A master-class in bizarre biology: itchy, incestuous, parasitic mites with adult offspring
(this post is co-written by Wayne Knee, an extraordinary Acarologist!)
Arthropods never cease to amaze: from twisted-wing parasites to nectar-feeding spiders, arthropods do anything and everything imaginable. That's what makes them a joy to study. Being an entomologist or arachnologist means being continuously confronted with the bizarre and the unimaginable. Case in point: mites in the genus Pyemotes.
Here's the take-home message: Pyemotes mites are parasitic on many insects, some of which are themselves economically important. After hanging onto their hosts, feeding on their haemolymph, and being good little parasites, females become engorged and have significant abdominal swelling. The eggs hatch in the female ovaries and the immatures develop develop inside their mothers, hatching as adults. When they emerge from their mothers, males swarm around the area where emergence is occurring, waiting for adult females to come out, and when they do so, they mate (briefly) with many females. Although humans aren't hosts for these mites, Pyemotes can bite us and cause pretty serious itching and potential for secondary infections, and some species of these 'itch mites' are becoming invasive species.
In more detail...
These mites (in the Subclass Acari) are in the family Pyemotidae and there are about 24 species known. As a group, these mites are parasites of other arthropods, from bark beetles, to bees, termites, and many other hosts (Cross & Moser 1975, Vaivanijkul and Haramoto1969). Thus, some have argued they may be used as biocontrol agents for economically important pest such as bark beetles (e.g., Moser et al 2005). The mites hang on to their hosts, feed on their haemolymph (blood) and generally be parasitic and nasty. Some species are also phoretic (hitch a ride with another insect), some have dimorphic females, and some are venomous (probably the only mites that are venomous!).
Their reproduction biology is mind-blowing. Females, after mating, will lay eggs within their own body, and these eggs will hatch within their mother, with the immatures growing without additional nourishment from the mother; a process known as ovoviviparity. As the immature mites grow, and as feeding continues, the mother's abdomens start to expand in a manner that might be described as grotesque:
If that wasn't weird enough, the offspring from the mother don't "hatch" until they are adults. This is *extremely* unusual, and rare within the world of arthropods - we cannot think of another example of this life history strategy (but we'd love to hear from you!). Many arthropods are known to exhibit ovolarviparous behaviour (i.e., larvae emerge from the mother, rather than eggs, as can be see with Tsetse flies, aphids, some beetles, and others) but having adults hatch from their mother is remarkable and unique. Here's an exert from a life history description of Pyemotes boylei by Vaivanijkul and Haramoto in 1969, describing the whole process, through to copulation. It’s all rather incestuous:
P. boylei was found to reproduce ovoviviparously. The eggs hatched within the ovaries and the immature stages retained there until they reached sexual maturity. These eggs and immature stages occupied the bulk of the inside of the enormously enlarged opisthosoma of the gravid females. When ready for birth, the offspring emerged in succession, anterior end first, through the mother's genital opening. In the case of mated females, the first 2 to 4 offspring were males and subsequent ones all females, whereas in the case of unmated females, all of the offspring were males. Upon emergence, the males congregated around the mother's genital opening and waited for the birth of the female offspring. They obtained their nourishment from their mothers and seldom wandered afar. As soon as the females were ready to emerge, the males vigorously vibrated their front pair of legs up and down. As the females emerged, the males grasped them with their well-developed hind legs and copulated. The females were released after a brief mating period of 10 to 30 seconds. One of the males mated with as many as 8 females during a period of 3 minutes and most of them mated with as many as 100 females during their normal life span of about 25 days. Also, some males mated with females that have been mated previously by them or by other males. Despite the habit of P. boylei males standing watch around the mother's genital opening, some females escaped mating.
The unusual biology of Pyemotes mites has been known for a long time, although we don't find its strange biology to be widely known or appreciated. In 1853, for example, Newton described a new species of parasitic mite, Heteropus ventricosus (now known as Pyemotes ventricosus) and provides this outstanding drawing of the female body and associated stages.
The story doesn't end here, because some Pyemotes mites are also of significant medical importance for humans. The well-known 'straw itch mite' (P. tritici) or ‘hay itch mite’ (the aforementioned P. ventricosus, as described by Newton) is a serious nuisance, and when in high numbers (due, of course, to a high number of insect hosts in straw or other materials), can bite and cause significant skin irritations in humans, sometimes important enough for ‘mite shower’ warnings. Vaivanijkul and Haramoto (1969) describe the symptoms, and apparently these symptoms are common for many species of Pyemotes: "Pruritic skin lesion which is rosy red and about 0.5 cm in diameter develops on the site of each bite. Because of the intense itching, the lesion is often excoriated by scratching and thus allowing secondary infection to set in...". In fact, any literature search on Pyemotes is heavily biased towards effects on humans (e.g., see Del Giudice et al. 2008), and the fascinating biology of the species is largely ignored in more recent publications. Some Pyemotes are also relevant as invasive species, as the European species Pyemotes herfsi (the Oak Leaf Itch Mite, which preys upon gall-making flies) is now in North America (Broce et al. 2006).
Pyemotes mites provide a master-class in bizarre biology. You just can't make this stuff up.
This is not science fiction: this is life, on a small, arachnological scale.
Broce AB, Zurek L, Kalisch JA, Brown R, Keith DL, Gordon D, Goedeke J, Welbourn C, Moser J, Ochoa R, Azziz-Baumgartner E, Yip F, & Weber J (2006). Pyemotes herfsi (Acari: Pyemotidae), a mite new to North America as the cause of bite outbreaks. Journal of medical entomology, 43 (3), 610-3 PMID: 16739423
Del Giudice P, Blanc-Amrane V, Bahadoran P, Caumes E, Marty P, Lazar M, Boissy C, Desruelles F, Izri A, Ortonne JP, Counillon E, Chosidow O, & Delaunay P (2008). Pyemotes ventricosus dermatitis, southeastern France. Emerging infectious diseases, 14 (11), 1759-61 PMID: 18976564
Cross & Moser (1975). A New, Dimorphic Species of Pyemotes and a Key to Previously-Described Forms (Acarina: Tarsonemoidea). Annals of the Ent. Soc of America.
Moser et al. (2005). Phoretic mites and nematode associates of Scolytus multistriatus and Scolytus pygmaeus (Coleoptera : Scolytidae) in Austria Agriculture & Forest Entomology DOI: 10.1111/j.1461-9555.2005.00261.x
Newton, G (1853). Further Observations on the Zabits of Monodontomerus ; with some Account of Acarus (Heteropus ventricosus), a Parasite in the Nests of Anthophora retusa. Transactions of the Linnean Society of London
Vaivanijkul P, Haramoto FH. 1969. The biology of Pyemotes boylei Krczal (Acarina: Pyemotidae). Proc Hawaiian Entomol Soc 20:443-454.