Monday, November 14, 2011
The Chironomids Part I: Marvellous Midges
(Photo by zh3nG 正)
Most of us are familiar with mosquitoes, and public service messages constantly remind us of the fact that there are commensal species which breed in stagnant water in and around our homes. In one aspect of their life cycle, mosquitoes (F. Culicidae) are not unique; numerous other species of flies have aquatic larvae, and the adults are often referred to as midges. One family in particular, commonly known as the chironomids or non-biting midges (F. Chironomidae), is well-represented throughout the world.
Here's the life cycle of a typical chironomid.
(Image from Environmental Changes at Port au Choix as Reconstructed from Fossil Midges)
The bulk of a midge's life is spent as a larva, and so I'll focus a lot on this stage in the life cycle. Besides, I often deal with chironomid larvae in my current field of work.
Hre's a diagram to help you make sense of the general external anatomy of a chironomid larva.
(Image from Biota of Foundry and South Coves)
Chironomid larvae are often referred to as bloodworms, due to the presence of haemoglobin in many species, which gives them a reddish coloration.
(Photo by North Carolina Museum of Natural Sciences)
This makes it easier to absorb oxygen from their surroundings, enabling them to survive in oxygen-poor environments that would kill other aquatic invertebrates.
(Photo by venwu225)
(Photo by JFCART)
However, they seem to lose the bright red colours once they're preserved in alcohol, and eventually turn green. This change in colour, combined with their tiny size, can make sorting of samples an extremely challenging task, especially if the bloodworms are entangled amongst tufts of algae.
(Photo by river.relief)
(Photo by henricksrobert)
(Photo by NOAA Great Lakes Environmental Research Laboratory)
However, not all chironomid species possess haemoglobin in the first place. These tend to be greenish, even when alive.
(Photo by Neil Phillips)
These macro shots are really quite impressive, since bloodworms are actually quite tiny creatures. The following photos help to illustrate this point.
(Photo by tantramarwetlands)
(Photo by Ohio Sea Grant and Stone Laboratory)
(Photo by piscator_4)
Habitats and Habits
The vast majority of midges have larvae that develop in freshwater, but there exists a wide variety of habitat preferences. Flowing water is preferred by a number of species, while others thrive in the still waters of ponds, lakes, and swamps. Some are specialised to live in thin films of water a few millimetres thick, such as those along the banks of streams or the shores of lakes, or near waterfalls or rapids, where the rocks are perpetually wet. Small amounts of water that accumulate in leaf axils may become home to chironomid larvae, and some live exclusively inside the pitfall traps of pitcher plants, where they somehow manage to avoid being digested.
Chironomid larvae spend most of their time on the bottom, where different species are adapted to feed in different ways, ranging from those that gather organic material and detritus from the sediments, to filter-feeders that cast a small net made from saliva to trap plankton and suspended particles from the water. Other species scrape algae from the surface of rocks and debris, or use their jaws to chew aquatic plants, wood, algae or leaf litter. Some larvae are predatory, engulfing planktonic crustaceans or piercing and sucking out the fluids of annelid worms, or other aquatic insects, including smaller chironomids.
Head of a chironomid larva;
(Photo by Specious Reasons)
Although larvae can actively swim in the water, they spend most of their time either crawling about on the bottom, or hanging by their tails from hard surfaces.
(Photo by Neil Phillips)
Many species construct a tube out of saliva and sediment, for added protection against predators. Others burrow and tunnel into wood and plants.
(Photo by debunix)
Pupa and Adulthood
The pupa has a bizarre, almost alien appearance.
(Photo by Wontolla65)
The white tufts near what passes for the 'head' of the pupa are the thoracic horns. These function like gills in absorbing oxygen, which is required in large quantities for metamorphosis to occur.
(Photo by Darren Post)
Unlike the pupae of other insects such as butterflies, those of chironomids are highly motile, and can swim. This is especially apparent when the midge is ready to progress to the next stage in its life cycle; the pupa swims all the way from the bottom to the water surface, where the skin splits, revealing the wispy winged adult.
