Instarfly fishing glossary

A developmental stage of an insect between two molts, particularly used to describe the growth stages of nymphs and larvae.

Understanding Instar

In the intricate world of aquatic insects, which form the cornerstone of a trout's diet, understanding their life cycles is paramount for any successful fly angler. The term instar refers to a specific developmental stage of an insect between two molts. It's particularly relevant when describing the growth phases of nymphs and larvae, which are the immature, aquatic forms of insects like mayflies, caddisflies, and stoneflies.

Imagine an insect encased in a rigid, non-expanding exoskeleton. To grow, it must shed this outer layer in a process called molting. Each time an insect molts and sheds its old skin, it enters a new instar. This new stage often brings subtle, or sometimes significant, changes in size, color, and even body proportions, as the insect grows larger and develops further towards its adult form.

For example, a mayfly nymph might go through 15 to 20 instars during its life in the water, starting as a tiny, almost microscopic creature and progressively growing larger with each molt. Each instar represents a distinct "size class" of that particular insect species. While the general body plan remains the same, a first-instar nymph will be minuscule and pale, whereas a late-instar nymph, nearing emergence, will be considerably larger, darker, and often show visible wing pads.

Why does this matter to the fly angler? Because fish, especially discerning brown trout and rainbow trout, become highly selective feeders. They key into the dominant size and color of the available food source. If a specific hatch is underway, understanding the instar stage the fish are feeding on can be the difference between a full landing net and an empty one.

How It Works on the Water

The practical application of understanding instars lies in observation and imitation. When you're on the water, your goal is to match the hatch, and that includes matching the size and color of the aquatic insects currently available to the fish. This isn't just about identifying the species; it's about identifying its developmental stage.

Observing Instars

Under Rocks: The simplest way to observe instars is to turn over rocks in the riffles and runs where fish feed. Collect a few nymphs or larvae in a small container or your fishing vest pocket with some water. Pay close attention to their size, overall color, and subtle features. Are they tiny and pale, or larger with distinct dark markings? This tells you which instars are currently active.
Water Column: Sometimes, fish will be feeding on instars that are drifting freely in the current, perhaps dislodged by high flows or preparing to emerge. Observing fish rise forms can give clues, but often, it's about what you find under the rocks.
Stomach Samples: If you keep a fish, a quick look at its stomach contents can be incredibly informative, revealing exactly which instar sizes and species it's been targeting.

Selecting Your Flies

Once you've identified the dominant instar, you can choose your fly pattern. Many popular nymph patterns, such as the Pheasant Tail Nymph or Hare's Ear Nymph, are tied in a wide range of sizes (e.g., #20 to #10) and often with varying amounts of weight to imitate different instars. For example, if you find tiny, early-instar BWO nymphs under rocks, you'll want to tie on a small, unweighted BWO nymph imitation, perhaps on a #20 hook, rather than a larger, bead-head version.

Understanding instars is also crucial for nymphing techniques like Euro Nymphing, where precise depth, drift, and fly size are critical. A slight change in the size or weight of your Mop Fly or Pat's Rubber Legs can often make a significant difference, reflecting the specific instar the fish are keying into.

Pro Tips

Always carry a diverse selection of nymph and larva patterns in various sizes, from tiny #22 midges to chunky #8 stonefly nymphs. Fish can be incredibly selective, and often, it's not the pattern itself, but the size and the subtle color variations that mimic a particular instar that will seal the deal. Don't be afraid to go smaller than you think – sometimes, those early instars are what the fish are truly focused on, especially during periods of low current or sparse hatches. A small change in tippet size can also help present those tiny instars more naturally.

Molting: The process by which an insect sheds its exoskeleton to grow. Each molt signals the transition to a new instar.
Nymph: The immature, aquatic stage of insects with incomplete metamorphosis (e.g., mayflies, stoneflies). Nymphs grow through multiple instars.
Larva: The immature, aquatic stage of insects with complete metamorphosis (e.g., caddisflies, midges). Larvae also progress through various instars.
Pupa: The transitional stage between larva and adult in insects with complete metamorphosis (e.g., caddisflies, midges). Pupae also undergo internal changes that can be considered instar-like, though the term is less commonly applied externally.
Emerger: The stage where an aquatic insect transitions from its nymphal or pupal form to its adult winged form, often ascending through the water column. While not an instar itself, the late instars of nymphs and pupae are often mistaken for or immediately precede the emerger stage.
Hatch: The mass emergence of aquatic insects from the water, often triggering intense feeding activity by fish. Understanding which instars precede a hatch can be key to success.

On the Water

Understanding instars is a subtle but powerful tool in your fly fishing arsenal, providing critical insights into fish behavior:

The Early Season Midge Game: In early spring, during cold winter flow conditions, larger aquatic insects may be scarce. However, tiny midges are often active year-round. Fish might be exclusively feeding on the earliest instars of midge larvae and pupae, which are barely visible. If you're seeing fish rise but can't discern what they're eating, try a tiny Red Zebra Midge or an Iced Out Midge Pupa in a size #20 or #22. This often happens in still waters or slow pools where fish have ample time to inspect their food.
Post-Hatch Lull: You just experienced a fantastic BWO hatch, but now the fish have stopped rising to your BWO Comparadun. Instead of switching to a completely different dry fly, consider that the fish might have moved to feeding on the remaining late-instar nymphs that didn't emerge, or perhaps spent spinners. Or, they could be targeting the next wave of instars of a different insect altogether. Turning a few rocks can quickly reveal if smaller, earlier instars of the same mayfly are still present and drifting.
Seasonal Transitions: As seasons change, so do the dominant instars. In late spring or early summer, as water temperatures rise, larger stonefly nymphs might be nearing emergence, representing a significant food source. Conversely, in the heat of summer, smaller, faster-cycling insects like Tricos or PMDs might be more prevalent, with fish keying into their smaller instars. Knowing this helps you prepare your fly box for specific times of the year, like during prime Washington May Fly Fishing.