Insecta (Aquatic)

Introduction

Insects are arthropods in the class Insecta. There were more than 750,000 described species as of 1996 and more are being added to this total daily. The membership of Insecta is three times larger than that of any other animal order. The insects have successfully colonized the entire planet with the exception of the subtidal zone of the oceans.

Insects are characterized by three pairs of legs, two pairs of wings, a single pair of antennae, and a pair of compound eyes. Their bodies are protected by an exoskeleton of scleretized cuticle and is divided into three regions: head, thorax and abdomen.

They have a ventral nerve cord and a dorsally located heart which pumps blood through an open circulation system. The blood is usually colourless or green and is rarely needed for oxygen circulation in the body. Trachea regulate air exchange and free amino acids are the major players in osmoregulation. Malphigian tubules rid the body of metabolic wastes.

The sexes are separate in all species, and copulation or the production of sperm atophores are often the rule. Eggs are laid, but brooding by one parent is a specialization found in a few families. Development can be ametabolous, paurometabolous, or holometabolous.

The importance of insects is almost unfathomable. They are a needed link on a number of food webs world wide. Two thirds of the world's flowering plants use insects as pollinators. Many insects are either parasitic or vectors of diseases, such as malaria and sleeping sickness, that cause human mortality. And they make great pets.

Coleoptera

Introduction

Coleopterans or beetles, comprise the largest order of insects in the world. There are 5,000 aquatic beetles described and 1100 North American species that have at least one aquatic lifestage. It is hard to generalize about the life styles of beetles since they have invaded freshwater a number of times. Each time a group invades water it finds a different way to adapt to the new conditions. Adaptations can be of a physiological, structural, or behavioural nature. In this way each species develops in a slightly different way than a species that invaded the same habitat earlier. This is one of the reasons why the aquatic beetles are so diverse.

Morphology

Coleopterans are holometabolous group, meaning that its members all transform from larvae to adults via a pupal stage. The aquatic larvae are distinguishable from those of other insects by their sclerotized head capsule, by the structure of their mouthparts, and by the possession of two or three segmented antennae. Larvae within the Coleoptera are separated into species based solely on the structure of the mouthparts. Three pairs of segmented thoracic legs are thoracic and gills are often present ventrally or laterally on the abdomen. Aquatic adults are all heavily sclerotized with fairly compact bodies. Heavy, often patterned, elytra cover a pair of membranous wings. The form of the antennae are integral to discerning taxonomy of each species. Most aquatic beetles have elongated swimming hairs on their forelegs to help catch water and force it behind them for movement. Others posses hairs as well as flattened forelegs that work in the same manner as oars on a boat. Still others do not swim at all, but crawl along the bottom or on vegetation searching out food. Some larvae have swimming hairs and can flex their bodies to move through the water. However, many are adapted strictly for crawling on submerged surfaces.

Metabolism

To aid in respiration while underwater, some species will capture an air bubble, treating it as a physical gill. Others rely on the collophore to meet their respiratory and osmoregulatory needs.

Reproduction

All beetles employ complete metamorphosis. Because of their diversity, it is impossible to describe specifics, even to the level of family, of the egg laying and larval development stages, locations, and timing. The most accurate descriptions are at the level of sub-order. The most primitive sub-order of beetles, Adephaga, has the most number of families that are truely aquatic, meaning that eggs and larvae both develop underwater. The sub-order Myxophaga is mostly aquatic but limited to fast flowing, shallow stream habitats. Very few families of the suborder Polyphaga are completely aquatic. Many live in the water during one stage of their life, be it as an egg, larva or adult, but the other stages are restricted to land.

Ecology

The larvae and adults of the suborder Adephaga are predators on other insects and small crustaceans. The larger adults can even kill tadpoles and small fish. Most chew their prey, but others have piercing/sucking mouthparts to feed on prey body fluids. The latter inject digestive enzymes into their prey and then wait for its tissues to dissolve. The Myxophaga feed exclusively on blue-green algae which they scrape from rocks using their mouthparts. The Polyphaga ("Poly"=many, "phaga"=feed) are true to the description of the name. Aquatic species in this suborder employ all types of feeding strategies including parasitism, predation, and herbivory to acquire food.

Beetles occupy a wide variety of both standing and flowing freshwater systems. The majority of species dwell on the substrate. Many live in the crevices of rocks or in self-constructed burrows, never venturing into open water. However, some adults are exceptional swimmers and use this talent to capture prey.

