Dolomedes
Encyclopedia
Dolomedes is a genus
of large spider
s of the family Pisauridae
. They are also known as fishing spiders, raft spiders, dock spiders or wharf spiders. Almost all Dolomedes species are semi-aquatic, with the exception of the tree-dwelling D. albineus of the southwestern United States
. Many species have a striking pale stripe down each side of the body.
They hunt by waiting at the edge of a pool or stream, then when they detect the ripples from prey, they run across the surface to subdue it using their foremost legs, which are tipped with small claws; like other spiders they then inject venom with their hollow jaws to kill and digest the prey. They mainly eat insects, but some larger species are able to catch small fish. They can also climb beneath the water, when they become encased in a silvery film of air.
There are over a hundred species of Dolomedes throughout the world; examples include Dolomedes aquaticus
, a forest-stream species of New Zealand, the raft spider
(D. fimbriatus), which lives in bogs in Europe, and the great raft spider
(D. plantarius), which lives in fens, also in Europe. Many species are large, some with females up to 26 mm (1 in) long with a leg span of 80 mm (3.1 in).
to stand or run on the water, like pond skaters. They can also climb beneath the water, and then air becomes trapped in the body hairs and forms a thin film over the whole surface of the body and legs, giving them the appearance of fine polished silver. Like other spiders, Dolomedes breathe with book lung
s beneath their abdomens, and these open into the air film, allowing the spiders to breathe while submerged. The trapped air makes them very buoyant and if they do not hold onto a rock or a plant stem they float to the surface where they pop onto the surface film, completely dry.
, other aquatic insects, and even small fish
. For fishing spiders, the water surface serves the same function as a web does for other spiders. They extend their legs onto the surface, feeling for vibrations given off by prey.
Dolomedes are nocturnal hunters, feeding when birds, their main predators, are sleeping. The method they use to fish for insects is to hold on to the shore with their back legs while the rest of their body lies on the water, with legs stretched out. Dolomedes species tend to be robust with thickset legs that allow them to tackle prey larger than themselves. They stretch out their front legs and wait, as if listening. Their front legs feel the vibrations carried on the water, just as other spiders feel the vibrations in a web. They are able to tell what is causing the vibrations that the water is carrying - to distinguish the drawn-out, erratic vibrations of a struggling insect from the one-off vibrations caused by falling leaves or the background noise of the wind or the flow of the water around rocks and other obstacles. As well as identifying the source of the vibrations, the spiders are also able to discern the distance and direction to the source. To this end they have a range of vibration-detecting organs, including very sensitive hairs (trichobothria
) on their legs and feet. Their eyes play a secondary role - experiments on related species show that touch is the main sense these spiders use to catch their prey. Their eyes are of little use for nocturnal hunting. These vibration detectors also serve to warn the spider of predators such as trout
.
As soon as the vibrations reveal that there is a floundering insect within range, some fishing spiders may take direct action - they run at pace across the surface of the water and grab the insect before it extracts itself from the water and flies to safety. Some fishing spiders use silk
draglines to prevent themselves from speeding past the prey.
One researcher, Michelle Greenwood, fed a large moth to a small juvenile fishing spider and observed its behaviour: "On my first fieldtrip I discovered the fun of hand feeding them. I caught a large moth and offered it, struggling in my fingers, to a relatively small juvenile fishing spider. The spider daintily took it, moved itself to dry land and proceeded to eat. I watched this process for almost half an hour; fascinated by the way the spider stood on the tips of its articulated legs and deftly manoeuvred the moth with its mouthparts. She then delicately cleaned all her legs, passing each between her fangs; carefully scraping them clean before wiping each front leg over her eyes." The spider then returned to the riverside to await further victims.
