Aquatic Life:

Arthropods - Insects

Insects have three pairs of legs, three distinct body segments, antennae, and either one or two pairs of wings, although some species lack wings. The insects are the single largest class of animals in the world with over a million species. Because we are only looking at aquatic insects, the list gets narrowed down quite a bit. Lots of insects spend part or all of their life cycle in water. Probably the most common ones we think about are dragonflies and mosquitoes. You would probably be bored to tears if we talked about every single aquatic insect that lives in our streams, therefore we will focus on a few of the more interesting ones. This list was narrowed down based on significance in relation to aquatic pollution and also on interesting qualities.

 

The first insect to consider is the water strider. Water striders are flat and long and lack wings. The second and third pairs of legs are long and are used to move the insect across the surface of the water. The forelegs are short and are used to attack and hold prey. Water striders live on the surface of ponds and slow streams and often gather in groups. They are both predators and scavengers, feeding on a variety of aquatic invertebrates, including mosquito larvae. It locates its prey by feeling the slightest vibration on the surface of the water. Water striders' legs have tiny, water-repellent hairs that hold tiny air bubbles. This allows them to skate across the surface of the water. They move with a jerking movement of the middle legs, and steer with the back legs. Their bodies are covered with soft, velvety hairs that hold a thin film of air through which they breathe. To escape predators, water striders will dive underwater until the danger is past and then pop back up to the surface and stand back up on their legs. Water striders are pollution tolerant because they live on the surface and are therefore not heavily influenced by pollution.

 

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Hellgrammites are the larvae of the dobsonfly, a rather large insect (2 to 4 inches long) with large, mottled wings that are held flat over the back and extend beyond the abdomen when at rest. The males have long, extended curved mandibles that are used to grasp the female during mating. The aquatic larvae are elongate (about the same size as the adult), dull colored, and have gill filaments and gill clusters along the sides of the abdomen. They have two hooks on the end of the abdomen. Most notable on the hellgrammite is the large jaw that is used to grasp prey in the water. Adult dobsonflies probably do not feed, as their long mandibles (especially the males) would make it very hard to eat. The hellgrammites on the other hand, are voracious predators that attack other aquatic insects and other organisms. Most of the dobsonfly's life cycle is spent in the larval stage in well-oxygenated, high quality water. Hellgrammites can be found in riffle areas of streams, hiding among rocks. Hellgrammites are a favorite food for many fish, including bass and catfish. Because of this they are often collected for fish bait. As mentioned before, hellgrammites are fierce biters, but are not dangerous. Hellgrammites are very sensitive to water pollution.

 

Another odd little critter found in our streams is the larvae of the caddisfly. Caddisflies are related to butterflies and moths. The larvae are caterpillar-like with three pairs of well-developed legs on the first three body segments and hooks on the last one. Caddisfly larvae live in a wide range of environments from fast moving streams to ponds. Their bodies are soft and are usually covered in a silky case of collected substrate such as gravel, twigs, or sand. They use the hooks at the end of their abdomen to hold on to their cases. The case is used for protection, shelter, and as an escape from predators. Some species do not live in cases and use their hooks to cling to the streambed and to drag themselves backward to escape predators. Caddisfly larvae eat algae, plants, and decaying plant matter. Some feed on other insects and spin silky nets to capture their prey. Caddisfly larvae cannot tolerate low oxygen levels and those species that break up leaf litter for food require vegetated streams where trees and shrubs overhang the water. Caddisfly larvae and adults are an important food source for many fish and are sensitive to water pollution.

 

Mayflies are the most primitive winged insects. They have four transparent wings, two or three long tail filaments, short antennae, and bulging eyes that are light sensitive. Mayflies spend the majority of their lives (up to three years) as nymphs and are aquatic and like the adult, have long tail filaments. They breathe through gills on the sides of their abdomens. Nymphs live under stones in fast moving streams or among plants in slower moving water. Most mayfly nymphs feed on plant matter but some are predators. They generally feed on top of stones and will retreat underneath to escape predators. As the nymphs eat they grow larger and shed their exoskeletons many times. The nymphs have pads on their backs where their future wings will grow. As they mature, they float to the surface of the water and begin to molt where it transforms into a winged, dull colored stage, which last for a day or more. It will then shed one last time into a shiny adult. Mayflies are the only insects that shed after having wings. As an adult, a mayfly will only live for a few hours because it doesn't have mouthparts to eat. Male and female adults will mate in flight, the male dies shortly after; the female will lay her eggs and then die. Mayflies are a favorite food of trout but are also eaten by birds, minnows, water striders, and frogs. They are also the favorite food of dragonfly nymphs. Mayfly nymphs are highly sensitive to low oxygen levels, preferring cool water where oxygen saturation is much higher. They are also highly sensitive to water pollution.

 

And last but not least, we have stoneflies. Stoneflies are insects often found resting on stones in the creek and look similar to dragonflies. Stonefly nymphs are often confused with mayfly nymphs because they also have tail filaments. Stonefly nymphs only have two, while most mayfly nymphs have three. Stonefly nymphs also have gills located on their thorax, while mayfly nymphs have their gills on their abdomens. Stoneflies have two claws on each leg for clinging to rocks and sticks and their bodies are streamlined so they don't get swept away in the current. Stonefly nymphs are only found in highly oxygenated, cold-water streams, living under stones. Often they are found on top of rocks or on submerged wood where they feed on dead plants and algae. Nymphs actually move to dry land just before they molt and the adult form emerges. Stoneflies take up to three years to develop into adults and like mayflies, will live only a short time as adults, usually long enough to mate and lay eggs. Stoneflies are also a preferred food of trout and other fish. They are extremely sensitive to low oxygen levels and pollution.