Mite Histiostoma palustre (Histiostomatidae, Astigmata), a mite species with two different male types

Current state of knowledge

Habitat and summary of general biology

I discovered the mite Histiostoma palustre in 2000 (and reared it at least until 2001) in the Berlin gravelpit area „Im Jagen 86“ at about 52°29’18.7″N 13°14’28.2″E and published the description in 2002 (2003). It is phoretically associated with at least two Hydrophilidae beetle species in bank mud (sapropel) around (in times of the sampling several) ponds. The phoretic juvenile instar deutonymph attaches its host insect to be carried that way to a new suitable habitat.

SEM photograph of ventral side of the deutonymph of Histiostoma palustre. © Stefan F. Wirth

SEM photograph of ventral side of Histiostoma palustre deutonymph as negative version, emphasizing single sicking structures and shapes of hind legs, © Stefan F Wirth

Phoretic hosts and attachment site

I in those times determined two host beetles as belonging to the genera Coelostoma and Cercyon. In laboratory host choice tests, in which deutonymphs in a culture had the choice between three different potential host species, there was a significant preference for those Hydrophilidae beetles, while another offered beetle species sympatrically living the same habitat was almost not at all attractive for the phoresy instar. But the choice test was performed as a smaller side project only and offered not more than a host tendency, as for example not a representative variety of insect species from the same natural field habitat of the mite was involved in these choice tests. But indications were at least clear enough that a host specificy (on a higher taxonomic level) of H. palustre can be stated. Mites preferred according to these tests a specific area for attachment on their hosts, which was the dorsal head area.

Two different male types and the difference between polymorphism und polyphenism

The mite showed under laboratory conditions a male diphenism with highly modified gripping organs or normal legs2. While the modified leg can appear on both sides or only on the left or the right side.

„Polymorphism“ is used internationally as the overriding term for such a phenomenon. Here, following the teaching of my former research group at the FU Berlin, I distinguished between a purely genetically determined expression of distinctly deviating morphs and the determination of these morphs by the presence of certain environmental factors. In the latter case we are talking about polyphenism. Of course, genetics also play a certain role here additionally.

Studies with cultures grown at different temperatures suggested that males with modified legs 2 appeared significantly more frequently at higher temperatures than at lower temperatures.

Temperatures: 30-32 °C (measured in summer inside the culture dishes at room temperature) and 20°C (climate cabinet, temperature inside and outside the cultures almost equal). Experimental approaches and total numbers of males counted: 10 cultures with lower (don’t remember exactly) numbers of developing mite stages per each of the two different temperatures. n=237 males in total at low temperature, n=729 males in total at high temperature. The number of modified males grew significantly from 18% (20°C) to 45% (30°C), while normal males decreased significantly from 82% to 55%.

Therefore I called the mechanism for the formation of two morphs diphenism. These studies were also a besides project and the amount of data would need to be even bigger according to higher level statistical requirements.

SEM photos of a male morph with modified legs 2, © Stefan F Wirth

Detailed SEM view to a male with legs 2 on both sides modified into gripping organs, perspective almost from the front side. © Stefan F Wirth

About the function of the modified male legs

I could observe in the mite cultures in which H. palustre thrived very well on decomposing potato pieces at about 20°C room temperature that the modified legs of the heteromorphic males used as prehensile organs played a role in intraspecific competition for female mating partners. Here several males, with and without gripping organs, were usually wedged together in veritable clusters around female tritonymphs or adult females. This was because the fighting happened more or less in a slow motion speed with apparent longer lulls in between. During the fights, the legs of the opponents were seized with the prehensile organ and the competitor was slowly pulled away from the coveted female.