Pupa just before eclosion (adult emergence);
(Photo by servitude)
Adult midge emerging from pupa;
(Photo by CurlyToes)
As their common name (non-biting midge) indicates, chironomids, unlike mosquitoes and several other families of flies, are not bloodsuckers. It is commonly thought that these midges do not feed as adults; however, feeding has been recorded for many species, and feeding behaviour may be more prevalent amongst adult chironomids than we think. Among the natural substances consumed include nectar, pollen, honeydew (the sugar-rich liquid secreted by aphids and other sap-sucking insects), and even fresh fly droppings!
(Photo by giubit)
(Photo by entomopixel)
Midge Larvae are Hardcore
The midges possibly have the widest distribution of any insect family, and have an extremely high diversity of species. Many of them have evolved to survive in the most challenging environments, and their larvae can be found in quite extreme habitats.
For instance, there are some whose larvae are specialised to live in streams fed by glacial meltwater, where the water temperature remains just above freezing. Here they feed on organic particles released from the ice as it melts, or on the cyanobacteria that manage to colonise these channels. Other species living in temperate climates are tolerant of subzero temperatures, and can even survive being frozen. At the other extreme, some chironomid species have been recorded from hot springs, where the water temperature exceeds 40°C. A species of midge (Sergentia koschowi) has been found in the ooze at the bottom of Lake Baikal in Russia, at a depth of 1,360 metres.
Many species of chironomids living in arid regions have larvae capable of surviving drought and the disappearance of the temporary pools they live in, entering a state of suspended animation until the rains return. One African species in particular, known as the sleeping chironomid (Polypedilum vanderplanki), lives in depressions on rocks that fill up after the rain to form puddles, and has become notable for its ability to survive all sorts of extremes in its dormant state. When dessicated, they can survive temperatures from -270°C to 102°C, and even exposure to very high levels of gamma radiation, resuming activity and successfully undergoing metamorphosis after immersion in water. Dessicated larvae have even survived after spending more than a year in outer space. In the laboratory, it has been demonstrated that the larvae of this species can survive at least 10 cycles of dehydration and rehydration, with the record of one larva being revived after being stored in dry sediment for 17 years! It's not surprising that the sleeping chironomid is used as a model organism for astrobiological research.
Dessication-rehydration cycle of the sleeping chironomid;
(Diagram from Identification of Anhydrobiosis-related Genes from an Expressed Sequence Tag Database in the Cryptobiotic Midge Polypedilum vanderplanki (Diptera; Chironomidae))
The larvae of some species are fully terrestrial, living in soil and damp vegetation. One species (Camptocladius stercorarius) is a specialist, living exclusively in cow dung. Another species (Eretmoptera murphyi) has larvae that live in damp moss and peat. This particular species is unique, in that it lives on the sub-Antarctic island of South Georgia, a place inhabited by few other insects. The adult stage has only very rudimentary wings, and so these midges are flightless.
(Photo by Roger S Key)
Another flightless midge species (Belgica antarctica) is found even further south, on Antarctica itself. On the frozen continent, it is not only Antarctica's only insect species, but also its largest fully terrestrial animal, even though it grows to 6 millimetres at most (Penguins and seals are of course much larger, but they spend much of their time at sea).
Mating pair of Antarctic midges;
(Photo by Tasteofcrayons, from Wikipedia)
It too has terrestrial larvae, which live amongst algae, in mosses, and in the soil around the roots of grasses. The larvae may also be attracted to the nutrient-enriched soil associated with nests and breeding colonies of penguins, seabirds and seals. They can survive freezing and dehydration, and immersion in freshwater (melting ice) or saltwater (sea spray from storms), useful adaptations in such a hostile environment. The larvae of the Antarctic midge can even live without oxygen for up to a month, which comes in handy when you're encased in ice or buried in penguin guano full of bacteria that deplete all the available oxygen.
