Does a fly itch?

A couple of larvae from the Cohort Parasitengonina

A couple of larvae from the Cohort Parasitengonina

Trombiculoidea are mites with a dubious distinction – most languages have one or more common names for them.  In Australia we would tend to call the hexapod larval stage ‘scrub itch mites’ for the intensely annoying rash-like erruptions they leave after biting – an all too common result of a pleasant stroll through the bush.  In North America they are better known as chiggers, red mites, or harvest mites (i.e. common in the Fall).  I imagine readers of this blog with a knowledge of non-English languages could supply a host of other sobriquets.  Anyone with actual experience of scrub itch, or as it is technically known – trombiculosis,  might be tempted to add some colourful modifiers.

Because a large wheal may develop at the spot that a chigger bites, some think that they are burrowing in the skin.  Alas, no, they are ectoparasites that digest our skin and lymph for their feed.  There’s nothing to dig out to reduce the itch and the best thing you can do is not scratch and help avoid secondary infections.  The first scratch is probably good, since it will crush or dislodge the mite, but the mite leaves behind all the enzymes it has been injecting into your skin, and that can take a week or two for your body to deal with.  The more you scratch, the longer it will take to heal. 

If you live in an area where the rickettsial disease scrub typhus is endemic, then it is a good idea to watch out for any large black scabs that form on a chigger bite.  A large black scab or eschar is a sign of infection and all it takes is one infected bite.  In Queensland scrub typhus is a problem, but one good thing about Edmonton is that both chiggers and scrub typhus seem to be absent.

It may be of some comfort to know that only two families of chiggers feed on vertebrates – about a dozen other families of related mites feed on insects, millipedes, spiders, harvestmen, scorpions, and the like.  The two-winged flies (Diptera) seem to be especially lucky when it comes to collecting chiggers.  A possible example of one is above on the viewers right (Microtrombidiidae?).

Only the larval stage (i.e. the first active stage which has only 3 pairs of legs) is parasitic in the Cohort Parasitengonina (chiggers and their relatives and the water mites).  The nymphs and adults are predators – and they may have common names too.  In English, the terrestrial species are often called Red Velvet Mites and they can be quite large (up to 16 mm – the largest known mites outside the ticks) and are covered in a dense pelage of hairs, typically red but sometimes red & white patterned, orange, or another colour or combination.  Alex Wild at MyrmecosBlog has posted a striking picture of one of the red species.

A headless mite - adult Calyptostoma sp.

A headless mite - adult Calyptostoma sp.

I don’t have any good SEMs of a red velvet mite – they are as hairy as a mammal and one would go bonkers trying to mask around all the hairs.  However, I do have an adequate picture of a member of the Subcohort Erythraiae, a species of Calyptostoma (Calyptostomatidae) with short setae.  These rather sluggish mites are thought to be predators of fly larvae as nymphs and adults.  The mouthparts (capitulum or gnathosoma) are retracted into the body and can be shot out to impale a maggot.  The mite larvae are parasites of adult Diptera (BugGuide has some good pictures of craneflies infested with the larvae).  So, the next time you come home with a case of scrub itch, think about all those poor flies out there that couldn’t scratch even if they wanted to – and emulate them, don’t scratch.

9 Responses to “Does a fly itch?”

  1. Gunnar Says:

    Are the parasitic larval stages of water mites identifyable to species level?

  2. macromite Says:

    Hi Gunnar:

    Usually you need an adult mite of a particular sex for species identifications. But, one of the strange aspects of parasitengonine taxonomy is that the larvae often are better known than the adults, and it is the latter that may be harder to identify if they haven’t been associated with a larva.

    Water mites are especially well known, though, thanks to the work of people like Dave Cook and Ian Smith (you can check out their section on water mites in the 3rd edition of the Manual of Acarology – the only section where there are keys to families of both adults and larvae).

    Being able to identify something to species, of coarse, depends on someone having worked on the fauna that you are interested in, but water mites are probably the best known of all large (6,000 described species) mite taxa.

    Cheers,

    Dave

  3. Marvin Says:

    Chiggers might win a prize for having the most misinformation, myths and useless home remedies circulating about them.

