what type of spider is black and has red circles (dots) in a row on its back and is it dangerous?
493101
Black Widow (?)\n\nThe red dots are said to resemble a violin.
680598
As far as I know, there is no species of spider commonly known as the "blue-back."  All spiders are technically poisonous, but if you mean "potentiall dangerous to humans" and the spider you're referring to is from the US, then the answer is probably no, since none of the dangerous US spiders that I know of could reasonably be called a "blue back" spider.
1124532
There are black widow spiders in southern Ontario.  Most of Canada is too cold for them.  \n\nThe brown recluse spider whose bite is quite bad,  is also found in Canada.  Some kind of spider can be found pretty much everywhere in Canada.  \n\nI hope this helps.
921541
I can't see your images (it says I am forbidden/don't have permission to view the site), but from the description it sounds like the spider is likely a spiny orbweaver or spinybacked garden spider (Genus Gasteracantha).\n\nHere's some pictures of spiny orbweavers http://www.floridanature.org/species.asp?species=Gasteracantha_cancriformis\n\nhttp://bugguide.net/node/view/1982/bgpage\n\nThese species are harmless, even beneficial garden spiders that build typical spider webs to capture flying insects, including flies, mosquitoes and other nuisance species.\n\nThey do possess venom, but their venom is formulated for effectiveness against insects, and is not very potent against vertebrates, even if their bite was strong enough to break human skin (which it may not be).\n\nThey are not aggressive, and would only bite if the spider were injured or undergoing extreme harassment.\n\nUnless you were allergic to the toxin (unlikely, but remotely possible) the bite would probably be no worse than a mosquito bite, at worst it would be similar in pain and severity to a wasp sting.  Annoying, but certainly not life threatening.  The greatest danger would be infection caused by scratching the wound with dirty fingernails.
518311
It may have been a harvestman (related to spiders, but included in order Opiliones). \nCheck this page, it has photographs of different types of Chelicerates and some identification tips:\nhttp://www.xs4all.nl/~ednieuw/Spiders/InfoNed/The_spider.html
1041169
It wasn't a black widow spider which are poisonous.They are dark black and shiny not hairy with white stripes on the leg. They have a red hour glass shape on the belly.I believe it might be just an ordinary jumping spider found in most homes non poisonous.
987078
it might be a brown recluse spider bite.  keep a watch on it.  if it continues to get infected you will have to go to the doctor.  let someone older look at it please.
443012
In general, Huntsman spiders are not regarded as dangerous. There are however reports of members of the genus Neosparassus (formerly called Olios) giving bites that have caused prolonged pain, inflammation, headache, vomiting, and irregular pulse rate. It is unclear under what circumstances these spiders bite people, but it is known that female members of this family are quite brave in defending their egg sacs and their young.\n\nHuntsman spiders is a common name given to the family Sparassidae (formerly Heteropodidae). These spiders are found in Australia, New Zealand, South-east Asia, the Middle East in areas near the Mediterranean Sea, Florida and Hawaii in the US, and quite possibly in many other tropical and semi-tropical areas.
1040058
The red spotted ant mimic spider, like all spiders, possesses venom glands, and is predatory, however its venom is not dangerous to humans.\n\nI'm not sure why they're actually called ant mimics, since they don't look much like an ant to me.  They look like a pretty black spider with a red stripe down the back of its abdomen to me.
1119778
SPIDERS! They freak me out, too many legs and too hard to tell apart. I can never remember which ones are deadly!\nRoaches are just nasty. Not scary.
1018179
There are 600 different types of spiders in Britain - five of which are commonly found in houses. They range in size from the tiny money spiders to the cardinal spider and the great raft spider which is the biggest in Britain and can be over 13cm across!\n\nMore about the raft spider:\nFound in marshes or swamps, usually in Southern England.\nFemales are usually about 22mm, males 10-13mm.\nRaft spiders are very big and strong, they eat prey such as damselflies, bluebottles and even small fish!
594597
The funnel web spider is the most deadly in Australia.  After being bitten there is little hope.  Death occurs with in an hour.  There is an anti venom if you can get it.  The funnel web lives in a underground tunnel and when the trip line is snagged the spider will charge out at its prey and repeatedly bite until it is dead.
774839
Some spiders spin funnel-shaped webs, others make sheet webs, spiders like the black widow make tangled, maze-like, webs, and still others make the spiral "orb" webs that are most commonly associated with spiders. These webs may be made with sticky capture silk, or with "fluffy" capture silk, depending on the type of spider. Webs may be in a vertical plane (most orb webs), a horizontal plane (sheet webs), or at any angle in between. Most commonly found in the sheet-web spider families, some webs will have loose, irregular tangles of silk above them. These tangled obstacle courses serve to disorient and knock down flying insects, making them more vulnerable to being trapped on the web below. They may also help to protect the spider from aerial predators such as birds and wasps.
1076628
I think there called zebra jumpimg spiders--- http://en.wikipedia.org/wiki/Zebra_spider
1071424
A few kinds of spiders are harmful to human beings. In North America, six kinds of spiders harm people, they are - the Brown Recluse, Sac, Black Widow, Brown Widow, Red-legged Widow and the Varied Widow. \n\nWater spiders are the only spiders that live most of their life underwater. The water spider breathes underwater from air bubbles that it holds close to its body. Its underwater nest is a silk web shaped like a small bell. The spider fills the web with air bubbles, which gradually push all the water out of the bell. The animal can live on this air for several months. Water spiders are found only in Europe and parts of Asia.
650854
It sounds like a jumping spider. If it is a hairy matte black then Im sure it is. They are mostly harmless prefering to leave you alone and eat houseflys. They are not generaly agressive unless provoked.
902800
There are probably a lot of spiders that would fit\nyour description. More information is needed, such\nas where (geographically) the spider was seen, in\nwhat kind of habitat and at what time of year.  The\nvarious other answers mostly mention the garden\nor writing spider (Argiope), but it might qualify as\na large spider, depending on what you consider\nlarge.  Another eastern U.S. spider that sometimes\nhas a bright yellow mark and is smaller is another\nof the orb spiders, Verrucosa, which has a triangular abdomen and gets about a half inch long\nin body length.