Entomology II

Baby Bugs: The nymphs, pupae and larvae

The instructions that come with a stomach pump read something like this:
A). Catch a fish.
B). Pump its stomach.
C). Match your fly to the stomach contents.
D). Go fishing.

Like walking a skyline to expose snipers; it kind of makes sense until you think about it. Let’s try to figure out what the fish are eating before we start fishing. In entomological terms we’re going to sample the river.

Leave the stomach pump at home (or better yet on the shelf in the flyshop), put the rod back in the truck (for the time being), and bring a wire mesh kitchen strainer and Tupperware container down to the river side. An aquarium net is often reccommended, but it is only useful for skimming flotsam from the film... a minute segment of trout food habitat. We want something sturdy that will stand up to extreme collecting. A serious strainer is a sign of serious intent. A few scoops with a good strainer will give a quick approximation of not only the bugs present, but more importantly, in which ratios.

A basic tenant of fish behavior is that fish tend to eat what there is most of. It’s wonderful when browns are detonating beneath giant fluttering stoneflies, but more often than not they’re rising amid the big stuff and sipping spinners. It’s not that fish prefer the taste of mayflies, it’s simply because spinners normally outnumber the stoneflies a hundred to one.

Pull the stickers off your new strainer, stomp on it to make it look used, and wade bravely into the nearest riffle. Next, brace the strainer against the river bed and tip over a few rocks immediately upcurrent from it. The dislodged and disoriented critters that were hiding under said rocks will sweep helplessly into your trap (I am not a fan of kick screens because far too much of the streambed gets unnecessarily trashed in the process of sampling).

Inspect the strainer and you’ll likely see a few creatures, but mostly it will be filled with gravel, sticks and waterlogged leaves. Scoop a couple inches of water into the TupperwareTM and add the booty from the strainer. If you collected from anything like a normal trout stream you will be blown away by the number and diversity of creatures moving about. Freed from the pull of gravity and the slime of detritus, the sticks and gravel will have transformed into life.

Find the bugs with three distinct body parts; a head, thorax and abdomen. The head will have obvious eyes and mouth parts and many will have antennae. The thorax is the segment where the legs stick out and the abdomen is, well, the ass end of the bug and it will usually have two or three tails sticking out of it. You have just identified the nymphs, the immature stage of some very important aquatic insects.

Now, find the nymphs with small structures attached along the sides of the abdomen. These might be small rods, cups, or discs or they might look like feathers. These appendages are gills. If you didn’t smash the nymphs with your macho strainer, most of them will be pumping their gills to better extract oxygen from the water.You have now identified the mayfly nymphs. I don’t care if they’re big or small, thin or squat, or if they swim or don’t swim; they’re all immature mayflies. All mayfly nymphs have gills on their abdomens. Pretty easy, huh?

Mayfly nymph. Note the HEAD, the THORAX with all the legs sticking out, and the ABDOMEN to which the tail is attached. Notice the fringe of GILLS along the abdomen.

If you got the bugs from a river, in all likely hood the remainder of the nymphs are stoneflies. Stonefly nymphs may have rod-like or feathery gills between their legs or even on their neck, but never* on the abdomen (*Never is a dangerous word. There is one stonefly, the Oroperla barbara, that does have gills on its abdomen, but unless you’re exceedingly lucky, you’ll never encounter one of these rare and beautiful insects).If you got the nymphs from a pond or spring creek, it gets a bit trickier. Pick the nymph up and give it a gentle squeeze. If it bites and draws blood it was probably a dragonfly nymph. These guys are mean.

Maybe a better idea would be to look at it’s tail first. If it doesn’t have an obvious tail it’s likely a dragonfly nymph, a water bug, or water beetle. The dragonfly nymph has a hinged, clubby looking device under its face (it’s called a labium and contains those nasty mouth parts). A water bug carries its forearms cocked in front of its face as if its looking for a fight and a beetle looks like a beetle; no problem there. All these guys bite so be careful.

Another common and very beautiful nymph is the damselfly. These are easily and immediately identified by their three leaf-like tails called lamellae. These lamellae not only aid in propulsion but are part of the damselfly nymph’s respiratory system.

Back to the Tupperware™. See any pink, slender, slimy, creatures that look just like worms? These are called worms. There are often dozens of worms caught in the strainer. The density of worms in a streambed can easily exceed that of any garden. The San Juan Worm isn’t such a ridiculous fly after all.

The other common worm-shaped item in the strainer is likely to be Diptera larvae. Diptera is a vast group of flies that covers everything from minute midges to giant craneflies. Don’t even begin to try to figure out the various Diptera larvae, even the pros get stumped. Just recognize their size, color, and relative abundance. They may come in handy some day.

By now some of the sticks and gravel clumps in the TupperwareTM have probably sprouted heads and legs and are dragging themselves about. These, of course, are the caddisfly larvae. Gently remove one of the larvae from its house and examine it. It probably curled up in its nakedness, but the small anal hooks should be readily apparent. All caddisflies have these claws at the tip of the abdomen to help them hang on to their house or the streambed. This frees up those three pairs of legs you’ll note just behind the head, so they can motate the caddis and manipulate food. The head itself is sclerotized or decorated with dark shiny shield. Like the anal hooks, all caddis larvae have one or more sclerotic plates and, in the future, these will help you identify the various caddis species.

Very likely there will be caddis larvae roaming about the TupperwareTM without homes. They may simply have become homeless, but more than likely these are species of free roaming caddis that never build a case. Again, note the anal hooks, the legs, and the sclerotic plate(s) on or behind the head.

Did you find anything looking like yellow or green vitamin E gell caps? These are caddisfly puparium.. Look closely and through the translucent skin, you’ll see the developing pupae within.

From our brief sweep of the river, we have identified the sub adult forms of all the major commonly important aquatic insect families: mayflies, stoneflies, Diptera, caddisflies, dragonflies and damselflies. As a bonus we’re going to learn one more, the hellgrammite.

At one time or another, every bug we’ve identified today has been described to me as a hellgrammite. I don’t know what made them so famous, but for being such famous bugs, no one seems to know what they look like.

Hellgrammites are the larval stage of alderflies and dobsonflies. They are the Doberman pinchers of the water world; their bite makes a dragonfly nymph’s seem like a pat on the butt. Hellgrammites live in leaf packets, usually in mid elevation streams. They are long fleshy creatures with legs and soft pointed tubercles running along each flank. Their head is flat and sclerotic and they sport massive jaws which they gnash in a most menacing way. If the Japanese knew hellgrammites, Godzilla would have had six legs.

Of course there will be a few assorted odds and ends in the strainer we didn’t identify. You will stumble across water pennies, various pupa, beetle larvae, and even terrestrial insects that fell in the water. Set them aside for later, today you learned a bunch.

Entomology III


Stop Pebble Mine