Anatomy of Bloodfeeding
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Leeches are best known for their bloodfeeding habits and for their use in the art of phlebotomy or bloodletting.
Few people ever take a good close look at a leech when it's feeding, and even fewer have stopped to ponder the complexity of this behaviour. Next time you are fortunate enough to have been found to be a suitable meal by a poor hungry leech, take a good close look before you reach for the salt-shaker. First of all, leeches are remarkably colorful. To the right here can be seen Macrobdella decora with a row of orange dots down the center of its back, as well as Hirudo medicinalis with a brilliant emerald and ruby color patterning on its dorsal surface.
When a leech is attached to your skin, it does not necessarily start feeding straight away. First it has to find a nice soft place to feed from that has to "taste" just right. The palm of your hand for example, it not really a good place for them since the skin is usually quite thick. The classic characteristic of a medicinal leech feeding is this strongly arched neck seen to the right here.
Medicinal leeches have three muscular "jaws", one dorsal and two ventrolateral jaws, each of which has a row of fine "teeth" or denticles on it. By moving those jaws back and forth across your skin, they make a fine incision and cut the underlying capillary beds, thus causing blood to pool. The outer rim of the oral sucker is very muscular and the leech uses this to create a firm seal around the wound so that is can effectively suck your blood into its gastric caeca (gut).
The resulting wound from a leech bite is not very large, and nor is it very deep. However, because of the circular oral sucker and because of the arrangement of the three jaws, it does provide you with a rather prestigious Mercedes-Benz-like mark on your skin!
Leeches, like Haemadipsa picta on the left, only have to feed once every few months because they fill themselves up and then hide under rocks or sticks as they digest this meal slowly. Without the anticoagulants leeches have in their saliva, they would turn into a little sausage-shaped brick if your blood clotted inside of them. As well, the leech takes about half an hour or more to finish feeding and normally this would be plenty of time for your clotting system to stop the available flow. The powerful anticoagulants that leeches have remain active in the wound for hours after the leech has finished. This is why you bleed for some time afterwards.
Leeches, of course, don't spend a good deal of time looking for humans to feed on. After all, we don't really spend a great deal of time in the water so a leech might wait a long time. Medicinal leeches like Macrobdella decora in North America and Hirudo medicinalis in Europe normally feed on frogs and other amphibians. A recent study by undergraduate students here at the University of Michigan (Maria Estigarribia, Leanne Miller, and Nathan Teismann) showed that temperature (37 C) and movement did not significantly shorten the time it takes M. decora to feed on blood in a sheep-skin condom, but that rubbing the condom with a frog first caused the leeches to attach and feed almost immediately!
If you try to get leeches to feed on you by picking them up and applying them to your skin, you'll run into the same problem that Hepburn and Bogart ran into when Huston was filming them in the African Queen. They won't readily oblige because they are used to being stealthy, and generally will be more interested in running away if something as big as you picks them up.
The leeches that were feeding on Alnut (played by Bogart) in the film probably belonged to one of these two species known from the Congo basin. To the left here is Limnatis michaelseni and to the right is Limnatis africana. Both of them I think you'll agree are more beautiful than Allnut suggests in the movie where he needlessly worries about them poisoning the blood. They do not.
Of course, you could always get-your-own-back:
I thank Jessica Light and Stacie Novakovic who volunteered their exposed skin for the bloodfeeding depicted here, Lisa Curran who provided the photo of Cam Webb's foot (shot in Borneo), and Ms. Hepburn for not suing me for using the photo from her book