Fishing spiders' main prey is aquatic insects, but they are opportunistic feeders and will eat anything suitable that happens within range. Dolomedes in North America have been observed catching and eating small goldfish.
have also been observed catching young spiders. Species parasitic on the spiders include a wasp of the Pompilidae family that stings the spider to paralyse it before carrying it off and laying an egg in its abdomen. The larvae of the wasp hatch and proceed to eat the spider from the inside out. One escape technique the spiders use is to disappear beneath the surface tension of the water. However some wasps counter the disappearing trick by going into the water to sting the spider and drag them out of the water.
The North American species D. triton has been observed jumping to get away from an attacking frog. The researchers concluded that the spider's vibration-detecting hairs felt the rush of air as the frog lunged to attack.
Female fishing spiders are larger than the males. If a female chooses to eat a male after mating, there is usually little he can do about it. This behaviour may help the female by providing the nutrition she needs to produce healthy young, while the male gets the benefit of passing his genes on to a well-fed next generation.
) lives primarily in small lakes and ponds. This spider consumes mostly water striders (pond skaters), but like all Dolomedes, it is an opportunistic ambush hunter that will eat anything that it can capture. Other species include the bog-dwelling D. striatus, and four species living by streams: D. scriptus
, D. vittatus, D. gertschi and D. holti. Two North American species, D. tenebrosus
and D. okefinokensis, exhibit female giganticism and/or male dwarfism, with their males being less than half the size of the females. The ninth species is the arboreal D. albineus.
. The raft spider
(D. fimbriatus) is widespread on the surface of bog
pools and in boggy grassland
. The great raft spider
, (D. plantarius) lives in fen
s, and is listed as endangered in Great Britain
and is globally vulnerable
.
. Two are widespread: the large Dolomedes aquaticus
of open riverbanks, and another as yet undescribed species of similar size, specialising in forested riverbanks; this is known as Dolomedes III or the New Zealand forest fishing spider. The largest of the New Zealand species Dolomedes schauinslandi or Rangatira spider occurs in the Chatham Islands
, where it occurs on rodent-free islands where running water is rare. The fourth species, D. minor
, is also not restricted to rivers. Known as the Nursery Web Spider, it makes white nursery webs on shrubs, but is still capable of fishing behaviour.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of large spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s of the family Pisauridae
Nursery web spider
Nursery web spiders are spiders of the family Pisauridae. They resemble wolf spiders , but they carry their egg sacs by means of their jaws and pedipalps . When the eggs are about to hatch, a mother spider will build a nursery "tent," put her egg sac inside, and mount guard outside...
. They are also known as fishing spiders, raft spiders, dock spiders or wharf spiders. Almost all Dolomedes species are semi-aquatic, with the exception of the tree-dwelling D. albineus of the southwestern United States
Southwestern United States
The Southwestern United States is a region defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas and Utah...
. Many species have a striking pale stripe down each side of the body.
They hunt by waiting at the edge of a pool or stream, then when they detect the ripples from prey, they run across the surface to subdue it using their foremost legs, which are tipped with small claws; like other spiders they then inject venom with their hollow jaws to kill and digest the prey. They mainly eat insects, but some larger species are able to catch small fish. They can also climb beneath the water, when they become encased in a silvery film of air.
There are over a hundred species of Dolomedes throughout the world; examples include Dolomedes aquaticus
Dolomedes aquaticus
Dolomedes aquaticus is a fishing spider that lives and hunts along the gravel banks of unforested New Zealand rivers. It prefers open riverbanks where it lives under rocks, usually less than 5 metres from the river. Its colouring allows it to blend in with river stones...
, a forest-stream species of New Zealand, the raft spider
Raft spider
The raft spider, Dolomedes fimbriatus, is a European spider of the family Pisauridae. The raft spider is one of the two largest spiders in the United Kingdom...
(D. fimbriatus), which lives in bogs in Europe, and the great raft spider
Great raft spider
The great raft spider or fen raft spider is a European species of spider in the Pisauridae family. Like other Dolomedes spiders, it is semi-aquatic, hunting its prey on the surface of water...
