Dry fly patterns can be broken down into three categories.
Attractors: patterns that don’t represent a specific insect.
Impressionistic: patterns that resemble a certain type of insect.
Imitative: patterns that resemble a specific insect species and life stage.
Attractor Fly Patterns:
Attractor dry fly patterns are used to get the attention of nearby trout that don't appear to be interested in feeding at the surface. Attractors don't attempt to imitate an natural insect that a trout would eat, but instead entice strikes from trout by appealing to their predatory instincts. Most attractor patterns are tied with some type of flash or trigger material in an effort to encourage a trout to bite.
These are the patterns that look like they were developed in someone’s basement out of sheer curiosity. They're often constructed with unnatural colors and materials, and if you held them up to a real insect, they would look nothing alike. Hundreds of attractor patterns have been created, but only a few have withstood the test of time.
Examples of attractor patterns:
Why they work:
No one seems to have a definitive answer. Some studies have shown it has to do with the unique way trout see and are attracted to ultraviolet light. These studies conclude that the ultraviolet light triggers a predatory response in trout that results in a strike. There're several other theories out there, but none have proven to be 100% accurate. This has been, and probably will for quite some time, one of the unsolved mysteries of fly fishing.
When to fish attractor patterns:
Attractor patterns are best used when there's no apparent insect hatch and no visible trout rising. If you're set on dry fly fishing but can't locate specific fish to target, an attractor pattern can sometimes coax a trout into coming to the surface to feed. These patterns can also work well for the dry fly on a dropper rig.
Impressionistic Fly Patterns
Impressionistic fly patterns appearance is similar to certain types of aquatic insects or terrestrials, but aren't designed to be an exact match of any specific insect species. Examples of these types are: mayflies, caddisflies, stoneflies, grasshoppers, and ants. Their ambiguity allows them to be fished effectively in several different trout fishing situations.
Examples of Impressionistic fly patterns:
Adams and Parachute Adams
Why they work:
Impressionistic fly pattern's effectiveness comes from their ability to represent a wide variety of species. For example: If you determine a trout is feeding on mayflies, but aren't sure which exact species it is, you can tie on a parachute adams knowing you're offering a reasonable imitation. Another reason is that they allow the trout to "fill in the blanks", which allows them to determine what they think your fly represents.
When to fish them:
Impressionistic fly patterns are best used in faster moving water when targeting sporadically feeding trout. The faster moving water gives the trout less time to get a good look at the fly, thus decreasing the likelihood of a refusal because of some minor imperfection. The fact that the trout is rising sporadically shows they aren't keyed in on a specific hatch, but instead are looking for anything that resembles food.
Although impressionistic patterns aren't meant to be exact replicas, some consideration as to season, stream type, temperature, etc., needs to be taken into account to determine which type and size of insects are most likely to be present on the stream. Because these patterns aren't suited for "matching the hatch", it may be difficult to visually see which insects are present. This is where hatch charts can come in handy.
*Here's an example of a hatch chart that would be useful.
Imitative Fly Patterns
Imitative fly patterns are used by fly fishermen in match the hatch situations. They are designed to be an exact replica of a specific aquatic insect species and their stage of emergence. To replicate every detail of the natural insect, careful considerations are taken into account as to size, shape, and color when tying imitative fly patterns.
How to match the hatch:
To help determine which imitative fly pattern to use, you first must figure out which insect species the trout are keyed in on. You don't need to know the Latin (or even common) name of the insects to be successful at this, but it is important to know what size, shape, and color they are. To figure these out:
Observe insects in the air. Although this is not a 100% guaranteed way to know what the trout are feeding on, it will give you a good idea of which insects are present.
Step into the water, stand still, and wait for an insect to float past you. Using either your hand or a small seining net, pick the insect up and examine it.
Open your fly box and select the pattern that most closely resembles the insect your captured.