Ancient harvestman had four eyes

A study of a 305 milion year old fossil harvestman from France has revealed that this specimen had four eyes. This is rather special, because all currently living harvestmen, as well as other fossilised harvestmen that have been studied, only have two eyes. The finding is important for understanding the evolutionary development of the harvestmen and related groups, and for studying how morphological characteristics are acquired and lost during evolution.

Hastocularis_argus

3D scan of the extinct fossil harvestman Hastocularis argus. Screen shot taken from a video made by Garwood et al. 2014 (see text).

The researchers made two 3D images of the fossil harvestman Hastocularis argus using microCT-scans at the Natural History Museum in London (they placed a very nice video online in which you can see the harvestman from all sides, to see it click the link and download movie S1). Apart from one pair of eyes on the top of the head, that almost all present-day harvestmen have, this specimen also had a pair of eyes at the side of its head, just above the first pair of legs. Interestingly, modern-day harvestmen still have genes that code for these lateral eyes, but the extra eyes are lost during embryonic development.

There are not many findings of fossilised harvestmen, because their exoskeleton is relatively fragile and doesn’t preserve well. According to researcher Garwood, the fossil of H. argus is unique because was preserved in three dimensions. That enabled the researchers to examine the details of its morphology more closely than scientists have been able to do in the past. According to the researchers, H. argus is closely related to Eophalangium sheari, which is the oldest harvestman fossil ever found and believed to be one of the earliest terrestial animals.

R.J. Garwood, P.P. Sharma, J.A. Dunlop, G.Giribet (2014). A paleozoic stem group to mite harvestmen revealed through integration of phylogenetics and development. Current Biology.

Odiellus spinosus

Last week I spotted my first two harvestmen of this year, a Rilaena triangularis and a Nemastoma lugubre sitting under a piece of bark. Unfortunately I didn’t have my camera with me. But I just remembered some pictures of last year that I wanted to share, so I’ll post them instead. It’s a Odiellus spinosus female sitting on a grape plant in my garden.

Odiellus spinosus female Odiellus spinosus female Odiellus spinosus female Odiellus spinosus female Odiellus spinosus female

3 species in one

Mitopus morio is one of the most widespread species of harvestman in the world (see also my post about harvestmen on Iceland). Now researchers have figured out that this species is actually at least 3 different ones.

It was already known that Mitopus morio has both short- and long-legged individuals. After someone observed a short- and a long-legged individual refusing to mate with each other in the field, researchers decided to study whether these two forms were in reality different species. They collected hundreds of individuals from two areas in Austria and carried out mating trials, measured nineteen different body parts and analysed DNA from multiple areas of the genome.

Mitopus morio male

In the end, they found out that there were not two, but three different species. Interestingly, these turned out to be ‘cryptic species’: indistinguishable for the human eye. The length of the legs that the study started out with did not differ among the species, each one having both long-, intermediate- and short-legged forms. But the mating trials and genetic differences were in line with each other, pointing out the existence of three distinct groups.

Using morphology for identification appeared to be possible, but only when taking into account several morpholocgical characters at the same time. To make it even more complicated, the characters are quite difficult to measure and the sets of characters to be used are different for females and males. For females you can for example look at the length of tibia of pedipalpus, the number of medial bristles on the tarsus of the pedipalpus and the number of spines on tibia II (still hanging in there?).The combination of these three characters gives almost 100% certainty on the identity of a specimen. For males a good set of characters is the interocular distance, the length of femur of pedipalpus and the number of distal bristles on the tarsus of pedipalpus. Of course this is based on Austrian specimens only – it is unknown how many ‘Mitopus morio-species’ exist in other parts of the world, let alone how to identify them.

So, I guess that for us opiliophiles there is not much that will change, unless we take along a portable lab into the field from now on. Anyway it’s fascinating to think how many more species might exist that we just don’t recognize, don’t you think? To read the full story, take a look here. There’s a video of two specimens of Mitopus morio mating during the trials available too. The scientific publication can be found here.

W. Arthofer, H. rauch, B. Thaler-Knoflach, K. Moder, C. Muster, B.C. Schlick-Steiner, F.M. Steiner. How diverse is Mitopus morio? Integrative taxonomy detects cryptic species in a small-scale sample of a widespread harvestman. Molecular Ecology 22, p. 3850-3863.

Platybunus pinetorum

In the last post on Rilaena triangularis, I mentioned that there are only very few species of harvestmen that are adult around this time of year. One of them is Platynunus pinetorum, but this species is not very common in most northern European countries. Today I went looking for it in the forest, and hurray, I found one!
Platybunus pinetorumIsn’t it a nice one? It came falling out a tree when I shook the lowest branch, and landed in my white umbrella. Once put on the grass it was very patient letting me take pictures. (Luckily, because my simple camera that only has auto-focus really likes to focus on everything except the thing that I have in mind…)

P. pinetorum originally occured in central Europe, but over the past two decades it has moved northwards. In Belgium it was first recorded in 1992 and in the Netherlands in 1998. Also in Germany, where it already occurred, the species is becoming more and more abundant. Only a few years ago (2010) there was a first sighting in Sheffield in the UK.