Less is known about which selective pressure would favor the morph with gripping organ

Males with normally developed legs were at least seemingly in my observations clearly at a disadvantage here, since their second legs in particular could be easily grasped and they were hardly capable of defense and counterattacks. Nevertheless, random counts at that time did not provide any clear indication that heteromorphic males were possibly found significantly more frequently in the final mating position. The same applies to expectations regarding an unequal sex ratio in favor of males. Instead, males and females appeared to be almost evenly distributed in random evaluations. Additionally males with modified legs on both sides and with only one modified leg on the right and the left were in randomly counted cultures seemingly equally distributed.

Still many questions unanswered about the biology of H. palustre

It was unfortunately not tested at all, whether females mated by heteromorphic males and those mated by homomorphic ones had different reproduction successes. In connection with this biologically highly interesting mite species, many questions about its life strategies could not be examined deep enough during times of my PhD thesis. There was no time for this, since my research at the time was geared towards systematic comparative studies of numerous species. Unfortunately, the species could so far not be found again in its habitat „Im Jagen 86“ – at least not by the means of substrate samplings with only accidental Hydrophilidae inside – until today so that further studies were not yet carried out. This seeming lack of the species‚ presence in its former habitat is probably due to ecological changes in this area, which originally had several ponds, from which now only one remained.

A normal leg 2 of H. palustre compared with a modified leg indicating homolog leg articles and corresponding setation.

What makes this male diphenism interesting from the evolutionary point of view

Two distinct morphs within the same sex of a species is the result of evolution. It does at first not matter, whether it is a dimorphism or a diphenism. A fundamental question, which cannot be answered at this time, is: does the trait of these two morphs indicate that the species is on the way to finally having exclusively only males with prehensile organs, and is the species even on the verge of becoming the to cross the species boundary into a new species? Or is the characteristic of two male types a permanently stabled character, because exactly this has strong selection advantage?

In this context, fossil finds of closely related species (in amber) would be of interest, but unfortunately these are not yet known. It would also be interesting to compare different populations of this species, which are also not yet available. It furthermore would be of interest to understand, what exactly the advantage of modified males versus non modified males might be. Is it possible that an advantage of the modified-legged males is simply that they copulate more frequently, although smaller datasets have not yet confirmed this? And/or do females mated by modified males produce significantly more offspring?

And in the special context of the knowledge at that time that it is a temperature-dependent diphenism, one could raise the question of whether the species „evolutionally prepared itself“ by developing two male forms to cope better with temperature fluctuations. In view of increasing global warming, this may be of general biological interest.

A similar species from Egypt

A publication about a species with modified legs 2 as in H. palustre

In a conference proceedings contribution Bishlawy, S. E. M. O., and S. F. M. Allam published an article containing the description of a species of Histiostomatidae, which the authors have named Histiostoma egypti ( Proc.2nd Inter.Conf.Ent.Soc.Egypt, vol. 1, pp. 407-420, 2007). This is worth of being mentioned here more detailed in context of a possible close relationship of these species or a convergent evolution. But due to the lack of systematic facts, a decision hypothesis is not yet made. I can just emphasize that one showed me photos of that species on a conference in Cairo (Egypt) in 2003, and my first and superficial interpretation was that it even is Histiostoma palustre.

I have not checked whether the species Histiostoma egypti has been accepted internationally as a new species and is therefore also listed in the Zoological Record, as this is not relevant to the considerations here. It is interesting that a similar species was discovered in Egypt, about which there are some biological observations.

However, I only have a text version without illustrations, which is online published like that and which is why I cannot assess this species systematically/taxonomically on a deeper level. The authors refer to a similarity of their species with Histiostoma palustre including a similar diphenism of the males. I, as I already mentioned above, remember confirming this resemblance at an acarological conference in Cairo in 2003, where the co-author showed me photos of the later H. egypti.

Association with laboratory cultures of entomopathogenic nematodes

According to the authors, the species entered the laboratory with entomopathogenic nematode substrate and is doing well in the nematode cultures. The mite species is called semiaquatic, which quickly dies off at lower humidity. According to the authors, the thriving of the mites is to the detriment of the nematodes, which can die as a result. In this context, the authors mention having observed that the heteromorphic male form appears more frequently when live nematodes are present, while the homomorphic form and also the phoretic deutonymphs predominate when previously present nematodes have died.

