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This is a claim that I have seen on several "Did You Know?" lists so far. They all state that when a mosquito bites you, they also urinate on you at the same time and that is what causes the area around the bite to swell, rather than the actual bite itself.

From woman's day:

While the average mosquito weighs about 2.5 milligrams, according to the AMCA, when they bite, they drink more than 5 milligrams—twice their body weight—of blood. For a 150-pound person, "that's like drinking a 300-pound milkshake," says Dr. Sirot. Once filled to the brim with blood, mosquitoes often excrete waste so they can fly away. This process, known as diuresis, happens with all species—though the type of waste product varies. Mosquitoes that can transmit malaria (anopheles) excrete plasma, while those that can transmit dengue and yellow fever (aedes) excrete urine.

  1. Is it true that mosquitoes urinate on the victim when they bite?
  2. Does the urination cause an allergic reaction leading to swelling around the bite area?
geoff
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Yes. Mosquito Peeing, Photo credit: James Gathany on Public Health Image Library (PHIL), Centers for Disease Control and Prevention (Photo by James Gathany, Public Health Image Library (PHIL), Centers for Disease Control and Prevention)

But it could be worse.

Most blood-sucking insects urinate while they feed so they can avoid filling up on fluid and get more nutrients out of their meal

But some species of mosquito also do what is called preurination — they excrete drops of freshly ingested blood without extracting any of the nourishing blood cells.

Source: New York Times article on Mosquitoes Cool Down during Blood Feeding to Avoid Overheating - Lahondère and Lazzari, Curr Biol. 2012

The pee is not the cause of the allergical 'itch'. The mosquito's saliva is the cause of that effect - it injects it to prevent coagulation in the blood.

Spork
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    Why not citing the original paper instead of the NYT? [Mosquitoes Cool Down during Blood Feeding to Avoid Overheating](http://www.cell.com/current-biology/abstract/S0960-9822%2811%2901311-X) - Lahondère and Lazzari, Curr Biol. 2012 – nico Sep 01 '14 at 16:01
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    "evaporative cooling of fluid droplets, which are excreted and maintained at the end of the abdomen" Some doubts remain to me: 1) is this excretion (I guess it corresponds to the photograph) considered urine? 2) there seems to be two different explanations here: "evaporative cooling", and making "more space" – leonbloy Sep 01 '14 at 19:31
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    @leonbloy My understanding of insect anatomy is that it cannot be urination by definition. Insects have a cloaca, which is an opening for urine, and also feces, and reproductive tracts. My understanding of insect physiology is that insects cannot control their waste secretions and feces and urine are excreted simultaneously because there is only one tract for them both. I am an amateur honeybee keeper, so I have studied honeybee and general insect biology. –  Sep 02 '14 at 05:23
  • [Here's an image of a generalized insect's cloacal tracts.](http://file.answcdn.com/answ-cld/image/upload/h_320,c_fill,g_face:center,q_60/v1401349487/moxwynm4crheuinuua0e.jpg) You can see that the Anal tube is the only waste related tract, because the urine and feces are mixed long before excretion. –  Sep 02 '14 at 05:30
  • I didn't use 'urine' in my answer, and I felt it is distracting from the question to be too nit-picky. But I can be, if you point me to a resource like 'insects cannot urinate'. For now I'm finding the opposite: http://www.pollinatorparadise.com/solitary_bees/faq5.htm – Spork Sep 02 '14 at 09:27
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    @leonbloy: whether that is considered urine or not is irrelevant, the physiology of insects are far too different than ours that using human terminologies to refer to anything it does is either misleading or at best just a very rough analogy. The point is that mosquitos excretes "liquid waste byproduct" during a meal. – Lie Ryan Sep 02 '14 at 15:55
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    @LieRyan If to call "urine" the excretion of an insect is " either misleading or at best just a very rough analogy" then, _that_ is a relevant fact, in light the original question. The "yes" answer should be qualified thus. – leonbloy Sep 02 '14 at 16:17
  • BTW, I insist on my second doubt: we have two very different explanations here, I'd like to know which one is the correct one. – leonbloy Sep 02 '14 at 16:18
  • I've never heard insect excrement secretions called urine. I *have* heard dejecta, ejecta, excrement, waste, fecal pellets, and frass. [This introductory text book prefers the term frass](http://books.google.com/books?id=S7yGZasJ7nEC&pg=PA82&lpg=PA82&dq=what+are+insect+waste+excretions+called&source=bl&ots=zIclwItqtN&sig=XzO_tcAQu5-HFL8lnylc5wyT2jk&hl=en&sa=X&ei=5-QFVNO1HMbJgwTomYKADQ&ved=0CDgQ6AEwBA#v=onepage&q=urine&f=false) on page 82, suggesting that "urine" might be an acceptable term informally, but frass is preferred academically, as it is more accurate. –  Sep 02 '14 at 16:52
  • @Spork The question was "Do mosquitoes urinate?" In which you answer yes. That is not actually the case. Yes, they do secrete a fluid, but it should not be called urine. –  Sep 02 '14 at 16:53
  • @leonbloy Yes, it is called urine by at least some researchers, e.g. http://physiologyonline.physiology.org/content/2/5/171, more sources are easy to find. Other synonyms are used too, of course. The several reasons it happens are described in the referenced material. Interesting, but not an impact to this answer. – Spork Sep 02 '14 at 17:04
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    @Spork I stand corrected. You should have lead with that article or a similar one. The fact that the mosquito is actually excreting only urine is a unique adaptation. Most other insects cannot excrete only urine. You can understand now why we pressed the issue. This is an exception to a rule of insect physiology. –  Sep 02 '14 at 17:07
  • @leonbloy My understanding of the newly linked article in the comments above is that it is a space issue. It removes fluids and salts before gorging. –  Sep 02 '14 at 17:08
  • There can be multiple reasons... Guys.... Sigh. – Spork Sep 02 '14 at 17:18