Idiosyncratic Inverts

Larvae of the family Chrysomelidae obtain their oxygen by piercing the tissues of aquatic plants and drawing out oxygen stored within. These larvae are also leaf feeders so the plant provides all the necessities of life: substrate, protection, food, and oxygen.

Diptera

Introduction

Dipterans, or true flies, are the most common insect order in aquatic environments, accounting for 40% of all freshwater insects. The name of the order is self descriptive ("di"=two, "ptera"=wing) as adults of this order bear only two membranous wings. The larval stage is the aquatic stage; adults typically only immerse themselves in water in order to lay eggs. Mosquitoes, black flies and many biting midges are just a few of the groups of flies whose larvae are aquatic.

Morphology

Fly larvae are very diverse in form, as expected in a group that has invaded so many different habitats. The head is often a complete capsule but the thorax and abdomen are usually soft and flexible, only rarely covered with sclerotized plates. The thorax has three segments excepting the Culicidae (mosquitoes) which have one giant thoracic segment that forms the widest part of the body. The abdomen has either 8 or 9 segment, depending upon the species. Adult flies range in form from the thin, long legged mosquitoes, to bulky, stout-bodied horse and deer flies. One characteristic structure of all flies is halteres, which are the remnants of a second pair of wings. They have been reduced to a thin stalk with a bulbous head and are used for balance in flight. The wing venation and location of specific hairs are the structures used to distinguish different species of fly.

Dipteran larvae are distinguished from other aquatic insect larvae by their lack of jointed thoracic legs. Locomotion is facilitated by three types of appendage: creeping welts, prolegs, and suctorial discs. These appendages are manipulated by muscle movement and turgor pressure. Creeping welts are swollen ridges found longitudinally along the abdomen of house fly larvae. They house one or more modified spines or setae that push against substrate to aid motion. Prolegs are not true legs, although they are paired, but are fleshy processes of the body that can be moved and retracted for the purpose of locomotion. Suctorial discs occur on the ventral surface of species from running water. They allow the larva to cement itself to the substrate so it will not get pulled away by the swift currents. Discs are put down alternately in locomotion.

Metabolism

Where a dipteran larva lives is a direct reflection of its oxygen needs. Many stream dwelling larvae, such as the black flies, simply exchange gases through their cuticle. Highly oxygenated water is always washing over them so is always available. Deep-water lentic species like the chironimids can also satisfy their respiratory needs through their body wall. Mosquito larvae live in shallow, standing water and simply reach the water's surface with their abdomen to draw a breath. Other species have air tubes that they use as snorkels, that can be extended up to three body lengths.

Reproduction

Eggs are often either laid on the surface of the water or cemented to a submerged substrate. Eggs hatch a few days to a few weeks after being laid. The larvae progress through 3 to 4 larval instars, sometimes more in the case of black flies and a few other species. In general the first instar is the shortest and the last instar is the longest as the larva needs to acquire the most energy in preparation for pupation. There are four different types of pupae: (1) an anchored cocoon where the emerging adult must swim to the surface; (2) a pupa that will float to the surface of the water when the adult is ready to emerge (usually found in the families with free-swimming larvae); (3) larvae crawl onto the shoreline and burrow into the substrate to pupate; and (4) the body of the last larval instar acts as the pupal case.

Ecology

The feeding strategies of these larval insects are just as diverse as their other attributes. Black fly and mosquito larvae are filter feeders with modified mouthparts called labrial fans that direct organic particles suspended in the water column into their mouths.

Members of the Syrphidae collect organic matter from the substrate for ingestion. The Tipulidae are shredders, as they break down living and dead plant matter for food. Chironimids, among others, are predacious on other insect larvae and other small animals. The adults that reside on land feed in a variety of ways. Female adult black flies and mosquitoes require blood meals in order to produce and lay eggs, while the males feed only on plant nectars.

The larvae of true flies have invaded nearly every aquatic habitat with the exception of the open ocean. Lifestyles in freshwater include free-living crawling/swimming (Culicidae, Simulidae); those that are buried underneath rocks or sediment (Tabanidae); and those that dwell inside silk tubes that they have spun and attached to submerged rocks or plants (Chironomidae).