My general experiences with histiostomatid mites and free living (phoretic) nematodes

A connection between the development of different mite morphs and the presence or absence of living nematodes never became particularly obvious to me in my studies at the time on species with different male types. Therefore, I have never specifically investigated such a connection and can therefore judge the correctness of such a connection as neither improbable nor very probable.

I can confirm, however, that basically all Histiostomatidae species that I examined always appeared together with nematodes, which were usually free-living, microorganism-eating representatives of the Rhabditidae or Diplogastrinae, which are also phoretic and possibly even were spread by the same hosts as the mites.

In this context, I am aware of the phenomenon that nematodes and mites can either compete with each other for food or reduce each other, for example chemically. In any case, it can often be observed that after some time in a culture vessel in which both animal groups were initially numerous, only the mites or the nematodes thrived in large numbers.

General information about the feeding behavior in Histiostomatidae

According to my recent research, based on frame-by-frame videographic analysis and SEM studies, Histiostomatidae species use their filter-feeding mouthparts to feed on decomposing fungal material that is also bacteria-rich. It is highly probable that the fungi are brought by the mites themselves into their habitat, which seems to happen hyperphoretically via fungal spores. The mites probably also control the growth and partial death of the fungus themselves in the form of their fungicidal gland secretions. Specific studies about the feeding behavior of H. palustre do not exist on a more advanced level. Slides and numerous SEM shots might indicate the transport of spores, but I so far had no time to examine those specimen closer. Such results thus need to be presented at another occasion.

Information about nomenclature used in the 2003 publication and about the citation of this species description

Nomenclature for dorsal/ventral setation of juveniles and adults used in this old species description of mite H. palustre was my own invention and is no longer used by me for practical/systematic reasons of comparison.

The nomenclature I introduced at the time of that species description for the pattern of individual fields that cover the thickening of the cuticle (proterosoma shield) on the anterior dorsal side of the proterosoma is still retained by me to this day. I still consider this characteristic in adults to be systematically relevant.

Cuticular shield of H. palustre stabilizes muscle origins at the dorsal proterosoma, close to the mouthparts, © Stefan F. Wirth

The original species description is cited as a 2001 publication on the Acarologia abstract page and is cited as 2002 publication in the PDF of the article provided by Acarologia as open access publication. In fact it was accepted for publication in 2001 and finally published in 2003. Thus I sometimes cited it as 2003 publication, but 2002 would be a correcter citation too. At least the full species name must be Histiostoma palustre Wirth, 2002.

Link to the original species description:

https://www1.montpellier.inra.fr/CBGP/acarologia/article.php?id=1849

© Stefan F. Wirth 2023, Berlin

Short story: The human nature – A Fairy Tale?

A father lived deep in the woods in a small house with his two growing children, a daughter and a son. The mother did not love the woods and had therefore left her husband and children many years ago.

The father taught his children to be one with nature. And so the son knew all the birds that lived in the forest, not only the many species, but also each individual, which he could always recognize by the sound of its personal melody. The daughter loved the trees and she too knew all kinds and every single tree.

The father had once moved his family from the big city to the woods because he believed that it was the big cities that robbed people of their minds and natural instincts, thus inciting destruction and violence.

In seclusion, he wanted to allow himself and his family to become part of nature again, because he hoped that it would be his descendants who could survive the fate of the world. It quickly became too much for the woman. She missed the narrow streets, the crowded markets, the idleness and the noise of the busy city. And so she fled one dark night and never came back.

The man taught the children how to make fire with stones, and which berries, fruits, and mushrooms are edible. And for a while the father lived with the son and daughter in happy and calm harmony. Only the wind occasionally swept through the green treetops and became the accompanying music in the symphony of birdsongs and the whispering, greening and howling of the wild animals.