Larvae of Antarctic midge. The dark coloration might help to maximise absorption of the sun's heat, as well as provide protection against ultraviolet radiation;
(Photo from Studying Belgica at the Bottom of the World)
Another account of chironomids' ability to survive extreme environmental conditions comes from reports of live larvae of a particular species (Chironomus salinarius) being discovered in fresh droppings of the black-tailed godwit (Limosa limosa), having presumably been eaten by the birds and passed through the digestive system alive. It's not exactly adapting to a particular habitat, and only a small minority of bloodworms found in the faeces were alive, but it does illustrate just how tough the larvae of some species can be in withstanding otherwise deadly surroundings.
Most insects do not live in marine environments, but once again, the chironomids prove to be exceptional. The larvae of many species are capable of developing in highly saline waters, while others actually grow on rocky shores in the intertidal zone. However, the marine midges (Pontomyia spp.) can be considered to be fully marine insects, though they are restricted to tropical and subtropical coastal areas in the Indo-Pacific and Caribbean. They have also been recorded in Singapore. As larvae, they inhabit tidal pools and lagoons, living in sand, seagrass and algae beds, and even amongst coral rubble on reefs up to depths of 30 metres. They've even been found in tufts of algae growing on the shells of sea turtles!
Once metamorphosis is complete, the pupae float up to the surface. Eclosion is synchronised, and might be influenced by the tides, the time of sunset, and the phases of the moon; one species (Pontomyia oceana) appears only around the new moon or full moon. As dusk falls, the males emerge en masse, and their presence somehow triggers the emergence of the females an hour later. The urge to reproduce is so strong that some males will even attempt to mate with a female before she can fully emerge from the pupal skin.
The adults are unique in their own way; the females lack mouthparts, wings, antennae, and possess only 2 pairs of legs, which are little more than stumps. The males too lack mouthparts, and their wings are too small to enable them to fly. However, by fluttering their wings rapidly, the males can still skate across the surface of the water, with the extremely long first pair of legs functioning as 'outriggers' to help maintain balance.
Adult male marine midges, Pulau Tekukor;
(Photo by James)
Once the male has found a female, copulation takes place rear to rear, with him dragging her along as he continues to skitter on the water. Once mating is finished, he drops her, and he may go off in search of another female. Soon after, the female lays a sticky string of eggs, which sink and adhere to pieces of coral or other hard surfaces. The female then dies after laying her clutch of eggs, with the males, eventually spent, dying as well. The entire orgy, from emergence to mating to death, lasts 1-2 hours at most, a ridiculously short adult lifespan by any measure. No wonder the adult marine midges don't need mouthparts at all, since they don't even have time to feed!
Adult male marine midge, Pulau Subar Laut (Big Sister's Island);
(Photo by James)
Because they can be very abundant in aquatic habitats, chironomid larvae and pupae are important prey for many species, from other invertebrates such as water beetles and dragonfly nymphs, to fishes, amphibians, and birds.
Pied wagtail (Motacilla alba yarrellii) with bloodworm;
(Photo by Andyincheshire)
Grey wagtail (Motacilla cinerea cinerea) with bloodworm;
(Photo by Alshappy 1)
As adults, chironomids may be fed upon by other insects, spiders, frogs, birds, and bats.
Jumping spider (F. Salticidae) feeding on midge;
(Photo by J. Coelho)
The predator-prey relationship between fishes and midges has been exploited by anglers, who create artificial lures to tempt salmon and trout (among other species) to take a bite of what appears to be a chironomid larva, pupa, or a newly-emerged adult floating on the water.
This lure is known as the LiquidLace-Bloodworm;
(Photo by 'Scratch')
In Europe, carp ponds are sometimes deliberately enriched with farmyard manure to stimulate the growth of chironomid larvae, which in turn provide a source of natural food for the fish. In Hong Kong, chironomids are even actively farmed: a field is specially set aside and a layer of chicken manure is spread onto it, before it is turned into a pond. Adult midges soon lay their eggs in the water, and the pond eventually becomes filled with bloodworms. These are then harvested and used as fish food.