Idiosyncratic Inverts

Members of the fly family Nymphomyiidae have very short but fulfilling adult lives. They emerge from their pupae underneath the water and immediately take to the air in search of a mate. A successful search may take only a matter of minutes. Once this task is complete, the couple dive back beneath the water to search for a place to lay their eggs. The female then lays the eggs in a flower petal pattern around her and her mate. After the last egg is laid, both parents die...still attached. Still, I feel sorry for the female: her wings get ripped off on the dive into the water.

Ephemeroptera

Introduction

The larvae of mayflies are aquatic, but the adults definitely are not. Every year millions of mayflies form mating swarms, mate and die. These insects can be a nuisance near lakes where they are so abundant that their dead bodies can make the roads slippery and cause accidents.

Mayflies are the only insect that has a winged immature stage.

Morphology

Mayfly larvae have both compound and simple eyes, filamentous antennae and usually three long caudal filaments. Gills occur either laterally or ventrolaterally on most abdominal segments. Larvae have 3 pairs of legs with a single claw on the end of their one-segmented tarsus that they use to crawl along the substrate. Wing pads are usually present on the thorax. Each thoracic segment bears a pair of legs that are often modified for burrowing, filtering food, grooming and gill protection.

Adult mayflies have two pairs of transparent wings that are held vertically above their body at rest. Males usually possess larger compound eyes than females. Adults have no mouthparts or gills because their function is to reproduce. After they reproduce, they die. Two or three caudal filaments extend from the posterior end of the abdomen.

Metabolism

Respiration takes place through gills which are found along the abdomen and at the base of the legs of ephemeropteran larvae. Some groups can be identified by gills on other parts of their body, such as the maxillae.

Reproduction

The eggs of mayflies usually hatch shortly after being laid. The number of larval moults can range from 12 to 45 depending on the species. At the end of their development, juveniles swim to the surface where they moult into a sexually immature subimago stage. The subimago is covered with tiny water-repellent hairs that make it almost waterproof. Mayflies remain in this stage for a few minutes to 48 hours and then moult into the adult stage. Adult males form mating swarms above water, but they can also be attracted to bright lights. Males grab the females as they enter the swarm and copulate with them. The eggs are usually deposited on the surface of the water. As adult mayflies lack mouthparts and respiration organs, they reproduce and die after a few days. In fact, females often die on the water surface as they are laying their eggs.

Ecology

Larval mayflies are an important food source for fish in aquatic habitats. Most larvae are herbivores or detritivores, but some larger species are carnivores. All mayflies have aquatic larvae; most occur in streams, but a few species are found in still waters. Different species have different oxygen requirements, so mayfly larvae are often used to monitor water quality. By determining which species are the most abundant in a given environment, the "health" of the ecosystem can be determined.

Idiosyncratic Inverts

Mayflies often mistakenly lay their eggs on dry asphalt roads where all the eggs will die. Why do they do this? The polarized light reflected off a smooth road surface mimics the light reflected off smooth water surfaces where mayflies aim to lay their eggs. As a result, paved roads are a death trap for these unsuspecting insects.

Heteroptera

Introduction

There are about 3,800 species of fully or semi-aquatic heteropterans around the world. The North American contingent of 68 genera and 412 species which include the truee water bugs, which hunt and feed under the water, and the water striders, adapted for life on the water's surface.

Morphology

Juvenile heteropterans look like miniature versions of the adults except that they lack wings and reproductive organs. Unfortunately, many species of water strider are wingless even as adults. Juveniles can still be recognized because they have one-segmented tarsi, while adults have the two-segmented compliment. All heteropterans have a beak-like mouth (rostrum) that originates at the anterior tip of the head and lies posteriorly underneath the body when not in use. Well developed eyes are a standard feature; and an asset to their predatory lifestyle. Water striders have conspicuous three to five segmented antennae while the true water bugs have tiny hidden or no antennae. The reduction in this structure is a lifestyle adaptation, as large antennae would reduce swimming speed during the hunting dives. The wings of heteropterans, when present, are distinct from all other insect groups. The hind wings are completely membranous and concealed by the forewings which have a leathery anterior portion and membranous posterior portion.

The water striders have "unwettable" hairs on their tarsi that do not break the surface tension of the water, so the bugs can walk on top of it. Some species have accessory wettable tarsi that are extendable and break the surface tension to give the insect traction for locomotion. Steering is controlled by the hind legs and assisted by unequal strokes of the middle legs. In families without tarsal structures, members move by having only three legs touching the water at any one time (aka tripodal). Some groups use saliva from their beaks to make the water's surface more slippery for a quick escape.

