Leiobunum blackwalli

Sometimes I see a specimen of Leiobunum blackwalli in my garden. The females of this species have a dark saddle that widens towards the back part of the abdomen and then terminates abruptly: Leiobunum blackwalli femaleLeiobunum blackwalli femaleLeiobunum blackwalli femaleThe males do not have such a pronounced saddle, but are easily distinguished from L. rotundum -which they resemble very much- by the pale ring around each eye where L. rotundum has an entirely dark/black eye hill. (Click to enlarge pictures)

Leiobunum blackwalli maleLeiobunum blackwalli maleL. blackwalli is often found in deciduous forest with dense undergrowth, along forest edges and waterways, in open fields and along the coast. In cities and villages L. blackwalli is much less common than L. rotundum, but not rare either. It is thought that L. blackwalli has decreased in numbers after the invasion of Opilio canestrinii (see last post), but unfortunately its distribution is not monitored well enough to know for sure.

Opilio canestrinii

Opilio canestrinii on treeEven though I have just recently become interested of Opiliones, there is one species I am already a bit tired of: Opilio canestrinii. In itself it is quite a pretty species to look at, but the thing is that you see it literally e-ve-ry-where. One reason is that it is very conspicious: large-bodied and often sitting openly on vertical structures such as trees and walls. The other reason is that it simply is very abundant.

Opilio canestriniiis believed to originate in Italy, but has colonized northwestern Europe with amazing speed over the past decades. In the Netherlands the first specimen was found in 19911 and it already is one of the most abundant species. In all countries where it appears this goes at the cost of other species, that apparently loose the competition and disappear. In the Netherlands as well as in the UK and Germany Opilio parietinus has seriously declined after the establishment of O. canestrinii. The last sighting of this species in the Netherlands was in 2006 and it was believed to be extinct, but in 2012 another one was found. In northern Germany Leiobunum tisciae has also disappeared after O. canestrinii set foot, and L. rotundum and L. blackwalliare probably negatively affected as well, although this has not been proven yet2.

Opilio canestrinii is easily recognizable in the field by it’s large (4-8 mm) orange to greyish-brown body with two rows of lighter-coloured spots on the back and darker-coloured legs:

Opilio canestrinii maleEven though they are similarly coloured, the difference between males and females is quite easy to see in this species. The one on the picture above is a male: it has a relatively small body that is a bit truncated at the back. Due to the body being so small, the trochanters (the light orange ‘bulbs’ between the black legs and the body) seem very prominent:

Opilio canestrinii maleOpilio canestrinii maleThe females on the other hand have much larger bodies with a rounded shape. Especially when they are about to lay eggs the body is very large and swollen.

Opilio canestrinii female

Another female from the front, showing the pedipalps and raised tubercle (eye hill):

Opilio canestrinii female frontThe last one has a more greyish colour, but also is an Opilio canestrinii female:

Opilio canestrinii femaleSee how she’s staring into the camera, inspectingly tapping it with her second pair of legs? More about the sensory legs of harvestmen in a next post!

1. R. van der Weele (1993) Opilio canestrinii nieuw voor de Nederlandse fauna (Opilionida: Phalangiidae), Entomologische Berichten 53: 91
2. H. Wijnhoven (2010) Hooiwagens op een Nijmeegse muur (Arachnida, Opiliones), II. Nieuwsbrief Spined 28: 32-35. Link to the article.