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.
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.
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!
Isn’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.
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.
P. 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.
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!
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.
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.
This month a giant harvestman has been found in a cave in Laos by Dr. Peter Jäger of the Senckenberg institute. They have not yet been able to identify it to the species level, but it probably is a member of the genus Gagrella in the Sclerosomatidae family.
While the body of this harvestman is not even 1 cm in length, the legs have a span of 33 cm. That means there is only one harvestman ever found that is larger: a species from Brazil with a span of 34 cm. (Unfortunately I can’t find out which species. Do you know? Please leave me a message!)
The interesting thing is that also other invertebrates reach giant dimensions in those caves in Laos, for example the Laotian huntsman spider Heteropoda maxima (span 30 cm), the whip scorpion Typopeltis magnificus (span 26 cm) and the predatory centipede Thereuopoda longicornis (40 cm). The reason why ‘giantism’ has developed there is still unclear. It is expected however that the invertebrates cannot become much larger than this. Either the oxygen supply would become a problem, or, in the case of harvestmen, the legs would become too long for fast movement.