Many of the true water bugs have modified legs to facilitate locomotion underwater. Many species have enlarged hind legs that move like oars to drive them through the water. Others use their hind and middle pair of legs, alternating on each side, when relaxed and working in tandem for higher speeds.

Metabolism

Water striders live above the water and obtain oxygen via a tracheal systerm as in other terrestrial insects. The true water bugs carry around their own stores of air for use when they are underwater. Air bubbles are either trapped beneath the wings and pressed against the dorsal side of the abdomen, or held on the ventral side of the abdomen by specialized hairs. These are called physical gills or plastrons. Different species have different structures for refreshing the air bubble in the plastron. Belostomatids have air straps, nepids have abdominal tubes, and notonectids poke the tip of their abdomen out of the water. Juveniles lack these adaptations for breathing in water, but they are small enough to exchange gases across their body wall

Heteropterans, like other insects, have a waxy cuticle that repels water to reduce the need for active osmoregulation. They absorb a lot of water while feeding, so their faeces are very dilute in comparison with terrestrial heteropterans. Hypotonic urine, to aid osmoregulation, is produced with needed ions being reabsorbed in the midgut.

Reproduction

Aquatic heteropterans lay their eggs in the spring, gluing them to submerged vegetation or rocks. Other species lay their eggs on plants that overhang a body of water. When these eggs hatch, the nymphs fall into the water where they start to feed. Development consists of 5 juvenile molts of increasing size.

Ecology

The majority of these species are fairly large and very efficient predators on both vertebrates and invertebrates. Some of the largest species (belastomatids) prey upon fish and frogs in addition to insects and tadpoles.

These insects occur in a variety of freshwater habitats including large rivers, springs, and lakes.

Heteropterans are fairly resistant to predation. Water striders are agile escape artists; while true water bugs are more easily captured but often bite. Both groups have scent glands on the abdomen which contain a potent chemical deterrent to ward off enemies.

Idiosyncratic Inverts

Water striders and the true water bugs are effective predators on mosquito larvae and adults. Some species prefer a good mosquito larva over any other prey. This is good news for researchers in the field of biological control, who seek to use the natural enemies to control pest populations instead of resorting to chemical pesticides. Incidentally, some of these mosquito predators are eagerly eaten by humans—the big belastomatids are a culinary delight in some parts of Asia.

Lepidoptera

Introduction

Although butterflies, moths, and skippers are all terrestrial, the larvae of some species feed on aquatic plants. All lepidopteran species that lay their eggs in water are members of the family Pyralidae.

Morphology

Caterpillars have a very distinctive set of characters although some are often modified by lifestyle. The head is distinct and bears a ring of ocelli (simple eyes), shortened antennae, chewing mouthparts, and a spinneret. This spinneret creates the silk that is needed for the caterpillar to spin a cocoon in which it will pupate. Larvae have three pairs of jointed legs and may bear gills on the thoracic segments. A pair of prolegs (unjointed legs) is found on each of the abdominal segments 3, 4, 5, 6, and 10. These prolegs have tiny crochets (strong hooked hairs) underneath, used to grip plant stems or rock surfaces. Some of the lentic species have long hairs over their body to help them swim.

Adult moths have four wings, body and legs all adorned with overlapping scales. Pyralid adults can be identified by their wings that are held roof-like over the body when at rest. All have compound eyes and may or may not have a single ocellus over each eye. Sensory antennae come in many shapes and sizes depending on sex and species. Species can often be identified by the color pattern on the wings although some species have wingless forms that are fully functional in every other way.

Metabolism

Thoracic gills that facilitate oxygen exchange can be present in all instars or develop in later molts. Behavioral adaptations to oxygen collection are developed by different species. The larvae of Parapoynx maculalis will vibrate its abdomen when they need oxygen quickly. This movement churns the water bringing oxygenated water to the caterpillar. Other larvae become surface feeders as they become larger, taking in air as they feed. The tiny first instars can exchange gases cutaneously.

There are no particular modifications, from the basic insect body plan, for circulation, excretion or osmoregulation that have been recognized in these moth species.

Reproduction

Although adults are terrestrial, they always live near a permanent water source and will dive beneath the surface of the water to lay eggs. Species with larvae adapted for life in running waters often place their eggs on the undersides of rocks for protection from the current. Those in still water lay eggs on submerged plant stems.

