Anyone know what these are? I found them in our local park today attached to a wooden earth-retaining wall.I was thinking some kind of insect nest/egg sac (spider?) They're about the size of a medium egg (hen's egg!)
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3$\begingroup$ Welcome to Biology Stack Exchange. It would help if this question had a few more details: where was this photo taken (country / state is probably enough)? What season was it taken in? What was the weather like? What's the vegetation like in the local area? $\endgroup$– bshaneCommented Jul 21, 2016 at 8:53
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$\begingroup$ A Google Image search for "wasp nest" returns a lot of similar looking photos. $\endgroup$– A LCommented Jul 21, 2016 at 20:38
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$\begingroup$ It looks like you've opened it up, did you see anything inside it? $\endgroup$– C_Z_Commented Jul 22, 2016 at 16:35
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1$\begingroup$ @C_Z_ Wasp nests and some similar things already have openings on the bottom or side. Would be safer to keep a distance and watch if anything goes in or out. Or maybe poke it with a long stick and be prepared to run away. $\endgroup$– A LCommented Jul 22, 2016 at 16:54
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$\begingroup$ @AL Good point, I was thinking it was probably an egg sac but if there's a possibility that it's a wasp nest you definitely don't want to go poking around in there. $\endgroup$– C_Z_Commented Jul 22, 2016 at 17:37
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1 Answer
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Updated Answer:
Why it's not the egg sac of Argiope aurantia:
Upon knowing your location to UK, we can rule out quite a few spiders, especially the Argiope aurantia since it is endemic to Northern and Central America. The spider's egg sacs are also noticeably smaller than "a hen's egg" as this post describes.
Why it's not a gall:
We can rule this out since it appears to be located on a wooden fence post. Galls require living plant matter to grow.
Why it's not a wasp nest:
There are not wasps that produce threadlike materials. Wasp nests are made from fibers of dead wood and plant stems mixed with saliva of the wasps, and no webbing or threadlike material. In the picture, we can clearly see some white 'webbing' between the gap of the fence posts directly underneath the mass.
So I really think it is an...
Aethalium of a slime mold:
Given the location, described area, spreading pattern in the picture, and apparent spongy texture of the mass, this looks to be the aethalium (or fruiting body) of a slime mold. My guess would be in the genus Reticularia or Enteridium, which are typically seen in its reproductive phase as a white 'swelling' on standing dead trees in the spring, or on large pieces of fallen wood. For example, Enteridium lycoperdon are common throughout the UK and are commonly associated with pine trees, the most common wood used to make fences in the UK.
*This post was edited from my original response. Please refer to the edit history concerning the previous answer.
answered Jul 21, 2016 at 20:21
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$\begingroup$ There is no web in the photo in the question though, wouldn't that rule out the spider? $\endgroup$– A LCommented Jul 21, 2016 at 20:39
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$\begingroup$ There appears to be some webbing directly below the mass (in between the fence posts). $\endgroup$ Commented Jul 21, 2016 at 20:46
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1$\begingroup$ To me that looks more like the building material of the best than it does a spider web. $\endgroup$– A LCommented Jul 21, 2016 at 20:47
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$\begingroup$ There are not wasps that produce threadlike materials. Wasp nests are made from fibers of dead wood and plant stems mixed with saliva of the wasps, and no threadlike material. My vote still stands as a spider egg sac or a fungi of some sort, such as a puffball. $\endgroup$ Commented Jul 22, 2016 at 13:49
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1$\begingroup$ I agree. This looks lika a slime mold to me $\endgroup$ Commented Aug 2, 2016 at 0:15