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Story by Harvey Thommasen, with Photography by Mike Wigle There are two kinds of insect life cycles: complete and incomplete. Both are important to fly fishermen. In the complete life cycle a wormlike larvae hatches from a fertilized egg. Larvae are commonly known as worms, grubs, maggots, and caterpillars depending on the kind of insect you are talking about. The larvae are voracious eaters and they grow rapidly. Once fully grown, the larvae changes its skin and becomes a pupa, a resting stage in which it develops its wings. Once it is fully formed,the adult emerges and flies or crawls off to find a mate. The butterfly is probably the best known example of an insect with a complete life cycle. The caterpillar is the larval stage, the cocoon is part of the pupal stage, and the butterfly fluttering around your garden is the adult.
Butterflies aren’t important trout food, because they pursue their life cycle on land and infrequently become available to fish. One doubts that a big, hungry trout would pass up a Monarch if it got knocked down onto the surface, but rarely is such a rise seen. Caddisflies, beetles, true flies (dipterans), alderflies, and dobsonflies all have a complete life cycle - and all are preyed on hungrily by trout when the opportunity arises. In the incomplete life cycle the juvenile that hatches from an egg is called a nymph, rather than a larvae. Nymphs resemble the adult forms in appearance, they are just smaller and lack wings and functioning reproductive organs. Nymphs grow directly into adults, they do not go through a pupal stage. When fully grown, the nymph sheds its outer skin and a fully winged adult emerges. Mayflies, stoneflies, dragonflies, damselflies, and the waterbugs (Hemiptera) all have an incomplete type life cycle. All are commonly available to trout in their nymphal stage. If they are less available as adults, it is only because, in most cases, they move away from water at least temporarily once they’ve transformed. The grasshopper is probably the best known example of an insect with an incomplete life cycle. Early in summer the wingless, miniature grasshopper you see jumping about as you stride through a field are nymphs - busily feeding on lush green vegetation. By the end of summer the same fields contain only adult sized, winged grasshoppers - adults that leap high in the air and fly away. If they splat down on a river surface - as either fully grown adults, or immature nymphs - they won't last long if there are any trout about. All juvenile insects molt by shedding their stiff, outer body covering - the exoskeleton - crawling out reveal a soft, elastic outer skin. Most aquatic insects molt 20 to 35 times before they are fully grown. Is it possible that trout recognize when insects have undergone a molt, and key on them? The vast majority of aquatic insects spend all but a few days to weeks of their lives underwater, residing in or on the silt, gravel, and rocks of stream bottom. It’s no wonder that they are important to both fishermen and fish.
Some of the larger aquatic insects, like the Giant Stonefly (Pteronarcys californicus) and the Giant Caddisfly (Dicosmoecus atripes) spend two to four years underwater before emerging to transform into adults. Other species like mosquitoes, No-See-Ums, blackflies, and the small Baetis and Callibaetis mayflies, have much shorter underwater life cycles and can produce two or more hatching broods in a single season. Trout may not be aware of such details, but they do know when the insects they feed on are undergoing transformation, by either molting or moving to the surface to change into adults. Such events are of enormous importance to trout - and anyone who's on the stream when a hatch comes off knows just how significant it can be for fishermen too. |
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