SCUDS, ISOPODS AND WORMS OH MY
Some of the most important trout foods are not mayflies, caddis flies, stone flies or even the ubiquitous midge. While these insects may constitute the bulk of the trout’s diet on freestone streams in spring creeks and many stillwaters there importance pales in comparison to scuds and isopods.
Scuds are in reality a type of fresh water shrimp. They are found in the class of Crustacea which includes such familiar animals as crabs, lobsters, and crayfish. They belong to the order of Amphipoda that includes of 7,000 described species of shrimp-like crustaceans. The word Amphipod is an interesting word. Amphi is Latin for ‘of both kinds’ and refers to the legs of these creatures. Amphipods have legs that are specialized for various activities; jumping, walking, swimming, and feeding. Podos is Greek for ‘foot.’ Thus we have Amphipod – a creature with both kinds of feet.
These creatures have a shrimp-like body that is flattened from side to side. Unlike other crustaceans they have no hard shell like carapace over the thorax, and the abdomen is not clearly divided from the thorax. The head displays two antennae, and a pair of compound eyes. They have eight pairs of legs, the first pair is modified to act as mouthparts, and two pair is used to help gather food and the rest of the legs are used for locomotion.
What is important to the angler is that these creatures are very common in many places, and are a very recognizable food form to many species of fish. They are herbivores consuming a variety of aquatic plants, but they will also scavenge on animal matter. Because they are semi-transparent the food that they eat influences the color of their body. They live among the aquatic plants that provide their primary food source, and since they assume the color of the food they ingest they are well camouflaged. Fish will often root around in aquatic weeds and flush them out so they can be eaten.
Scuds are not strong swimmers. They swim for a short distance and when they stop swimming they fall to the bottom, usually in a curled position. This movement is very erratic and the angler should attempt to make their imitation act like the natural. The angler should use short erratic strips and then allow their imitation to sink before repeating the action.
The legs of scuds are constantly in motion moving water over their gills so the angler should use imitations that incorporate materials that move easily when they are in the water. Mating scuds form a tandem unit with one on top of the other. Pregnant females have an orange spot that represent the developing eggs within the body of the female. Anglers should consider tying some imitations that incorporate some orange material to imitate this feature.
Scuds do not go through the larva, pupae, and adult stages that insects undergo. When they hatch from the egg they are merely a smaller version of the adult. They do not hatch into something else and they spend their entire live as an aquatic creature.
Since baby scuds are merely a smaller version of the adult the angler should have a variety of sizes in their fly box. In addition, since scuds are basically the color of what they eat the angler should have imitations in several shades of color.
My favorite scud pattern consists of a dubbed body of the appropriate color with a strip of clear plastic from a sandwich bag pulled over the body and held in place by monofilament ribbing. I use a dubbing brush or bodkin to pick out the dubbing on the underside of the fly to imitate the legs. I tie them in a variety of sizes and various shades of olives. I tie some weighted and some unweighted. I tie them on curved nymph hooks in sizes from 12 to 18. On the larger sizes I tie some with a bit of orange fur in the abdomen to imitate the pregnant female.
Isopods are an aquatic form of sowbug, and like the scud they are members of the class of Crustacea. They belong to the order of Isopoda which derives from the Greek. Iso means ‘same’ and pod means foot. Unlike scuds, which are flattened side to side, isopods are flattened top to bottom. Like scuds they come in a variety of colors depending upon their habitat. The standard pattern for imitating Isopods is the Ray Charles. This fly consists of an ostrich herl body and pearl tinsel pulled over the back. The ostrich herl forms the body and the legs and the tinsel represents the shell-like body.
Aquatic worms are close relatives to the common earthworm except they live underwater. Like their land based relatives they burrow around in and ingest mud, removing and digesting bits of vegetative and animal matter. They are found in a variety of fresh water habitats including lakes, ponds and streams. When anglers encounter them they presume that they are merely earthworms that have gotten washed into the stream, but aquatic worms are very common. Some researchers have suggested that they are one of the most common animals in the water.
Several years ago my nephew and I made our annual spring pilgrimage to the Big Horn River in Montana. Due to a higher than normal winter snowpack they were releasing lots of water from the Yellowtail Dam, and the river was quite high. This sudden increase in the volume of water had disrupted the normal hatches, and the increased flow required us to change our normal tactics. My nephew, who carries more flies than most fly shops, had a box filled with San Juan Worms that looked like they were on steroids. Tied on size 8’s and 10’s they had a large brass bead tied in the middle of the hook. Boy did the Big Horn trout love those worms. The brass bead had enough weight to get the fly down to the bottom, and the increased flows had flushed lots of aquatic and non-aquatic worms into the river. For the three days we were there we consistently caught fish with these flies.
Unless you are a died in the wool dry fly angler your fly box should have a good selection of scuds, sowbugs, and yes - even worms.