    • macromite Says:

      Hi Marvin,
      Yes, chiggers do seem to attract misinformation. I’m amazed at the number of people who think they are burrowing in their skin or are afraid of bringing them back to infest their home – not very likely unless their home has suitable habitat and prey for the nymphal and adult stages (however, there is some data indicating that chiggers colonize home gardens in Europe via dispersal on birds and rodents). Also, there is a famous Australian blowhard with a specious story in one of his books about chiggers breeding on his scrotum in New Guinea. I used to use the story on Med-Vet Entomology exams. But to be completely honest, or thereabouts, I’ve passed on a dubious chigger story in the past.

      Many chiggers are host generalists (perhaps microhabitat specialists) that feed on a variety of vertebrates (lizards, birds, mammals) in a particular habitat – and other chiggers specialize on particular host types. In the epidemiology of scrub typhus (caused by the rickettsial bacterium Orientia tsutsugamushi), only a few of the mammal specialist are considered important reservoirs/vectors. There is a story that the bites of the mammal specialists are less itchy than those of a generalist chigger (in one version, the lizard chiggers have the itchiest bites). Sounds reasonable, but when I was challenged on it by a student and tried to find supporting data, I came up empty handed. Oh well, at least the student kept me from passing it on in a book chapter.

  4. Marvin Says:

    Well, heck! There’s another chigger myth I need to forget. I’ve encountered variations of the specialist/generalist difference and it’s relationship to itching many times — often in general interest publications that seemed well-written and accurate in other respects — so I’ve accepted it as fact. Usually the author is claiming the reason chigger bites itch so much is because humans are not the intended host.

    This statement from a wildlife biologist at the the site you want to link to is typical:

    … North American chiggers only bite humans by accident. Although our chiggers can feed on most animals, they are really looking for reptiles and birds, their preferred hosts. The itching reaction human skin has to chigger bites occurs because we are not their correct hosts. Chiggers that specifically prey on humans in Asia and Pacific Islands cause no itching!

    Causing less itching does seem as if it would be a survival mechanism when the host is any creature with the ability to scratch. Chiggers should look into evolving that adaptation. 🙂

    • macromite Says:

      “chiggers dine on us only in their childhood, and later become vegetarians that live on the soil”

      Hi Marvin – Yes, even well written and mostly informative articles like the one you link to can be riddled with misinformation. As far as is known, nymphal and adult chiggers and their relatives are predators: none are vegetarians.

      This critical paragraph is almost entirely anthropocentric and misleading (but sounds so reasonable):

      “It is of little comfort to learn that North American chiggers only bite humans by accident. Although our chiggers can feed on most animals, they are really looking for reptiles and birds, their preferred hosts. The itching reaction human skin has to chigger bites occurs because we are not their correct hosts. Chiggers that specifically prey on humans in Asia and Pacific Islands cause no itching!”

      Chiggers feed on us only because we are warm, emitting CO2, and perhaps, smell like dinner. Any accident is due to us walking through their habitat without an effective repellent. If there is any data supporting the itchiness hypothesis, anecdotes aside, I have not been able to find it. Actually, it is extremely difficult to find good recent data on what chiggers are actually biting people – a biting chigger is almost never identified (for an exception, see “A Fly for the Prosecution”, where Lee Goff – former chigger specialist turned forensic entomologist – tells a good story about how a chigger identification helped in a murder case). As far as I know there are no ‘[c]higgers that specifically prey on humans’ or require people in their life cycle (although habitat disturbance and encouraging rodent populations may benefit many chiggers).

      I guess one important point to remember about the itching story is that you can get scrub typhus from a single chigger bite (or at least with only a single eschar forming) – and one or two bites are not likely to be remembered a week or two later when an eschar or other sign or infection appears (especially if you had to deal with mosquito and midge bites at the same time). If the itching story were true, though, you’d think the bioprospectors would be on it. Anything that could inject that many foreign proteins and cause that much damage without inducing inflammation and itching would be worth investigating.

      PS – great blog you’ve got over at ‘Nature in the Ozarks’

  5. Marvin Says:

    Oops! The link above works but should say “Missouri Department of Conservation” instead of “the site you want to link to”.

  6. Adrian Thysse Says:

    …now I’m itchy…

  7. Linda Says:

    What an amazing site you have. I had chigger bites once…and it was enough to drive me mad (I recovered only slightly). Nice to know the story behind the myth. I used tobacco poultices, which did reduce the itching. However, I think they were best to keep me from scratching the itch. The bites, as you said, did stop itching after two weeks. That was a long two weeks.

    Beautiful photography. Stunning, riveting.

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