(D. plantarius), which lives in fens, also in Europe. Many species are large, some with females up to 26 mm (1 in) long with a leg span of 80 mm (3.1 in).
Aquatic adaptations
Dolomedes spiders are covered all over in short, velvety hairs which are unwettable (hydrophobic). This allows them to use surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
to stand or run on the water, like pond skaters. They can also climb beneath the water, and then air becomes trapped in the body hairs and forms a thin film over the whole surface of the body and legs, giving them the appearance of fine polished silver. Like other spiders, Dolomedes breathe with book lung
Book lung
A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...
s beneath their abdomens, and these open into the air film, allowing the spiders to breathe while submerged. The trapped air makes them very buoyant and if they do not hold onto a rock or a plant stem they float to the surface where they pop onto the surface film, completely dry.
Hunting behaviour
Rather than hunting on land or by waiting in a web, these spiders hunt on the water surface itself, preying on mayfliesMayfly
Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...
, other aquatic insects, and even small fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
. For fishing spiders, the water surface serves the same function as a web does for other spiders. They extend their legs onto the surface, feeling for vibrations given off by prey.
Dolomedes are nocturnal hunters, feeding when birds, their main predators, are sleeping. The method they use to fish for insects is to hold on to the shore with their back legs while the rest of their body lies on the water, with legs stretched out. Dolomedes species tend to be robust with thickset legs that allow them to tackle prey larger than themselves. They stretch out their front legs and wait, as if listening. Their front legs feel the vibrations carried on the water, just as other spiders feel the vibrations in a web. They are able to tell what is causing the vibrations that the water is carrying - to distinguish the drawn-out, erratic vibrations of a struggling insect from the one-off vibrations caused by falling leaves or the background noise of the wind or the flow of the water around rocks and other obstacles. As well as identifying the source of the vibrations, the spiders are also able to discern the distance and direction to the source. To this end they have a range of vibration-detecting organs, including very sensitive hairs (trichobothria
Trichobothria
Trichobothria are elongate setae, present in the arachnids, and various orders of insects, that function in the detection of airborne vibrations and currents.-Morphology:...
) on their legs and feet. Their eyes play a secondary role - experiments on related species show that touch is the main sense these spiders use to catch their prey. Their eyes are of little use for nocturnal hunting. These vibration detectors also serve to warn the spider of predators such as trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
.
As soon as the vibrations reveal that there is a floundering insect within range, some fishing spiders may take direct action - they run at pace across the surface of the water and grab the insect before it extracts itself from the water and flies to safety. Some fishing spiders use silk
Spider silk
Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring...
draglines to prevent themselves from speeding past the prey.
Fishing spiders' main prey is aquatic insects, but they are opportunistic feeders and will eat anything suitable that happens within range. Dolomedes in North America have been observed catching and eating small goldfish.
Predators
The main predators of fishing spiders are birds and snakes. DragonfliesDragonfly
A dragonfly is a winged insect belonging to the order Odonata, the suborder Epiprocta or, in the strict sense, the infraorder Anisoptera . It is characterized by large multifaceted eyes, two pairs of strong transparent wings, and an elongated body...
have also been observed catching young spiders. Species parasitic on the spiders include a wasp of the Pompilidae family that stings the spider to paralyse it before carrying it off and laying an egg in its abdomen. The larvae of the wasp hatch and proceed to eat the spider from the inside out. One escape technique the spiders use is to disappear beneath the surface tension of the water. However some wasps counter the disappearing trick by going into the water to sting the spider and drag them out of the water.
The North American species D. triton has been observed jumping to get away from an attacking frog. The researchers concluded that the spider's vibration-detecting hairs felt the rush of air as the frog lunged to attack.
Breeding
North America
Nine species of Dolomedes exist in North America. The six-spotted fishing spider (D. tritonDolomedes triton
The six-spotted fishing spider, Dolomedes triton, is an arachnid from the nursery web spider family Pisauridae. This species is from the genus Dolomedes, the fishing spiders. This species of fishing spider is named after the mythological Greek god Triton who is the messenger of the big sea and the...
