In streams and rivers there are hundreds of insect species of interest to anglers. The vast majority of these insect species belong to five Orders (Table 1). Aquatic insects are usually far more significant to fly-fishing than terrestrial insects, so most of the information I provide will focus on aquatic insects. Table 2 contains the names of several fly patterns used to imitate stoneflies, mayflies and caddisflies.
| Table 1: Major insect types (aquatic and terrestrial) important to fly fishing. | ||
| Common Name | Scientific Name (Order) | Life Cycle Type |
| Stonefly | Plecoptera | hemimetabolous |
| Mayfly | Ephemeroptera | hemimetabolous |
| Caddisfly | Trichoptera | holometabolous |
| Midge | Diptera | holometabolous |
| Grasshopper | Orthoptera | hemimetabolous |
| Table 2: A few fly patterns associated with stoneflies, mayflies and caddisflies. | ||
| Adult (dry fly) | Larva/Pupa* | |
| Stoneflies | Improved Golden Stone Stimulator Bird's Stone Clark' Little Yellow Stone Sofa Pillow Improved Sofa Pillow Fluttering Stone Whit's Adult Stone Langtry Special Little Yellow Stone Clark's Stonefly Lime Sally Elk Hopper Joe's Hopper |
Ted's Stone Matt's Fur Montana Stone Anderson's Brown Stone Box Canyon Stone Little Yellow Stone Whit's Golden Stone Brook's Stone Bitch Creek Dionne's Stone Kaufmann's Golden Stone Carrier's Stone |
| Mayflies | Adam's Royal Coachman Mosquito March Brown Blue Quill Quill Gordon Gray Fox Light Hendrickson Flick's March Brown Pale Morning Dun Borcher Special Baetis Zinger Western Green Drake Speckled-winged Parachute No Hackle Dark Haystack Big Yellow May Spinner Colorado Green Drake |
Kaufmann's Hare's Ear Black's Lite Brite Hare's Ear Gold Ribbed Hare's Ear Green Drake Lawson's Green Drake March Brown American March Brown Cate's Turkey Skip Nymph Lingren's Olive Catskill Hendrickson Janssen's Emerging Callibaetis Pale Morning Dun Emerger Blue Wing Olive Emerger |
| Caddisflies | Woodchuck Caddis Poly Caddis Elk Hair Caddis Deer Hair Caddis Colorado King Dancing Caddis Sparkle Caddis Fluttering Caddis Arctopsyche grandis Goddard Caddis Mitch's Caddis Clark's Deer Hair Caddis Deschutes Caddis Travelling Sedge Mitch's Sedge Flat Wing Caddis Hemmingway Caddis |
Cream Caddis Midge Nymph Latex Caddis Randall's Caddis Buckskin Caddis Peacock Larva Trueblood's Caddis Krystal Flash Pupa* Lafontaine Caddis Larva Deep Sparkle Pupa* Speckled Sedge Solomon's Caddis Pupa* Dicosmoecus Cased Caddis Salmon Candy |
* Pupa patterns are for Caddis only
Insect bodies are divided into three parts (tagmata): head, thorax and abdomen (Figure 1). Each tagma (part) has specific structures used to differentiate between Orders and ultimately species. Therefore, it is important to gain an appreciation for some major features.
Heptageniidae (a mayfly family) and perlidae (a stonefly family) are better adapted for avoiding current due to their general dorso-ventral flattening and smooth streamlined back (dorsum). Dorso-ventral flattening with legs originating on their side allows them to squeeze into calm habitat within the substrate. Aquatic insects are generally less than 4 mm thick therefore; they can avoid high water velocity by staying within the boundary layer. In areas with gravel in the soil, where water can percolate through, many aquatic insects have been found at 1 m depths. However, some stoneflies have been found at a depth of 4 m and as far away as 30 m from the stream.
Most insects can be divided into two groups based on stages in their life cycle - they are either hemimetabolous (Figures 2 and 3) or holometabolous (Figure 4). A hemimetabolous life cycle proceeds from egg to larva to adult (imago) and a holometabolous life cycle proceeds from egg to larva to pupa to adult. It is important to note that mayflies are unique among aquatic insects because they have two adult stages. The first is a non-reproductive subimago (dun) and the second is a sexually reproductive imago (spinner).
The larva that hatches out of an egg is termed a first instar larva. Growth occurs when the first instar larva moults its exoskeleton (cuticle). The cuticle is a hard, non-living structure that houses all the soft living tissues such as muscles, digestive tract, and circulatory system. After moulting, the shed cuticle is called an exuviae and the larva is now into its second instar. The exuviae includes all external structures as well as the lining of the foregut and hindgut. The new cuticle is soft and cream colored. Within an hour or two, the cuticle is hard and dark. During this period of hardening, the insect expands by taking in water or air. Because of the need to moult to increase size, larval growth follows a stair-step pattern, one step per instar, with the final moult resulting in an adult (Figure 5). The number of instars for an aquatic insect varies between species, ranging from approximately 8 to 18 before adult emergence.
