McKenzie Caddis
Arctopsyche grandis
Key Identification Characters - larva: 2
- Abdomen with row of gill tufts along underside.
- Dorsal surface of each thoracic segment with dark hardened (scleritized) plate.
- Body olive-brown dorsally and olive to green ventrally.
- Body length of mature larva 18-25mm.
Similar larvae: 2
This species has similar features as other species of Hydropsychidae. It is unique however in its size with mature larvae about one inch long (small larva to the right of the Arctopsyche in the photo is a typical Hydropsyche species). Identifying young Arctopsyche grandis from other Hydropsychidae species requires careful viewing under a microscope.
Key Identification Characters - adult: 2
- Large caddis with body length (to tip of wings) between 18-25mm; body length excluding wings 9-13mm. Females noticeably larger than males.
- Body color dirty to bright green when newly emerged, darkening to olive or olive-brown after several hours.
- Wings dark gray with some light speckling.
Similar adults: 2
Caddis adults all have the same general shape, but vary widely in size and color. Arctopsyche grandis is one of the largest caddis adults you will typically see, though not as large as the October caddis. Its large size, green to dark olive body, dark wings, and emergence from heavy fast water in cold water streams, all help narrow the list down to this species. If larvae have also been collected from the same area at an earlier date, all the better. Complete confirmation requires a careful look under a microscope.
Habitat & Distribution 2
Arctopsyche grandis is a widespread caddisfly in swift streams throughout the West, but its distribution is limited to streams with icy water and fast currents over large boulder substrate. Generally this means it is limited to higher elevation streams in mountainous regions, but I have collected it at relatively low elevation if other conditions are satisfactory.
In Oregon the best known populations of Arctopsyche grandis are found in the mid to upper reaches of the McKenzie River above Eugene. It is such an important hatch to fly anglers on the McKenzie that it is regionally called the McKenzie caddis. Throughout the rest of the state you are most likely to find it in the mid to upper reaches of fast flowing cold-water streams. With the exception of the McKenzie River it is usually sparse in numbers.
Life Cycle - Emergence 2
Because of the wide distribution of this species emergence periods vary, and adult activity may be spread out over several months. That said massive hatches are well known on many western streams in April and May, and as a result it is often called the Mother’s Day caddis due to the emergence occurring around Mother’s Day.
In Oregon the dominant emergence period for B. americanus is early April through May, but fall emergence (mid-September-October) can also occur. It is uncertain if this fall emergence is the same species.
Oregon Emergence Period: April-May (also Sept-Oct?)
Metolius: May-October
McKenzie: April-May.
Necanicum/Nehalem: ?
Sources and Credits
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
