Understanding fly hatches is one of the most useful skills an angler can develop. When trout and grayling begin feeding on insects at the surface, knowing what is happening can make the difference between choosing the right fly and spending the afternoon changing patterns without success.
A hatch is the emergence of aquatic insects from their underwater stages into winged adults. During this process, insects can become particularly vulnerable to feeding fish.
However, **a hatch doesn't necessarily mean fish will be taking adult insects from the surface**. They may be feeding on nymphs rising through the water, insects trapped in the surface film or adults drifting downstream.
Understanding these different stages is often more important than identifying the exact insect species.
## What Happens During a Hatch?
Many of the insects important to UK fly fishing spend most of their lives underwater.
Upwinged flies, for example, begin life as eggs before developing into aquatic nymphs. When they are ready to emerge, they move towards the surface, where the winged adult escapes from its nymphal skin.
This creates several opportunities for trout and grayling to feed.
Fish may take insects:
- As nymphs move towards the surface.
- While insects are emerging through the surface film.
- As newly emerged adults drift on the water.
- When adult insects return to the water to lay eggs.
- When spent adults become trapped or die on the surface.
Each stage presents a different appearance and behaviour, which is why several different artificial flies may be effective during the same hatch.
## The Main Insects That Hatch on UK Waters
Several groups of aquatic insects are particularly important to British fly anglers.
### Upwinged Flies
Upwinged flies are among the most recognisable insects on our rivers.
They include:
- Large Dark Olives.
- Blue Winged Olives.
- Iron Blues.
- Pale Watery flies.
- Medium Olives.
These insects have upright wings when resting and are often seen drifting downstream after emerging.
Their nymphs, emergers, newly emerged adults and returning spinners can all be important food sources for trout and grayling.
### Mayflies
In UK fly fishing, the term **Mayfly** generally refers to the large *Ephemera* species, particularly *Ephemera danica* and *Ephemera vulgata*.
Their emergence is one of the most celebrated events in the British fly fishing calendar, usually occurring during late spring and early summer.
The large insects can encourage trout to feed confidently at the surface, sometimes producing spectacular rises.
However, Mayfly fishing is only one part of the wider fly fishing season, and many other insects provide important food throughout the year.
### Caddis or Sedges
Caddis, commonly called sedges by UK anglers, are another important group.
Unlike upwinged flies, caddis undergo a complete metamorphosis, developing from larvae into pupae before emerging as adults.
Their emergence can be particularly important because fish may feed on the pupae as they move towards the surface.
Adult sedges are often active during the evening, and some species move across the surface rather than simply drifting motionless.
This can make presentation and movement particularly important when choosing an artificial.
### Midges
Midges are small two-winged insects that are extremely important to both river and stillwater fishing.
Their aquatic larvae and pupae are familiar to anglers as bloodworms and buzzers.
During emergence, the pupae rise towards the surface, where the adults escape.
Trout can feed heavily on emerging midges, sometimes producing very subtle rises that are difficult to interpret.
Midges can be active throughout much of the year, including periods when larger insects are scarce.
### Stoneflies
Stoneflies spend their immature stages underwater, usually in well-oxygenated rivers and streams.
Unlike many upwinged flies, stonefly nymphs generally crawl out of the water onto stones or vegetation before emerging.
As a result, their emergence does not usually produce the same surface-feeding opportunities as insects that hatch directly through the surface film.
However, stonefly nymphs and adult insects can still be important food for trout.
## What Is an Emerger?
An emerger is an insect in the process of changing from its underwater stage into a winged adult.
For many aquatic insects, this transition takes place at or near the surface.
The insect may struggle to break through the surface film, leaving part of its body submerged while its wings and thorax emerge above the water.
This is a particularly vulnerable stage.
Trout and grayling can become highly selective when feeding on emergers, sometimes ignoring fully emerged adults floating nearby.
Artificial patterns such as Klinkhamers and CDC Shuttlecocks are designed to represent insects during this transition.
Although these flies are often discussed alongside dry flies, **they are emerger patterns rather than conventional adult dry fly imitations**.
## What Are Duns and Spinners?
These terms are particularly important when discussing upwinged flies.
### Duns
A dun is the newly emerged winged stage of an upwinged fly.
After escaping from its nymphal skin, the dun usually rests briefly on the water before flying away.
During this period, it can be vulnerable to feeding trout and grayling.
Many traditional dry fly patterns imitate this stage.
### Spinners
The dun subsequently moults into the sexually mature adult, known as the spinner.
Spinners return to the water during mating and egg-laying activity.