Each species has a different set number of developmental instars that will range between three and five depending on the family. Lentic species will live on rocky bottom substrate or mine into the stems, leaves or roots of aquatic plants. Lotic species will often construct protective cases out of detritis and sand particles. They live and feed in these cases, emerging only to pupate. Pupation also takes place in the water. After the caterpillar spins its cocoon it chews a tiny escape slit along the length to help the adult break free when it emerges. The adult moth swims to the surface using its hind two pairs of legs and its wings.

Ecology

Stream dwellers feed on algae embedded directly in rock surfaces as well as on diatoms that have settled. Those in cases attached to plants feed on the plant tissues, while others mine inside the stem, root, or leaf itself directly protecting itself from predators and feeding at the same time.

Megaloptera

Introduction

Megalopteran adults are not often seen in nature because they are short-lived, secretive and often nocturnal.

Morphology

Megalopteran larvae have long, cylindrical, bodies and are easily distinguished from other aquatic larvae by the lateral filaments along their abdomen. Larvae in habitats with low oxygen levels may have long caudal respiratory tubes to obtain oxygen from the air above the surface of the water. Spiracles line both sides of the abdomen allowing the larvae to live out of water as long as the habitat is moist.

Adult megalopterans range from 1 to 7 centimeters (cm) in length. They hold their wings roof-like over their body and often have colored or mottled bodies. Males of some species have huge, curved mandibles to attract females.

Metabolism

Megalopteran larvae have long lateral filaments along the abdomen that are used to increase the surface area of the body for respiration.

Reproduction

Female megalopterans lay their eggs on plants or rocks overhanging the water. When the eggs hatch, the larvae fall into the water and hopefully find a suitable habitat. Larvae usually have 10 to 12 instars, but some species have as few as 3. Once mature, the larvae crawl out of the water and pupate in the soil. Generation times vary among species from a month to 5 years.

Ecology

Only the larval stages of megalopterans are aquatic, all other life stages (adults, pupae, eggs) are terrestrial. Most larvae respire by diffusion, so they are restricted to habitats with high oxygen concentrations. Species in areas of low oxygen concentrations have caudal respiratory tubes which allow them to obtain oxygen from above the surface of the water. Most larvae are predaceous and feed on most other small invertebrates. One group of larvae feed solely on freshwater sponges!

Idiosyncratic Inverts

The huge mandibles of male megalopterans are spectacular scimitar shaped weapons that make these creatures look truly ferocious!

Neuroptera

Introduction

The Neuroptera, called Spongillaflies for their favorite food, are often paired with the Megaloptera in discussions of aquatic insects because of their similar lifestyles. Both groups contain a total of less than 300 species world-wide. The majority of the neuropterans are solely terrestrial, but of two genera have larvae which develop in permanent freshwater sites. But, the adults, pupae, and eggs are all found near bodies of water.

Morphology

Adult neuropterans reach only six to eight millimeters in length. They have large, brown, membranous wings that are held roof-like over the body when at rest. Variation in venation patterns of the wings and diversity of genital structures are used to distinguish species.

The larvae are from four to eight millimeters in length, and ranges in color from dark green to dark yellow to brown. Mouthparts are elongated into stylets modified for feeding on sponges.

Metabolism

The second and third larval instars posses transparent abdominal gills which exchange gases with the environment. The adults use trachea to satisfy their oxygen requirements.

Prior to pupation, the larvae do not excrete any metabolic waste in the form of feces. This attribute is typical of all Neuroptera, but is not found in any other group of insects.

Reproduction

Although eggs are laid on vegetation above the water's surface, newly hatched larvae fall into the water. After passing through three larval instars, the last stage crawls onto land to spin a cocoon for pupation. The cocoon is double layered with a tight white inner layer and a mesh overlayer. During the pupal stage, the larval mouthparts transform into shearing tools so the adult can cut its way out of the cocoon.

Ecology

Neuropteran larvae are picky eaters—they feed only on freshwater sponges. They have unique sucking mouthparts that are made up of two stylets and covered with spiny setae. These mouthparts are adapted to reaching between the skeletal components of the sponge to suck up the succulent cells. Adult neuropterans feed only on nectar and even then will only eat for a few weeks before mating and dying. The adults of these species are rarely observed as they are short lived and nocturnal.

