‘Commensal’ is not the name of any one type of spider. Instead it’s a term to describe the fact that it gets its food from other spiders. These spiders are scavengers.
The spider in the main photo above was crawling across the edges of a large orb web. The thing that made me notice it was that it looked mighty “uncomfortable”, like it didn’t belong there. For starters, it appeared to be working hard not to get stuck in the web. Also the orb web was way too big for that little guy. It was fumbling along, picking at little trapped insects and remnants. You can see those food items in the photo.
It didn’t take long to find the true owner of the web. That’s it shown hunched up and hiding nearby in the inset photo.
Now I’m not saying that every web has commensal spiders helping themselves to the leftovers. However, the bigger orb weaving spiders are unlikely to be able to keep their webs free of the tiny insects that stuck — insects too small for the big spiders to eat. The commensal spiders therefore perform a kind of service by keeping the webs clean, while filling themselves with plenty of free feeds along the way. I love it how things rarely get wasted in nature.
Do things always go so easily for the owner of the web? Nope. Some spiders that hang around other spiders’ webs do so because they like to eat the other spider, not its leftovers. Life can be tough for spiders.
One of the tiny spiders feeding on the food scraps too small for the Golden Orb Weaver
Here’s another type of commensal spider. It was very small — its leg span would have been about 2 cm. I spotted this fellow living in the web of a giant Golden Orb Weaver spider in tropical northern Queensland. To compare sizes, the Golden Orb Weaver in the centre of the web had a leg span of about 15 cm. I counted 18 tiny commensal spiders on that one web, feasting on the prey that was too small for the Golden Orb Weaver. They moved slowly and carefully and the Golden Orb Weaver didn’t show the slightest interest in them.
Some Quicksilver Spiders (I’m pointing to them with the arrows) are dwarfed by the Golden Orb Weaver Spider. That’s one of the Quicksilver Spiders shown in more detail in the inset.
Another spider known to inhabit the web of Golden Orb Weaver spiders is the Quicksilver Spider. Resembling droplets of glistening molten metal or big drops of dew (they are sometimes called the Dewdrop Spider) these little guys make a feast of the tiny stuff that might otherwise go wasted and foul the web of the larger spiders.