) lives primarily in small lakes and ponds. This spider consumes mostly water striders (pond skaters), but like all Dolomedes, it is an opportunistic ambush hunter that will eat anything that it can capture. Other species include the bog-dwelling D. striatus, and four species living by streams: D. scriptus
Dolomedes scriptus
Dolomedes scriptus is a fishing spider found in central North America. Female spiders can grow to be over 6 cm in legspan. The spider is a pale brown colour with lighter stripes around its legs and a stripe down each side of the body. It is similar to D. tenebrosus.-References:* , The Nearctic...
, D. vittatus, D. gertschi and D. holti. Two North American species, D. tenebrosus
Dolomedes tenebrosus
The Dark Fishing Spider is a fishing spider found in northeastern North America.Female bodies are 15–26 mm; males are 7–13 mm. Legs range from 50–90 mm. The spider is a pale to dark brown colour with several chevron markers and lighter stripes around its legs. Similar to D....
and D. okefinokensis, exhibit female giganticism and/or male dwarfism, with their males being less than half the size of the females. The ninth species is the arboreal D. albineus.
Europe
Two Dolomedes species occur in EuropeEurope
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. The raft spider
Raft spider
The raft spider, Dolomedes fimbriatus, is a European spider of the family Pisauridae. The raft spider is one of the two largest spiders in the United Kingdom...
(D. fimbriatus) is widespread on the surface of bog
Bog
A bog, quagmire or mire is a wetland that accumulates acidic peat, a deposit of dead plant material—often mosses or, in Arctic climates, lichens....
pools and in boggy grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
. The great raft spider
Great raft spider
The great raft spider or fen raft spider is a European species of spider in the Pisauridae family. Like other Dolomedes spiders, it is semi-aquatic, hunting its prey on the surface of water...
, (D. plantarius) lives in fen
Fen
A fen is a type of wetland fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients...
s, and is listed as endangered in Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
and is globally vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
.
New Zealand
Four species occur in New ZealandNew Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. Two are widespread: the large Dolomedes aquaticus
Dolomedes aquaticus
Dolomedes aquaticus is a fishing spider that lives and hunts along the gravel banks of unforested New Zealand rivers. It prefers open riverbanks where it lives under rocks, usually less than 5 metres from the river. Its colouring allows it to blend in with river stones...
of open riverbanks, and another as yet undescribed species of similar size, specialising in forested riverbanks; this is known as Dolomedes III or the New Zealand forest fishing spider. The largest of the New Zealand species Dolomedes schauinslandi or Rangatira spider occurs in the Chatham Islands
Chatham Islands
The Chatham Islands are an archipelago and New Zealand territory in the Pacific Ocean consisting of about ten islands within a radius, the largest of which are Chatham Island and Pitt Island. Their name in the indigenous language, Moriori, means Misty Sun...
, where it occurs on rodent-free islands where running water is rare. The fourth species, D. minor
Dolomedes minor
Dolomedes minor, also known as the nursery web spider, is a large fishing spider endemic to New Zealand. Female spiders can have a legspan of over 60 mm....
, is also not restricted to rivers. Known as the Nursery Web Spider, it makes white nursery webs on shrubs, but is still capable of fishing behaviour.
Further reading
- Carico, James Edwin (1973): The Nearctic spiders of the genus Dolomedes (Araneae: Pisauridae). Bulletin of the Museum of Comparative Zoology (Harvard) 144 (7): 435-488.
External links
- Fishing spider Dolomedes tenebrosus
- Pictures of Dolomedes triton (free for noncommercial use)
- Jeffrey K. Barnes, Dark fishing spider, Arthropod Museum Notes, University of Arkansas.
- Richard Ford, The raft spider (Dolomedes fimbriatus), Digitalwildlife.co.uk. image and short description