Idiosyncratic Inverts

Adult neuropterans have giant wings that fold over their bodies when at rest. These wings are membranous, like those of a fly, but the veins are dotted with thousands of tiny hairs. This feature makes them unique in the insect world.

Odonata

Introduction

The Order Odonata includes the dragonflies and the damselflies. Members of these two groups can be easily distinguished as both juvenile and adult dragonflies have more robust bodies than damselflies.

Morphology


The larvae of odonates are easily distinguished from those of other aquatic larvae by their long, hinged labium that is used to seize prey. They have large compound eyes and short filamentous antennae. Damselfly larvae have a more slender body, wider head and longer antennae than the larvae of dragonflies. Their legs are slender and well designed for walking, though the first two pairs of legs may have burrowing hooks in the damselflies. The abdomen of most larval dragonflies ends in a sharp projection, while the damselflies have three specialized gills extending from the end of their abdomen.

The heads of adult odonates are dominated by huge, compound eyes. The space between the eyes is occupied with three small simple eyes and short antennae. The head is attached to the pronotum by a very slender neck. The shape of the pronotum (damselflies) and the head (dragonflies) is very important in reproduction, because the male grasps the female around her neck with appendages on the end of his abdomen. These appendages are made to fit the neck of females of the same species and are poorly suited to grasp the neck of females of other species. Most of the thorax is fused to form the pterothorax which is tilted to displace the legs forward and the wings backward. Since odonatans are predatory, forward projecting legs allows them to have an advantage catching airborne prey. Dragonflies hold their wings horizontally at rest, while most damselflies hold their wings together above the body.

Metabolism

Odonates respire not only through their gills, but also through their wing pads.

Reproduction

Male odonates patrol areas of shoreline and chase away other males, as they seek out females. Before mating, the male transfers sperm from the tip of its abdomen to the bottom of its second abdominal segment. When a female enters his territory, the male grasps her. The female loops the tip of her abdomen forward to retrieve sperm from the male's abdomen. The eggs are laid in plant tissue either above or below the water line. In many species the male continues to grasp the female and protect her while she deposits her eggs. This ensures that he fathers the eggs which she lays and that she is not grasped by another male on her way to deposit her eggs. Larval odonatans go through 11 or 12 instars before crawling out of the water and moulting into adults. Although adults live for just a few weeks, the entire life cycle for damselflies takes a year and dragonflies may be up to 4 years.

Ecology

Two-thirds of larval odonates occur in ponds, marshes, swamps and lakes, while the other species breed in lotic habitats such as streams. Larvae are predatory, feeding on other insects, and are the top predators in many habitats. Adult odonates are also predatory, feeding upon mosquitoes and other flying insects.

Idiosyncratic Inverts

Modern dragonflies are generally considered to be large insects nowadays, but fossil dragonflies have wing spans of up to 2 meters! These giant insects were able to sustain flight because oxygen levels were far higher at this time.

Plecoptera

Introduction

There are over 1700 stonefly species in freshwater habitats worldwide. Much work remains to be done in linking the larval and adult forms of these groups.

Morphology

Stoneflies are a good example of the primitive insect body form. Larvae have compound eyes, simple eyes and antennae on their heads. Their mouthparts vary; carnivorous larvae have a mouth adapted for stabbing and grasping, while herbivorous larvae have scraping or grinding mouthparts. Their pairs of legs occur on the thorax and have one or two claws on the end of the tarsi. Wings pads also occur on the thorax and they enlarge with each moult until they reach full size in the adult. Both larval and adult stoneflies have two long caudal cercae which are often modified into copulatory organs in males.

Adult stoneflies have two pairs of wings that fold flat over the abdomen. They look very similar to their larvae, except they have full wings, no gills and males have external copulatory organs.

Metabolism

Most plecopteran larvae use gills to respire, but those that don't have gills need to live in highly oxygenated water in order to be able to diffuse enough oxygen across their body surface.

Reproduction

Larval stoneflies moult between 10 and 30 times, requiring 1 to 3 years to mature. Larvae sustain growth in cold water, but diapause when the temperatures rise. In their last instar the larvae crawl out of the water and moult to the adult stage. The adults crawl or fly to the vegetation surrounding the water to find a mate. Males attract females by drumming on their abdomen. After mating, the female gathers her eggs and deposits them into the water. Females usually lays multiple eggs masses over the course of their one to four week lifespan. The eggs may hatch in two weeks or in several months.