But the older the children got, the more they bothered about the seclusion in which they lived. Increasingly, they no longer saw the wild nature around them as an asset, but as a constant threat.

The birds shit on their heads, ate the berry bushes and fruit trees bare. At night the ominous cry of the owls robbed them of their sleep. And the trees, bushes and weeds grew closer and closer to the family’s small wooden house with the crooked chimney.

And it seemed to the children as if the forest wanted to slowly and cruelly suffocate them and their house. And so, estranged from the father, both son and daughter conspired, met secretly in the forest clearing near the river and discussed that the father was probably not in his right mind to just let nature happen. And they realized that only those who know how to subjugate nature would survive.

And when the father went out to get food and firewood, the children began to tame, to control, to clean up wild nature. The son showed the daughter how to make a bow out of wood fibers and thin branches. The daughter explained to the brother which flints could be used most efficiently to quickly start a fire.

And every day, punctually when father left the house, they would meet in the forest clearing and together they would plan to shoot as many birds as possible with the bow. And it was not long before the magnificent symphony of the birds over the great, wide forest fell silent forever.

And brother and sister worked hard to carry the many small dead bodies to the forest clearing by the river, where in the late afternoon, before their father returned, they kindled a huge campfire in which they burned the dead birds of the forest in droves. And they couldn’t resist the smell of the simmering, tender meat, so they ate a feast that prevented them developing any appetite later when their father was preparing supper.

But he wasn’t surprised because he firmly believed that modesty always comes when people have found their way back to their original nature.

When there were no more birds in the whole forest, brother and sister hunted mice and rabbits and everything that was small and rustled or squeaked and frightened the siblings. Here, too, they ate and became fuller and fuller every day. But the father, exhausted from his long excursions and almost blind to everything that contradicted his ideals, continued to ignore the changes in his children and the forest.

And now it was only the increased hunger that moved the siblings to kill all the deer and pigs. And at the feast in the clearing by the river, they filled their bellies almost to the point of bursting. But the father, who was getting older and more tired, still didn’t notice any of this. And now the weasels, foxes, and wolves died of their own accord, so that the brother and sister laid aside their bows, because they only had to collect the dead animals.

There were so many that the siblings slept into the afternoon for many weeks and then indulged in gluttony while the father progressively lost his sight and noticed nothing.

Like pigs, brother and sister had become so fat that they had great difficulty making their way through dense undergrowth and over gnarled tree roots to the clearing by the river. And so they decided to clean up the forest and once and for all to remove all vegetation between the house and the forest clearing.

To this end, both set fire to different places. But they completely underestimated the destructive power of the flames. At first only individual trees burned, but then the flames combined and became a raging and violent conflagration, which first completely burned down the house with unspeakable heat and breakneck speed and then took hold of the entire forest. Brother and sister had no choice but to throw themselves into the river and stay there, mostly completely submerged, for two days until the fire died out, until the forest was completely burned down.

And when they emerged from the river, there was only a soot-blackened wasteland with gnarled skeletons crouching on the ground, the sad remnants of what was once the forest. A huge cloud of soot and water vapor covered the sky and the sun, everything was gray and dark and the smell of death and ash filled the air.

And the father? He was on his way back, with bundles of firewood hauling in a cart and bags full of fruit slung over his shoulder, when he saw the blaze racing toward him. He parked the cart and put his bags down. And in that moment he finally realized what had happened. He would never see his children growing adult, other people would never follow the example of his family and found a new dynasty of purer, nature-loving people with his children.

And just before the firestorm reached his body, shattering his head and evaporating his brain, just before his body thereafter completely crumbled to ashes, tears welled up in his eyes and he exclaimed in a hoarse voice, „So this isn’t a brain spectre, it’s indeed human nature, destruction and killing!“

Oh, if only brother and sister had known the way back to the city. But that was far away, and the path was completely burned and turned into a wasteland. The siblings had little strength left to anticipate and mourn the death of their father.