Adult emergence requires that an aquatic insect move from an aquatic to a terrestrial environment. This relocation can be accomplished by several means. Stoneflies tend to simply crawl along the stream bottom right to shore. Once on shore, they proceed with the final moult and emerge as adults. Once the adult's wings have dried, the adult moves to nearby vegetation in preparation for mating.
Mayflies and caddisflies usually use other strategies to complete emergence. When some of these final instar larva or pupa are ready to moult, they produce a gas bubble that will carry them to the water surface. Some moult while travelling to the water surface while others moult and emerge after reaching the surface. While on the surface, they may float downstream while their wings dry. Once their wings have dried, they fly away in preparation for mating.
The role of an adult aquatic insect is strictly reproduction. Most adults drink water, some do not eat, and others neither eat nor drink. The energy stored over the last part of larval life is converted to reproductive structures in the final instar larva (start of metamorphosis). The reproductive structures will include sperm, eggs, and genitalia. After adult emergence, males and females are ready for copulation. Shortly after copulation, females' deposit fertilized eggs into the water and then the adult dies, thereby completing the life cycle.
Life cycle lengths for aquatic insects are highly variable, with less than 2 weeks required for some mayflies (some Baetidae and Tricorythidae species) and several years required for some stoneflies (some Pteronarcyidae and Perlidae species). However, the annual life cycle (completed in one year) is by far the most common.
Insects, like trout, are ectotherms. Insects have specific temperature requirements that must be met to complete their life cycle. For ectotherms, temperature affects metabolism, growth, emergence, reproduction, and ultimately distribution. Most aquatic insects grow and develop during periods of warmer temperatures (summer). However, some Plecoptera (e.g. Nemouridae) and Diptera species have adapted to growth and development during periods of colder water temperatures (winter).
Globally, various aquatic insects have been found in temperatures between -30ºC (arctic ponds) and 47ºC (hot springs), though very few species are adapted to the extreme temperatures. Aquatic insects in headwater streams have adapted to "cooler" temperatures (generally less than 20ºC). In a Temperate Zone climate, aquatic insects have adapted various strategies to avoid freezing in the winter. Some will burrow into the substrate, others will build cocoons, others may go into diapause, and still others may use a combination of these and other strategies.
Figure 1: A stonefly larva (Claassenia sabulosa) with a few external body features identified. Dorsal view.
Figure 2: A representative of a hemimetabolous insect, the mayfly has egg, larva and adult stages. Mayflies have two adult stages - subimago (dun) a non-reproductive adult, and imago (spinner) a reproductive adult.
Figure 3: A representative of a hemimetabolous insect. Stoneflies have egg, larva, and adult (imago) stages. Grasshoppers are also hemimetabolous.
Figure 4: A representative of a holometabolous insect, the caddisfly has egg, larva, pupa, and adult (imago) stages. Most, but not all, caddisfly larvae construct a case in which they live. The cases can be made of various types of material, and can be a diagnostic characteristic for identification of some families.
Figure 5: Larval growth pattern for insects. There is a stair-step increase in size because of moulting.
Some life cycle stages (egg, larva and pupa) are synchronized so that growth, development and emergence (reproduction) avoid unfavourable conditions (e.g. winter). Photoperiod (day length) and temperature changes will often act as cues for the start or end of diapause. Diapause is a dormant state in which development and metabolic rate are reduced while growth, differentiation and metamorphosis are suspended.
Based on diet, larval aquatic insects can be placed into three general categories: herbivores (live plant material), detritivores (dead plant material and associated microbes), and carnivores (insects and other small animals). The mechanism by which they feed falls into four general categories: shredders, collectors (use a net), scrapers, and predators.
Aquatic insects generally feed and are most active from dusk until dawn. This strategy provides them with increased water temperature as well as the cover of darkness. The stonefly Pteronarcys californica is almost completely inactive during the day and has a dark body colour to further hide it from predators (e.g. trout and birds). Other aquatic insects also use body colour patterns to help them blend with the substrate. The use of colors to hide from predators is called cryptic coloration.
Aquatic insects, like trout, have preferential locations (habitats) in which they live. Logs, rot wads and vegetation (instream and bank) provide a suitable and often required habitat for insect colonization. Some aquatic insects prefer very fast flowing water with cobble like substrate while others prefer slow flowing water and sandy/muddy substrate. Stoneflies typically prefer areas with fast flowing, agitated water (highly oxygenated) with a substrate composed of larger sized particles (loose packed gravel to boulders). The dependence of stoneflies on this "pristine" environment is one reason why stoneflies are considered biological indicators of stream health.
Mayflies can be found in wide ranging habitats such as burrowed in gravel, sand or mud, living on submerged plants, and some are free swimming in quiet water.
Mayfly, stonefly and caddisfly larvae, in that order, are most likely to drift with the current. Drift is usually catastrophic (physical disturbance or pollution) or behavioural (movement to a new feeding location or escape from predators). Mayflies often exhibit behavioural drift and tend to drift more frequently at night.
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