Updated: Dec 22, 2022
Scuds, amphipods, side-swimmers, freshwater shrimp, or whatever you want to call them are important in calcium-rich, spring-fed waters. In fact, there is probably no more important aquatic invertebrate in the streams of the Driftless Area. It is certainly not that scuds are overlooked but they might be a bit underappreciated.
If I may put on my science-geek hat for a bit...
Amphipods (order Amphipoda) are related to insects in that they are both in the phylum Arthropoda (changed to Eurathropoda - I guess...) whose name comes from their jointed, segmented legs. Another character of the phylum is that they and round worms (phylum Nematoda; now Nemathelminthes) are ecdysozoans which means that they molt or shed their exoskeletons and replace them to be able to grow. Why any of this is important to the angler is that scuds have pretty substantial exoskeletons and molt them quite often so they exist largely in streams and lakes with an abundance of calcium. Spring creeks and spring ponds are the perfect habitat for these 'critters' as these waters are high in calcium and their temperatures stay relatively constant which allow scuds to reproduce year-round (less frequently in the winter).
Amphipods fairly typically make up a large proportion of the aquatic invertebrate biomass in many Driftless streams; at times more than 50% of the invertebrate biomass. Scuds - sometimes called 'side swimmers' - are quite active swimmers, something too many anglers overlook (more below). They tend to be most common in slow moving waters of pools - particularly those with a lot of aquatic vegetation - as well as along the margins of streams. They are however also found in the interstitial spaces between rocks within riffles. Quite frankly in most Driftless streams they are just about anywhere and everywhere.
The video above is scuds swimming - just scuds swimming - but it might be useful to see just how active they are and how they swim. I honestly do not fish scuds as much as I probably should. I am not a big nymph fisher (post on how I like to fish) but sometimes it is a necessary 'evil' (it is not evil, I just do not enjoy it as much). If I am being more honest, these days when I do fish scuds it is probably because little else is working - they are my fall back because I know how numerous and important they are. If I absolutely had to catch a fish on a Driftless stream at any time in the year, a scud or Milwaukee leech would be my flies of choice.
Scuds tend to be more active when light is low so fishing them early and late in the day and on overcast days is the most successful strategy. They will probably work on bright, sunny days but they are a much more active when the light is low. By active, I mean that they are actively moving and feeding and thus vulnerable to fish. As you can see from the video, they are pretty good swimmers and they tend to swim in short little bursts (more on that below).
Fly Patterns and How to Fish Them
There are a ton of different scud patterns out there but what most tend to have in common are a fairly heavily segmented body - often with a distinct shell - and something that hangs down to represent their legs. In my opinion, scud patterns can be extremely simple and still be effective so long as they get down fairly deep which is why they often have beads or a large lead or non-toxic wire underbody. My preference is for scud patterns I do not mind losing so I tie them quickly and simply. I have no idea if it makes any difference but I like to tie my scuds with orange, pink, or bright yellow thread which will show through the wet dubbing. The idea is that females often are carrying eggs and that may make them more visible, attractive to fish, or it is just what I hope to be true when I am fishing them. Scuds are often parasitized by an acanthocephalan worm that is bright orange within a scud. One of the most interesting things about these parasites is that they change the behavior of amphipods - they become "zombies" - which makes them more active and thus vulnerable to avian predators which they must pass through as part of their complex life cycle. I really think there is something to the idea of tying with some bright colors and many commercial patterns use orange or pink beads in the middle of the fly for the same purpose. One of the most popular Midwestern fly patterns is John Bethke's Pink Squirrel which is a very good scud imitation (or cased caddis, or just food).
My typical scud is a dubbed body (in a dubbing loop) that is ribbed with a fairly heavy wire and then the legs are picked out below and the top is trimmed. What I have always heard is that you should tie them in colors that match their substrates - olive for vegetation, tan in sandy areas, and dark gray/tan for rocky areas. I have no idea if there is much truth to that idea and I tie most of mine in tan, understanding they will get a little darker when the flies are wet. I have no preference for dubbing but generally like some natural fibers like rabbit fur to make the longer legs when the dubbing is picked out. I do not bother with antenna or a shell back unless I am trying to get fancy. I am not fancy all that often but a few flank fibers fore and aft and a shell back do make for a nice looking fly. I have used UV resin as a scud back as well and that looks nice - but I contend it does not make a difference to the fish. Scuds are one of the few fly patterns where smaller is not necessarily better in the Driftless Area. I normally start with a #12, often on a 2xl hook, and move down to a #14 if it seems necessary. Most scuds, particularly the parasitized and/or pregnant females are quite large.
Probably the most contentious debate around scuds is about what hook to tie them on - a scud hook or straight hook. After years of fishing and observing scuds (I have even published a few papers on scuds and their parasites), my preference is for scuds tied on straight hooks. When scuds are dead or buried in substrates they are often curled up but when they are actively swimming - again, see the video above - they are straightened out. When I am fishing scuds, I want to be able to move them. How I generally fish scuds is to let them sink deeply and then move them in twitches of a couple of inches by pulling on the fly line as the line is taut. Hits almost always come just after you twitch the fly. If you are not moving your scuds, you really need to give it a try.
University of Florida Entomology - Featured Critter
Orvis video: How to tie a simple scud
FlyFisherman magazine - Scud patterns for trout
Fly Craft Angling with Phillip Rowley - Scuds
Dally's Ozark Fly Fisher - Best way to fish scuds and sowbugs
RIO Products - Oh-my-scuds
Fly and Lure - 6 stunning freshwater shrimp or scud fly patterns
Missoulian Fly Angler - Top 4 scud patterns