Questions about Interspecies Communal Spiders

[LinotheleLover]

Hello, There has been an influx of flying insects at my job and I saw something today I've never seen before. There was a Tetragnathidae sp. , a Pholcidae sp., and a Latrodectus sp. Sharing a web. They were all in very close proximity, and upon getting near the web, one of them ran toward the Pholcid and they batted legs for a bit before they both ran off on opposite directions of the web. My question is, are interspecies communal webs common? Why would the larger Latrodectus/pholcids not prey on the much smaller Tetragnathidae? They could easily overpower them yet they seem to tolerate each other well. Do these spiders see each other as one of their own? Very fascinating!!

 

[AphonopelmaTX]

It is very common for cellar spiders (family Pholcidae) to take up residence in and build upon the webs of cobweb spiders (family Theridiidae). Despite its appearance that there is just one web, they are most definitely not communal and have their own territories. I haven't heard of or seen this in the Tetragnathidae, but that doesn't mean it isn't just as common though. I see this every year from summer to fall in my own apartment where males of Pholcus phalangioides temporarily inhabit the webs of a small unidentified cobweb spider as they wander around searching for females. The females of Pholcus phalangioides are the ones that build upon the webs of the cobweb spiders and remain in one place as long as food is available. The reason for this has eluded me for years since the cellar spiders dwarf the cobweb spiders in size and seem to be more successful at capturing prey. There does not seem to be a need for the cobweb spiders' web since they do not overstep the boundaries of the others' territory. However, the male cellar spiders might be taking over the webs of cob web spiders in an effort to capture food as they wander around, but I haven't made a direct observation of this.

Even though the cellar spiders and cobweb spiders don't attack, kill, and eat each other, I can certainly tell they do eat other spiders. The spider I find most often in these conjoined webs are a species of spitting spider (Scytodes sp.) which are typically being fed on by the cellar spiders. I'm pretty sure the web building spiders just don't interpret each other as prey since they can navigate the cob webs and don't register as food. This is opposed to something getting caught in the bottom of the cob webs and triggering the predatory behavior.

 

[LinotheleLover]

It is very common for cellar spiders (family Pholcidae) to take up residence in and build upon the webs of cobweb spiders (family Theridiidae). Despite its appearance that there is just one web, they are most definitely not communal and have their own territories. I haven't heard of or seen this in the Tetragnathidae, but that doesn't mean it isn't just as common though. I see this every year from summer to fall in my own apartment where males of Pholcus phalangioides temporarily inhabit the webs of a small unidentified cobweb spider as they wander around searching for females. The females of Pholcus phalangioides are the ones that build upon the webs of the cobweb spiders and remain in one place as long as food is available. The reason for this has eluded me for years since the cellar spiders dwarf the cobweb spiders in size and seem to be more successful at capturing prey. There does not seem to be a need for the cobweb spiders' web since they do not overstep the boundaries of the others' territory. However, the male cellar spiders might be taking over the webs of cob web spiders in an effort to capture food as they wander around, but I haven't made a direct observation of this.

Even though the cellar spiders and cobweb spiders don't attack, kill, and eat each other, I can certainly tell they do eat other spiders. The spider I find most often in these conjoined webs are a species of spitting spider (Scytodes sp.) which are typically being fed on by the cellar spiders. I'm pretty sure the web building spiders just don't interpret each other as prey since they can navigate the cob webs and don't register as food. This is opposed to something getting caught in the bottom of the cob webs and triggering the predatory behavior.

Wow! Thanks for the info! I might add that where I work there are millions of small midge flies and food is extremely plentiful, maybe that plays a part as well? I didn't even think about the fact that spiders wouldn't register each other as prey because of their ability to navigate the webs!! So cool!! Its just super cool to see non related species tolerate each other within such close proximity.

 

[AphonopelmaTX]

Wow! Thanks for the info! I might add that where I work there are millions of small midge flies and food is extremely plentiful, maybe that plays a part as well? I didn't even think about the fact that spiders wouldn't register each other as prey because of their ability to navigate the webs!! So cool!! Its just super cool to see non related species tolerate each other within such close proximity.

I think you are correct in thinking the plentiful food supply is what is bringing the three different species of web building spiders together. An abundance of prey will most certainly attract all of the predators to the same area.

I actually have a cobweb spider and a female cellar spider living next to each other in conjoined webs right now in my dining area. Unfortunately, I can't get a picture of them because they are behind some furniture and I can't get any lighting to them without tearing their webs apart. I can clearly see that they are both eating regularly in their respective territories. Good for them, but bad for me because they are making a big mess on my floor. If past observations hold consistent, then as soon as winter arrives and the temperature in my apartment drops, both spiders will separate and go into hiding when the prey disappears.

EDIT

I just remembered I asked a similar question about two years ago to the American Arachnological Society's e-mail group after watching a cobweb spider and cellar spider with conjoined webs with each having eggs that hatched in close proximity. I didn't get much of a response except for something along the lines of "it happens," but I did get a photo of it.

 

[LinotheleLover]

I think you are correct in thinking the plentiful food supply is what is bringing the three different species of web building spiders together. An abundance of prey will most certainly attract all of the predators to the same area.

I actually have a cobweb spider and a female cellar spider living next to each other in conjoined webs right now in my dining area. Unfortunately, I can't get a picture of them because they are behind some furniture and I can't get any lighting to them without tearing their webs apart. I can clearly see that they are both eating regularly in their respective territories. Good for them, but bad for me because they are making a big mess on my floor. If past observations hold consistent, then as soon as winter arrives and the temperature in my apartment drops, both spiders will separate and go into hiding when the prey disappears.

EDIT

I just remembered I asked a similar question about two years ago to the American Arachnological Society's e-mail group after watching a cobweb spider and cellar spider with conjoined webs with each having eggs that hatched in close proximity. I didn't get much of a response except for something along the lines of "it happens," but I did get a photo of it.

I didn't think to snap a pic, but thats a really cool one you got. I wonder what the primary vector for spider communication is and if there are subtle queues both spiders understand when they encounter each other..

 

[The Snark]

Tetragnathidae

Apply the basic scientific methodology here. Always throw out the top and bottom 10% of any experiment. Tetragnathidae are in the outlier percentage unless you can find multiple incidents of their presence. That leaves two species that prefer similar habitats. Latrodectus prefer cool and dark locations. Pholcids the opposite, they will set up shop anywhere and everywhere. It's inevitable their habitats will cross boundaries. Then look at the common traits of both species. Both are nearly blind. Both rely on their webs as sensory organs. It would be unusual if they were somehow able to detect the presence of the other species and choose to locate elsewhere.
As long as the Latrodectus keeps to it's common habit of being very reclusive - that is stays in it's hide until web vibrations indicate prey, the two species can be considered communal. If the Latro goes wandering it will likely become prey for the Pholcid. It is also unlikely the Latro will be able to mate as wandering males will easily become Pholcid food.

You can also take into account the prey capture methodology. Latro requires the sticky lines contained well within the boundary of the web. Pholcids are more than capable combatants; wrangle and wrap anywhere within their web with those long legs keeping their bodies well away from a nip of a victim spider.