation should reap rewards.



  • Two large upright transparent or opaque wings
  • Two or three long tails
  • A segmented body and a thorax
  • Almost all have two small hind wings
  • Examples
    Mayfly (Green drake)
    Blue winged olive
    Large dark olive

    Life cycle
    All upwinged flies go through a four-stage life cycle

    Egg –> Nymph –> Dun –> Spinner

    The first two stages are underwater while the last two are the adult winged stages.

    There are various forms of nymphs and these are generally classified by how they behave:

  • Burrowers: Live in tunnels in the mud, sand and gravel on the river bed eg, mayfly nymph
  • Silt crawlers: Tiny nymphs which crawl along the silty beds of rivers eg, caenis
  • Stone clingers: Spend most of their time on the undersides of stones and are rarely seen swimming around, eg, March brown nymph
  • Moss creepers: a group of weak-swimming nymphs who feed on mosses on the river bed, eg, Blue winged olive nymph
  • Laboured swimmers: Spend most of their time in the margins but can swim from place to place, eg, Claret nymph
  • Agile darters: Swim freely and rapidly from place to place and are therefore available for fish to feed on, eg, Dark olive nymph


    • Four wings, with the front pair usually being longer than the rear pair
    • When at rest the wings lie close along the body in an inverted V shape
    • No tails
    • Look similar to moths but have a coating of fine hairs on their wings whereas moths are coated in tiny scales

    Sedge or Caddis

    Life cycle
    Sedges also have a four-stage life cycle; however it is different to that of the upwinged flies

    Egg –> Larva –> Pupa –> Winged adult

    The majority of the larvae live in cases which they construct from materials found on the river bed such as grit, sticks, leaves etc. The remaining larvae are free swimming.

    Examples of free-swimming larvae
    Rhyacophila larva
    Hydropsyche larva


    • Two short transparent wings which mostly lie flat along the top of the body
    • No tails
    • Two of the most common species of this group are the house fly and mosquito

    Hawthorn fly

    Life cycle
    This is a large group and many have a life cycle which are land-based and therefore not of interest to the angler. Only the adults which are blown or drop onto the river will be of interest. There are however, two members of this group that do have aquatic life cycles and are therefore of interest: these are the Reed smuts and the Midges. The reed smuts are tiny black flies and almost too small for the angler to imitate.

    The midges are slightly larger. Their life cycle is:

    Eggs –> Larva –> Pupa –> Winged adult

    The larvae are worm-like and vary in colour from pale green through to bright red.


    • Four wings which are hard and shiny
    • Wings appear very narrow when the fly is at rest
    • Wings lie flat along and slightly over the body

    Willow fly
    Needle fly

    Life cycle
    Stoneflies have a three-stage life cycle:

    Egg –> Nymph –> Adult

    The nymphs have two tails and quite long antennae. The mature nymph crawls to the shore or finds a convenient rock or post to climb out of the water where it emerges into the adult. During this migration it is particularly vulnerable to the trout.


    This is a general idea of what you would expect to see on the River Derwent. Timing is deliberately vague as a number of factors will influence the hatches, including river and weather conditions.

    Click to enlarge