They laboriously built a small, shabby hut out of the burnt ruins of the house, in which they lived together in a very small space from then on. It wasn’t long before the brother knew every moss and lichen, while the sister knew every stone and every dry waterhole in this forest desert. And so they ate mosses and lichens, which they crushed to pulp with all kinds of suitable stones. And they drank the water from the shrunken river, which was more like thick, foul-smelling slime.

Both, brother and sister, grew thin as spindles, and days turned into months and months into years. Since we are in the year 2085 and cold winters have long since ceased to exist even far from the equator, the former forest slowly turned into a real desert of sand and stone. The rare rain filled the riverbed just enough for brother and sister to drink. Mosses had become rare. And so the siblings were eating lichens and the putrid bank mud of the river, when a dispute arose among the siblings over the privilege of eating.

So they divided the one shabby hut into two shabby huts, which they built along the river bank at a suitable distance from each other, so as to remain close to their feeding grounds but as far away from each other as possible. But occasional quarrels were not absent. Ultimately, the initial quarrel turned into deep anger, and then abysmal hatred. The brother, now a man, began throwing heavy stones at the sister whenever she tried to approach the succulent heap of putrid riverweed and filthy mud he had first spotted.

But the sun shone relentlessly and hot winds sanded the desert landscape more and more. The bed of the river shrank, and the huts of the quarreling scrawny siblings inevitably drew nearer and nearer to one another. In the end, the brother saw no other way out than to burn down the sister’s hut, whereupon she grabbed an old, rusty and long nail and drove it right into the brother’s skull.

She buried her dead brother where the forest clearing had once been, and yes, she shed a tear in the process. And more tears followed, day after day and month after month. After another year, the sister died, not of hunger, nor of thirst or a force of nature, but of loneliness. How can you go on living when there isn’t even someone to hate, she thought just before closing her eyes forever. Her body crumbled to dust that the wind carried up into the air. And the dust became one with the ashes of the forest and those of the father, and finally fell down on the brother’s grave. In the end there was nothing left but the desert.

© all copyrights (text, idea, drawings) by Stefan F Wirth, Berlin, 3 January 2023

New Year 2023, a further step into our future

It is important to follow the progress of time, to constantly develop further and to remain curious and emotional. At least, that's my idea of basic happiness in life.

Basically, there is no need for a specific point in time like New Year's Eve to decide on a bigger step forward, on new ideas, new projects and the processing of all the older ones, while also leaving enough space for an own experiencing of humanity, empathy and commitment.

But the beginning of the year, with its solemn tradition and thanks to the blaring staccato and howl of the firecrackers and bangers, almost forcefully calls us to wonder what we will experience in the new year. I am happy to indulge in this kind of thoughtfulness that is somehow predetermined. It seems to help  to additionally making use of this specific moment, once a year at the same time, in winter, at night, in the dark. A repeating and somehow well known environment offers more space for free and productive new thoughts.
 
Life is not a passive current, it can be shaped. I never want to stop learning, discovering, getting excited by the phenomena of nature, the world, the idea of a bright future and being inspired in my work. I see the future as an experience that I can only successfully go through based on what I learned from each of all the many days of my so far life. 

In my sense, I can only experience the future through steady personal progress based on the progress of my past. Also in the new year I have the need to recognize, to feel, to participate, to discover, to observe, to experiment and to communicate. I don't want to get tired, my strength will continue to flow out of myself to a large extent. I will be awake, want to be productive,  want to be there as the future gradually becomes the present in 2023. And I will and want to help shape this emerging present like a single powerful ant worker in its ant nest with my strength, with my individual contributions. 

I wish everyone a prosperous and peaceful New Year 2023.

© Stefan F Wirth, 1 January 2023