Once their reproductive activity is complete, many fall onto the water with their wings spread flat.
These are commonly known as **spent spinners**.
Fish may feed selectively on spent spinners, particularly during evening falls of Blue Winged Olives and other upwinged species.
Recognising this stage can be important because a conventional upright-wing dry fly may not provide the correct silhouette.
## Why Do Hatches Occur at Different Times?
Different insects emerge at different times of the year.
Their development and emergence are influenced by several factors, including:
- Water temperature.
- Seasonal changes in daylight.
- Weather conditions.
- The insect species.
- Local river conditions.
Some insects have relatively short and concentrated emergence periods, while others hatch over much longer periods.
For example, Large Dark Olives are particularly associated with the cooler parts of the trout season, while Blue Winged Olives are an important feature of summer fishing.
The large British Mayfly has a much more concentrated seasonal emergence.
These patterns provide a useful guide, but **hatch calendars are not fixed timetables**. Local conditions can alter both the timing and intensity of insect activity.
## Does a Hatch Always Bring Fish to the Surface?
No.
Even when insects are emerging in large numbers, trout and grayling may continue feeding beneath the surface.
There are several possible reasons.
Fish may be taking ascending nymphs before they reach the surface, or emerging insects may be more readily available beneath the surface film.
Water temperature, light conditions and the behaviour of the fish can also influence where feeding takes place.
This is why seeing insects on the water doesn't automatically mean a conventional dry fly will be effective.
If fish are producing bulges or swirls without clearly breaking the surface, an emerger or subsurface pattern may be a better choice.
## How Do You Match a Hatch?
Matching a hatch means selecting an artificial fly that reasonably represents the natural insects being eaten.
You don't necessarily need to identify the exact species.
Start by considering four things.
### 1. Size
Try to match the approximate size of the natural insect.
This is often more important than choosing an exact colour.
### 2. Shape and Silhouette
Consider whether the insect has upright wings, lies flat in the surface film or has a body hanging beneath the water.
The silhouette of your artificial can make a significant difference.
### 3. Stage of Emergence
Are fish taking nymphs, emergers, duns or spent spinners?
Choosing the correct stage may be more important than choosing a particular named fly pattern.
### 4. Presentation
Even a very accurate imitation can be refused if it behaves unnaturally.
On rivers, a drag-free drift is often essential when imitating insects being carried downstream by the current.
Some insects, particularly adult sedges, may require a different presentation involving movement.
## What If Several Insects Are Hatching?
This is common, particularly during warmer months.
You may see several different upwinged flies, sedges and midges active at the same time.
Rather than assuming fish are feeding on the most obvious insect, spend time watching individual fish.
Look at what is drifting towards them and whether they appear to be taking insects from the surface or just beneath it.
A fish may become focused on one particular insect or stage of emergence even when several other food sources are available.
This is often referred to as **selective feeding**.
## What If You Cannot Identify the Hatch?
Don't worry.
Even experienced anglers cannot identify every insect they encounter.
You can still make an informed choice by observing:
- The approximate size of the insect.
- Its general colour.
- Whether it has upright or flat wings.
- Whether it is emerging or fully developed.
- How it behaves on the water.
A small selection of general dry flies and emergers in several sizes can cover many common situations.
Patterns such as the F-Fly, Parachute Adams and Klinkhamer are useful because they can suggest a range of natural insects without requiring an exact identification.
## Do You Need to Know Latin Insect Names?
No.
Learning to recognise the main insect groups and their behaviour is much more useful for most anglers than memorising scientific names.
Knowing that trout are feeding on small olives or emerging sedges gives you a practical starting point.
As your experience develops, you may become interested in identifying insects more precisely, but this is not essential for successful fly fishing.
## Key Takeaways
- A hatch occurs when aquatic insects emerge from their underwater stages into winged adults.
- Trout and grayling may feed on several different stages of the same insect.
- Upwinged flies, Mayflies, sedges and midges are particularly important in UK fly fishing.
- Stoneflies generally emerge by crawling out of the water rather than hatching directly through the surface film.
- Emergers are often particularly vulnerable to feeding fish.
- Duns are newly emerged upwinged flies, while spinners are the sexually mature adults.
- The timing of hatches varies with the season, insect species and local conditions.
- A visible hatch does not guarantee that fish will feed on the surface.
- Matching the size, silhouette, lifecycle stage and presentation is often more useful than identifying the exact insect species.
- You do not need to become an entomologist to become a successful fly angler.
In the next article, we'll look at **how to choose a dry fly**, including the importance of size, silhouette, colour and presentation when selecting a pattern for trout and grayling.
