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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?
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7  
That source doesn't make the claim that the urine is the cause of the irritation. –  iamnotmaynard Sep 1 at 12:18
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I know I do when I bite them –  Adsy Sep 2 at 16:24

1 Answer 1

up vote 47 down vote accepted

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.

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4  
Why not citing the original paper instead of the NYT? Mosquitoes Cool Down during Blood Feeding to Avoid Overheating - Lahondère and Lazzari, Curr Biol. 2012 –  nico Sep 1 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 1 at 19:31
    
@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. –  fredsbend Sep 2 at 5:23
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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 2 at 16:17
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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. –  fredsbend Sep 2 at 17:07

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