Ecology

Stoneflies are commonest in cool oligotrophic streams or lakes. All stoneflies have aquatic larvae, which are important in the diet of fish and invertebrate predators. The larvae generally start their life as herbivores or detritivore, but shift to an omnivorous or carnivorous diet in later stages. Few adult stoneflies feed, but those that do eat algae and lichens.

Stonefly larvae almost always occur in clean, cool running water because they are intolerant of low oxygen levels. The larvae either lack gills or have small ones, obtaining oxygen through their cuticle. Their presence in aquatic systems is a sign that the ecosystem is healthy.

Idiosyncratic Inverts

Male stoneflies drum on their abdomen to attract a mate, but only virgin females will respond since they mate just once in their life!

Trichoptera

Introduction

Caddisflies are one of the largest groups of aquatic insects and their larvae have been very successful at adapting to different microhabitats.

Morphology

Larval caddisflies are very similar in appearance to lepidopteran caterpillars, but caddisflies have only one pair of prolegs on their abdominal segments while caterpillars have many. The head capsule of larval caddisflies and their first two thoracic segments are sclerotized. They have simple eyes, chewing mouthparts and very short antennae. Three pairs of thoracic legs, each bearing one tarsal segment and one claw, are present just behind the head. The last abdominal segment often bears a pair of fleshy prolegs which help the larvae to hold onto their portable case or helps their net-spinning.

Adult caddisflies look like moths, but their wings are covered with tiny hairs. They tend to be nocturnal and are generally drab in color. Compound eyes may be complemented with up to three ocelli. Adults do not have well-developed mouthparts and are only able to ingest liquids. The position and number of setal warts on their heads are often used to identify adults of different species.

Metabolism

Trichopteran larvae can survive in water with low oxygen concentrations even though they live in tubes. The larvae move around inside the tube to increase water flow and oxygen concentrations inside the tube. Larvae with tubes can live in water with much lower oxygen concentrations than those without tubes.

Reproduction

The eggs of caddisflies are laid in a gelatinous matrix either in or out of water. In the latter case, the larvae remain within the matrix until the pool is reflooded. Once the larvae break out of the matrix they spin a silken case or net. Larva usually have 5 instars. In their last instar the larvae either construct a pupal case, or simply seal off the portable case they have been using in their larval stage. The pupa stage has well developed mandibles which are used to cut open the case. Most pupal stages are 2 to 3 weeks in length, though some species overwinter as pupae. After the pupae cut themselves out of their case, they swim ashore or to the surface of the water where they moult. Adults live a few weeks to several months. Most species have more then one generation per year, but other need 2 years to complete development.

Mass emergences of caddisflies are often a nuisance to people because the adults are attracted to lights and some people are allergic to the setae on their wings.

Ecology

Although adult caddisflies are terrestrial, all North American species have aquatic larvae . Caddisflies are often classified according to the lifestyle of the larvae. Larval lifestyles are generally divided into 5 groups:

  1. Free-living forms - these larvae do not make a case until they pupate. They occur in cool running waters and are predaceous.
  2. Saddle-case makers - these larvae construct portable cases out of rock fragments. They occur in both running waters and on the wave-swept shores of lakes where they eat diatoms and particulate organic matter.
  3. Purse-case makers - these larvae are free-living in all but their last instar. The final larval stage constructs barrel- or purse-shaped cases. These larvae occur in permanent aquatic habitats where they eat algae and diatoms.
  4. Net-spinners or retreat-makers - these larvae construct retreats that are fastened to the substrate equipped with nets to collect food particles from the flowing water.
  5. Tube-case makers - these larvae construct tube-shaped cases in cool streams. They feed on detritus.

The larval cases protect them from predators, camouflage them and also act as a physical barrier between the abrasive substrate and their soft bodies.

Trichopteran larvae are very important in some streams because they dominate the insect biomass and are an important food source for fish. They have adopted every possible feeding strategy in aquatic systems, from predation to filter feeding.

Caddisflies have invaded many aquatic habitats, but are most diverse in cool, running waters.

Idiosyncratic Inverts

Caddisfly larvae usually build some sort of net or portable case. Many entomologists use these cases to classify larvae into groups.



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Glossary

Citation

Hebert, P., & Ontario, B. (2008). Insecta (Aquatic). Retrieved from http://www.eoearth.org/view/article/153843

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