Re: Which part of the human body is most commonly bitten by insects?

Thanks for the interesting question. The answer to this particular question is rather complex. What we need to consider is the state of the human body at the time, and when I say state, I mean type of clothing, what is clothed, activity state, what the person is wearing, and disease state. To further explain why the above is important, we must understand, as well as we can, how an insect sees the world. Now, I’m going to focus my attention on mosquitoes. The main reason why is because we have the greatest amount of research on mosquito attraction to a blood meal target than any other insect. Why is that you might asking? Well, here’s the answer, in short form. There has been a great deal of research on what mosquitoes are attracted to (and for good reason – mosquitoes are one of the most common vectors for disease). In many developing countries today mosquitoes are one of the largest threats to human health, especially community health. So if we know what makes a host so attractive to the mosquito we might be able to better create a product or use alternative means to make us look less attractive to our little mosquito "friends." And now for something completely different...namely science...(the next few paragraphs list out some studies that were done, below that is where I tie everything together and really get to the substance of the answer -- I put these research synopsese first so that you can get a good handle for what work has been done, and so you can better understand the answer below). There was a study done by De Jong and Knols where they noted that when naked individuals who remain motionless are housed in the presence of many mosquitoes the person is bit primarily on the feet and head. These areas are the same that exhibit the highest body temperature and odor. It’s interesting to note that upon washing the feet and removing the exhaled air that there was a dramatic change in these preferences [1,2]. In fact after removal of breath there was a decrease in the amount of insect bites on the head, from 49% to 20% [2]. In a study to see the effects of skin temperature on the landing preferences of mosquitoes it was noted that mosquitoes preferred to land on hands that were artificially warmer than normal (control) hands. And that there is a deterrent effect of cold hands. So the researchers took the next logical step, and allowed mosquitoes to choose their blood meal. They had animals whose body temperatures vary greatly. In this study the mosquitoes preferred to land on the animal who has the warmest body temperature [3]! In another study involving temperature preferences, it was shown that midge flies prefer to attack one of their hosts on the belly – the warmest part of this particular animal host [5]. It is also noted that in flies there is a preference to feed off of hosts which are more passive. That is, hosts which exhibit less movement (i.e. in the case of horses, fewer tail flicks, foot stomping, and the like) [4]. So now let’s collate all this data, and come up with a nice, smooth flowing hypothesis/interpretation. I must say, as interesting as it would be to perform the actual experiments to prove/disprove this hypothesis, I cannot fathom that any Institutional Review Board (the group at a University who approves research involving human beings) in the United States would allow research of the kind required. But that doesn’t stop me from dreaming up a hypothesis. It would appear that our insect “friends” are attracted to individuals through temperature and odor. So let’s take a rather normal human instance of hiking through the woods, or walking down the street, there isn’t much difference between the two activities (physiologically speaking). As one walks they are moving their legs. Their arms are not in as much motion, and the torso is rather motionless, and so is the head and neck. The body as a whole is producing a great deal of heat – especially the legs. However, the legs are moving, and an insect landing on a moving target like a leg will be more difficult than landing on a more relatively stationary object, say the upper body. As you walk, you expire more air than at rest, so more warm air moves out of the mouth and nose, so the legs and the head are really showing a great deal of heat. That will attract the insect more to those areas. Because of our ears, we can usually hear an insect near them. Because of our eyes, we can generally see an insect going for the face, and would bat it away. This leave the back of the head, neck, torso, and legs. The torso is usually more covered than the arms in warmer weather, and would be showing more heat than the torso (the clothes would hold some of the heat in, so that it’s not easily seen by the insect, compared to the naked arms). So the insect would prefer biting the arms in this case. Now let’s take the case of the person who is sleeping, covered, in a bed. This person is exhaling, and most of the body is at rest. The only part of the body that is exposed is the head. The head also receives a great deal of blood, and as a result loses a great deal of heat. So the head looks like a beacon of heat to the insect, and in this case would be the preferred site of biting. Besides, the rest of the body is covered, and probably blocking the insect anyways. Now let’s take the case of the person reclining in a chair just relaxing. If the person just came home from a hard day’s work, and is just reading the newspaper, or watching television or something else, the feet are very easily accessible to the insect. They are probably not going to be covered – who likes to relax with their shoes on? And the socks might be off, maybe not – that doesn’t matter as long as the socks are thin. The feet are a source of great friction and heat, and therefore are an area that tends to sweat a lot! Sweat has in it nutrients that bacteria just love! The bacteria are what cause the odor, the odor attracts the insect, and presto – the insect attacks! Now an interesting question that I’d love to know the answer to is are the mosquitoes MORE attracted to odor or to heat, given that the quantities were relatively equal, and the sources were both as easily as accessible, and all other variables being the same. So basically – give them a warm hand that doesn’t smell, give them a warm hand that does smell – see which they prefer. Then give another group a cold hand that smells, a cold hand that doesn’t smell, and see which they prefer (that one would probably be the smelly cold hand). Then, give them a choice between a cold smelly hand, and a warm non-smelly hand, and see which they prefer! Now that would be a cool experiment! And we might even be able to get an approval for that! So, by means of review, let’s wrap this all up. An insect’s preference for host and where to bite are dictated by many items. To make things even more complicated we must always remember to consider the host, and the environmental conditions. The activity level of the host, and the activities that said host is participating in are tantamount in determining the biting preferences for an insect. Pretty cool, eh? Thank you very much for the question, and I hope that sheds some light on what’s going on in the interactive world of insects and humans! Best regards, Lyle D. Burgoon Graduate Assistant Dept. of Pharmacology and Toxicology College of Human Medicine National Food Safety and Toxicology Center Michigan State University References: 1) De Jong R, Knols BG. Selection of biting sites on man by two malaria mosquito species. Experientia 1995 Jan 15;51(1):80-4. 2) Knols BG, Takken W, de Jong R. Influence of human breath on selection of biting sites by Anopheles albimanus. J Am Mosq Control Assoc 1994 Sep;10 (3):423-6. 3) Srinivas SD, Pandian RS, Dwarakanath SK. Biting behaviour of Armigeres subalbatus (Coquillett) with reference to host selection and landing. Indian J Exp Biol 1994 May;32(5):348-50. 4) Warnes ML, Finlayson LH. Effect of host behaviour on host preference in Stomoxys calcitrans. Med Vet Entomol 1987 Jan;1(1):53-7. 5) Braverman Y. Preferred landing sites of Culicoides species (Diptera: Ceratopogonidae) on a horse in Israel and its relevance to summer seasonal recurrent dermatitis (sweet itch). Equine Vet J 1988 Nov;20(6):426-9.