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.
In some species of harvestmen it is easy to distinguish females and males, while in other species they look more similar. But are there also differences in behaviour? One would expect some, such as that males are more involved in territorial behaviour, or that the females, who have larger bodies and need energy for the production of eggs, spend more time eating. Indeed this is confirmed by some studies.
In studies on both Neotropical harvestman Ilhaia cuspidata and the temperate species Phalangium opilio, the females spend more time feeding while the males were busier with ‘social interactions’: inspecting and attacking other males1/2. In another Neotropical species, Neosadocus maximus, females were resting more, while males spend more time exploring their surroundings, and, perhaps as a result of all that exploring, were busier cleaning themselves3.
Neosadocus maximus is an impressive-looking harvestman by the way, very different from all species that can be found in my regions. It is very large (male body length around 10 mm) and it is even reported to eat small frogs!4 Unfortunately I haven’t seen one myself; the two pictures in this post are made by João P. Burini (see his marvelous photo stream here).
There are indications that males and females also select different locations to rest during the day. In a study where a 100 m long brick wall in the Netherlands was examined on a weekly basis, the males of Opilio canestrinii on average sat higher up the wall than the females5. The author thinks that this distribution develops when males fight with each other for females during the night, thereby trying to get higher up the wall to be in a good position to attack other males from above. In this way, stronger males end up high on the wall and weaker ones on the bottom part. Females would then select the ‘higher’ males for mating, after which they leave the arena to rest on the lower parts of the wall. At dawn the males also go resting, staying at their higher positions.
No such micro-spatial pattern could be found however in a study with Phalangium opilioon soy bean plants: both males and females sat mainly on the bottom and middle parts of the plants, with only few visits to the top parts3. At dusk, all P. opilio became active and moved to the ground to forage and mate. Around 4 hours later, larger nymphs and females returned to the plants, while males did not return for another 3 hours, mainly spending the last hours walking.
1. Behavioral Repertory of the Neotropical Harvestman Ilhaia cuspidata (Opiliones, Gonyleptidae). W. Pereira, A. Elpino-Campos, K. Del-Claro and G. MacHado. 2004. Journal of Arachnology, 32 (1):22-30
2. Diel activity patterns and microspatial distribution of the harvestman Phalangium opilio (Opiliones, Phalangiidae) in soybeans. C.M. Allard and K.V. Yeargan. 2005. Journal of Arachnology, 33(3): 745-752.
3. Activity pattern of the Neotropical harvestman Neosadocus maximus (Opiliones, Gonyleptidae): sexual and temporal variations. F. Osses, T.M. Nazareth, G. Machado. Source: Journal of Arachnology, 36(3):518-526. 2008.
4. Harvestmen (Opiliones: Gonyleptidae) predating on treefrogs (Anura: Hylidae)
L.M. Castanho & R. Pinto da Rocha. Link to the article.
5. Hooiwagens op een Nijmeegse muur (Arachnida, Opiliones), II. H. Wijnhoven. Nieuwsbrief Spined 28: 32-35. Link to the article.
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: The 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)
L. 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.
Even 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:
Even 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:
Another female from the front, showing the pedipalps and raised tubercle (eye hill):
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.