Monthly Mystery #7: Clothesline Egg Sacs

On page 43 of Tracks & Sign of Insects, I included a photo of “a mysterious and very common egg sac, almost certainly of a long-jawed orbweaver (Tetragnathidae).”  Here is another example, showing the full length of the “clothesline” from which the egg sac is suspended, in this case between two striped maple leaves.

This is a timely post, because June is when I first start seeing these egg sacs.  The photo above was taken on June 2 last year in Upper Jay, New York.  I’ve found them in forests throughout the northeastern US, most often along streams or the edges of swamps (typical long-jawed orbweaver habitat).

The family level identification in the book is based on the spiderlings that emerged from an egg sac I collected.  Unfortunately, Noah and I were on the road working on the book at that point, and there was no way to keep them alive and find out exactly what they were. Here’s a shot looking into a jar full of them (taken July 4, 2008):

On May 1, 2010, I was doing some spring cleaning and found this ichneumon wasp in a vial in which I had placed another one of these egg sacs.

Bob Carlson examined the wasp and identified it as Tromatobia ovivora.  Could the wasp’s identity be a clue to that of the spider?  No such luck; Bob informed me that T. ovivora has been associated with 17 different spider hosts, none of which are tetragnathids.  I collected several egg sacs that spring to try my luck again, but nothing hatched.  So in the spring of 2011, determined to solve this mystery once and for all, I collected every one I found. Several of them produced spiderlings, the first emerging on July 7 from an egg sac collected in Colchester, Vermont.  Here one can be seen through the exit hole made by its siblings:

The newly emerged spiderlings measured about 1.3 mm.

I dumped them into a jar in which I had scooped up springtails and other tiny bugs from the lawn, as suggested by John Maxwell, a fellow BugGuide editor who is far more experienced at raising spiders than I am.  But they seemed most interested in cannibalism; one seized another as soon as I put them in the jar.

By the next day they were showing at least some interest in other food.  Here’s one with a leaf-mining fly (Agromyzidae: Phytomyza):

By July 11, many more spiderlings had hatched; I counted nearly 50 from one egg sac. The largest one from the original egg sac now measured about 1.9 mm, and its legs had grown much longer.

By July 23, only a single spider from the first egg sac was still alive (2.0 mm):

It died a few days later, but I was able to raise one from another egg sac to a later instar. Here it is on August 1 (4 mm):

And here it is stretched out along a twig, as you might find it in nature.

An adult would be needed to identify it to species, but I’m pretty sure it belongs to the genus Tetragnatha.  Anyone care to disagree?  The thing is, as far as I can tell, Tetragnatha species are recorded as making fluffy egg sacs, not papery ones suspended from clotheslines.  This kind has some fluffy material inside, though, which reminds me of another mystery.  When I opened one of these egg sacs up, I found a foreign object inside.

This seems like a good match for the parasitic frit fly (Chloropidae) puparia shown in a jumping spider egg sac on page 46 of my book.  So that’s not the mystery.  The mystery is what are the eggs (~0.6 mm long) on the surface of this egg sac, photographed in Pelham, Massachusetts on June 20, 2011.

I found another the next day.  These eggs appear completely smooth, other than being a bit dented.  I’m not sure if that’s from me damaging them when I collected them, or simply a sign of their having hatched.

The thing is, none of the spider egg sac parasitoids discussed in my book would lay an egg on the surface of the egg sac.  Frit flies lay eggs inside the sac while it is still being constructed.  Ichneumons and other wasps insert their ovipositors to lay eggs.  Mantidfly (Mantispidae) eggs are laid in clusters on random objects and the larvae have to find their own way to egg sacs, then bore into them.  I have a hunch, though, based on something that emerged from an egg sac I collected on July 11, 2011, belonging to a cobweb spider (Theridiidae: Euryopis).

This 1.5-mm scuttle fly (Phoridae) was identified by Brian V. Brown as a female Phalacrotophora epeirae, a species known to develop in spider egg sacs.  I’m not sure, but it may be the only scuttle fly known to do so.  I haven’t gotten around to looking up its life history yet, but it seems like a good candidate for the layer of those eggs.  I eventually tore open the egg sacs that had them, but found nothing identifiable inside.

I suppose this month’s mystery (or mysteries) might be solved to most people’s satisfaction, but I would love to have someone confirm that these are scuttle fly eggs, and would love it even more if this post inspires some of you to look for and collect these “clothesline” egg sacs and try to raise the spiders to adulthood.  (Catching a female in the act of making one would be fine too!)