Net Builder Caddis / Hydropsyche Caddis

Unlike many other caddis species, Hydropsyche Caddis (or net-spinning caddisflies) don’t build a case in which to live. Instead, they build a rough shelter of gravel and plant debris that they attach to the sides of rocks.

The term “net-spinner” derives from the spider-like web these caddis larvae weave at the front of their shelters. These silken nets strain the larvae’s food from the currents. The need for current to bring them food means that nearly all net-spinners live in flowing waters (a few species occur along wave-washed shores of lakes).

Among the net-spinning caddis, four genera from the Hydropsyche Caddis are most important in the West.

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Habitat and Life Cycle of Hydropsyche Caddis

The Hydropsyche Caddis prefer moderate to large sized streams with warmer temperatures, somewhat slower currents, and smaller substrate.

Like all caddis, net-spinners pass through four stages of development: egg, larva, pupa, and adult. Most Hydrospyche Caddis require one year to develop from egg to adult. Some Hydropsyche Caddis pass through two generations in a single year.

The exact emergence periods are difficult to predict because of the variety of species and habitats. In general, hatches are sparse in early spring. Often, the first large emergences occur during the last weeks of May through June. The heaviest activity often occurs in July and August and some continue emerging into September and October.

Larval Stage – Hydropsyche Caddis

The larvae of net-spinning caddis periodically crawl out of their shelters, let go, and drift downstream 40, 50, or even 100 feet. This activity occurs on a daily cycle, and peaks near sunrise and sunset. Entomologists call this “behavioral drift” and speculate that it functions to disperse insect populations, thereby relieving competition and allowing the colonization of underutilized areas.

For the fishermen, it means increased food for trout, making nymph fishing during periods of peak drift very effective.

Some Hydropsyche larvae throw a twist into normal drift behavior. Instead of simply letting go of the substrate, they attach a silk thread to the bottom and lower themselves downstream on a “life line.” In his book “Caddisflies”, Gary LaFontaine discusses his increased success fishing with net-spinning caddis larva patterns when he colored the last 18 inches of his leader white to suggest this silk anchor line of the natural.

Pupal Stage – Hydropsyche Caddis

Once the larvae are mature, they seal themselves inside their shelter and transform into pupae. The pupae remain sealed inside until ready to emerge into adults. The complete development of the pupae typically requires four to six weeks.

When ready to emerge, the pupae swim to the surface, which is perhaps the most vulnerable period of the insect’s life cycle. Trout feed selectively on the rising pupae and imitating them is one of the most effective methods to use during a caddis hatch.

Peak emergence activity occurs in the late morning or early afternoon during the spring and fall. In mid-summer heavy hatches occur in the late afternoon and evening.

Adult Stage – Hydropsyche Caddis

Adult Hydropsyche Caddis spend most of their time hiding on streamside vegetation. Mating occurs on the foliage, and unless a wind blows them over the water they are unavailable to fish.

Once egg laying begins, however, their vulnerability increases dramatically.

Large swarms of gravid females congregate over the water from afternoon to late evening. To lay their eggs, they dive into the water and swim to the bottom, where they deposit strings of eggs on the substrate. Once egg laying is complete, they swim feebly back to the surface.

Such behavior makes them easy targets for feeding trout and an important stage for fly fishers to imitate. On rivers like the South Platte or Arkansas, some of the fastest and most consistent fishing of the season occurs during the last hour of light when the Hydropsyche’s lay their eggs.