Platybunus pinetorum

As already indicated by its name, this species is often found on coniferous trees, such as pine. This is the area where I was today, with nice low pine trees so that it’s easy to shake the branches.

Forest with platybunus pinetorumP. pinetorum is a very good-looking harvestman, if I may say so. It is rather large (5-8 mm) and has a very dark colour. The body of a male is completely black, with brown legs, while the female is dark brown with a saddle of black and white (so, the one pictured here is a female). They have a row of white spikes running along the underside of the palps, pointing forward when the palps are drawn in (visible on the first two pictures above). On the picture below a view of it’s cute white belly.

Platybunus pinetorum

Rilaena triangularis

Rilaena triangularis is a harvestman with a life cycle that is different from most other harvestmen in temperate regions. While most species pass the winter either as egg (mostly long-legged species) or as adult (mostly short-legged species that live close to the soil), Rilaena triangularis does this being a juvenile. This means that it can be found as a (sub)adult very early in the year: from late March onwards, when most other species are in the juvenile stage. This characteristic of R. triangularis has given it the common name of spring harvestman. So, have a look in your garden today, and if you see a fully grown harvestman you know who it is!

Rilaena triangularis The juveniles survive autumn and winter living in the upper soil layer, in leaf litter or under pieces of dead wood. Adults can be found higher up, on trees or walls, in bushes, or on grasses and herbs, and die already around the end of july. R. triangularis shares this biology with a few other species that are closely related within the genera of Platybunus and Megabunus. But these species look different and are less widespread.

The spring harvestman is best recognized by its unusually large ocularium (eye hill) with large black eyes and two rows of blunt spikes running on top. Males and female are both light to medium dark brown and often have a darker saddle with a pale outline. It is usually the females who have the most pronounced saddle, and they are a bit larger too (5-7 mm, males being around 3-5 mm). The places where the legs are attached to the body have a light greyish or beige colour.

Rilaena triangularis3

The spring harvestman is very widespread not only in Europe – with the exception of northern Scandinavia, the high Alps and the Mediterranean – but also in other parts of the world, including the USA.Rilaena triangularis

If I were…

If I were an opiliophile in Iceland… I would only encounter four species1. And of these four, only Mitopus morio is widely spread over the island, living at the seashore, in the mountains and on the many islands and islets. The other three species, Nemastoma bimaculatum, Oligolophus tridens and Megabunus diadema, have only been found in the southernmost part of Iceland. This can probably explained by a combination of their introduction (see last paragraph) and the climate. Winters are relatively mild in the southern part, with temperatures of 0°C on average. In the central highlands, the mean winter temperature is around -10°C and in the north even -25°C. Apparently M. morio can handle such low temperatures!

Figure copied from I. Agnarsson, 1998

Distrubution of Mitopus morio in Iceland. Figure copied from I. Agnarsson, 1998

Despite the narrow distribution range of N. bimaculatum, O. tridens and M. diadema, they can locally be fairly common. N. bimaculatum seems to select south-facing slopes, an indication that the temperature is a limiting factor for this species. M. diadema is most common near the sea, where it lives on moist cliffs. The North Atlantic Current brings warmth to this part of the world, making the climate of Iceland more temperate than would be expected so close to the Arctic circle.

Distribution of Megabunus diadema in Iceland

Distribution of Megabunus diadema in Iceland. Figure copied from I. Agnarsson, 1998.

Most likely, dispersal is the main reason why so few species of harvestmen occur in Iceland, compared to other areas in Europe with a similar climate. In a recent paper, I. Agnarsson explains that M. morio probably is the only species that has reached Iceland without the help of men. It has been present in Iceland for a long time (a report from 1772 mentions that it already was widespread at that time) and it also occurs on locations north of Iceland, including Greenland, where it is the only species of harvestmen. Nevertheless, I think that the possibility cannot be ruled out that this species was hitchhiking with earlier settlers, for example the Celtic monks that came to Iceland in the seventh or eighth century, or Norse settlers that arrived from the ninth century onwards. The recent discovery (1929) and limited but expanding distribution of the other three species points towards their dispersal by men.

1. This post was entirely based on the publication Íslenskar langfætlur og drekar by Ingi Agnarsson (1998). A complete checklist of all Nordic Opiliones was published by Ingvar Stol in the Norwegian Journal of Entomology (2007). Click the links for PDF’s.

Excursion materials

One of the nice things about harvestmen is that some species can be found year-round. So excursions can be undertaken even in the midst of winter! I have a special bag containing the things I need when I go look for harvestmen. This is the equipment:

Excursion materials

  1. Notebook and pen
  2. Plastic containers
    For examining or taking home specimens
  3. Photo camera
    I take a picture of every specimen I find, so that more experienced people can check the determination before they are entered into the database of the Netherlands. Luckily there are so few species living here that they mostly can be recognized from a picture.
  4. White umbrella
    A light-coloured umbrella is very handy to find specimens that sit in bushes or in the lower parts of trees. Open up the umbrella and place it upside down under the branches, then shake the braches or hit them a few times with a stick so that all invertebrates fall into the umbrella (like this).
  5. Handheld GPS for taking coordinates
  6. Spare batteries for the GPS
  7. 10x magnifier
  8. Determination guide

So, I’m off for today!