Bloodworms are highly prized by aquarists, who use them as live food for their pets. Since live bloodworms are not always available, there are also frozen and freeze-dried options.
(Photo by Ninja T. Penguin)
Panda corydoras (Corydoras panda) with bloodworm;
(Photo by Azaezl)
Turtles (F. Emydidae) feeding on bloodworms;
(Photo by DrChino)
Axolotl (Ambystoma mexicanum) with bloodworms;
(Photo by sour_bunny)
African dwarf frog (Hymenochirus sp.) with bloodworms;
(Photo by cathleenarnerich)
Midges are a very important component of the aquatic biota, and anyone studying freshwater ecology cannot afford to ignore them. Where eutrophication occurs due to high nutrient levels as a result of sewage or fertiliser runoff, the dissolved oxygen content of the water may be abysmally low, creating a habitat that is inhospitable to all but the hardiest of aquatic organisms. More often than not, chironomids are not only able to survive in such heavily polluted water, they manage to flourish, thanks to the haemoglobin found in the larvae of many species. They soon end up dominating these water bodies in terms of sheer numbers. Even sewage pools may be colonised by those species adapted to living under such challenging conditions.
Because chironomids are so abundant, and because one often finds multiple species coexisting in the same water body, they can fulfill an important function as biological indicators of water quality. Each species may have its own degree of tolerance to specific water quality parameters; for instance, some species may be more sensitive to changes in the levels of dissolved oxygen, organic matter, pH or heavy metals. And even though we have seen how some species can be tolerant of extremes, most chironomids have an optimal set of environmental conditions under which they can develop. If something happens to alter those conditions, for instance a chemical spill, only those capable of tolerating the new surroundings will survive. Hence, by sampling and examining the numbers and diversity of chironomids, one can monitor changes in the environment, and find evidence for deterioration or improvement in the habitat.
I mentioned earlier on that adult midges do not feed on blood, but aren't necessarily completely harmless to humans.
For many chironomid species, the adults in an area all emerge simultaneously. Then, as if triggered by an unseen cue, the males all take off and congregate in a particular location, forming a swarm. It does not take long before the females see the swarm and join in, and mating takes place in the air. From a distance, this hub of activity may look like a plume of smoke rising from the water.
(Photo by Deborah~)
(Photo by Bruce Bolin)
(Photo by kuddlyteddybear2004)
These mating swarms can have severe negative consequences for people in the vicinity. The clouds of midges make it difficult to stay outside without inadvertently inhaling them, or having lots of tiny insects flying into your mouth, eyes, and ears. Visibility can be poor, making driving through a swarm a very dangerous prospect. Where the aggregations of midges land to rest, their faeces may stain buildings so badly that repainting is necessary. Many chironomid species emerge and swarm at dusk, and are attracted to sources of light. As a result, swarms will enter homes, contaminating food, staining the walls, and making life generally very uncomfortable for the residents.
(Photo by AMICHAELMURRAY)
The midges die in their millions, and their bodies accumulate everywhere, requiring frequent cleaning to get rid of the unsightly piles. They clog up machinery, such as sensitive aviation equipment and air-conditioning units, and the body fragments may stick to car paint, damaging the paintwork. The dead midges themselves give off a foul stench which may persist for several days, even after the bodies have been cleared.
(Photo by zh3nG 正)
To make matters worse, chironomids may trigger allergic reactions in some people.
Information about chironomids is available in most guides on freshwater life, but where it comes to detailed information about the family as a whole, there is no better resource than The Chironomidae: The Biology and Ecology of Non-Biting Midges.
Coming up in the next post, we'll be taking a look at one particular midge swarming event that took place earlier this year, one that's particularly interesting because it occurred in a place that's been in the news lately for all the wrong reasons: Bedok Reservoir.
This is Part 1 of a 2-part series on the Chironomidae, or non-biting midges.
The Chironomids Part I: Marvellous Midges (this post)
The Chironomids Part II: Millions of Midges