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As life returns to the gardens this spring, it’s not just the plants that are stirring.

Spiderlings emerging on a warm spring afternoon (click any image to enlarge)

At a turn in our driveway there is a large volcanic rock, no doubt put there by the previous owners, to prevent vehicles from cutting the corner.  Typical of lava rock, it’s riddled with various sized holes.  This rock seems to be a favored habitat of our resident lizards, and our bees enjoy drinking water that becomes trapped in the holes in winter and spring.

This large rock is popular with lizards, and bees alike...and now spiders!

Yesterday, while out in the garden, checking first for bees, I sat on the rock for few minutes during a brief break.  As I looked down though I noticed something was moving, just below my knee.  No…wait, it’s lots of somethings moving. A mass of recently hatched spiderlings!

A few separate clusters of spiderlings were gathered on the face of the rock

We have no shortage of orb weaving spiders here, as becomes apparent during the fall months when almost every tree seems to be draped with spider webs.

Orb weaver webs, like these, are abundant here during the fall months

Because of the vivid yellow color, and the markings, at first I thought these might be Argiope sp. spiderlings.  We found two species here last autumn out in the gardens, Argiope aurantia

Argiope aurantia

…and a few weeks later, Argiope trifasciata.

Argiope trifasciata

After some reading I felt fairly confident ruling out Argiope aurantia spiderlings though.  The yellow coloring of these spiderlings might seem convincing, but it turns out that Argiope aurantia spiderlings are typically more white in coloration, not yellow.

Behaviorally something was apparently amiss too.  Argiope aurantia spiderlings emerge gradually over a period of days, not in a matter of hours en masse. [1]

That said, Argiope trifasciata spiderlings reportedly do all hatch out together though, and engage in clustering behavior, much like these spiderlings, when they first hatch.

These spiderlings clearly were clustering together in groups

The clusters tend to be tighter in cooler weather.

As the temperature warms though, the cluster becomes more open.

The clusters become much more open during warmer weather

The spiderlings clearly had constructed a communal tangle of web material.  Argiope aurantia spiderlings do not typically form tangles of web material, but Argiope trifasciata spiderlings do, so perhaps now I had my answer?

Each spiderling contributes threads of silk to the 'tangle' to support the cluster

Perhaps not.  I finally convinced myself  that these spiderlings likely belong to neither Argiope species!

So which species of spider is this?

Argiope sp. spiders generally construct their webs, and position their egg sacs, amidst vegetation.  We watched the females build, and rebuild their webs repeatedly last year.

Last fall the Argiope sp. webs were built in the gardens, between plants, not on rocks or walls

The egg sac producing these spiderlings wasn’t located in vegetation though, but was clearly embedded deep within one of the holes in the lava rock, along with what looks like the remains of someone’s meal.

The egg sac the spiderlings were emerging from was recessed deep in a hole in the rock

Honestly, I couldn’t even envision an Argiope spider actually fitting inside this hole, as they are substiantially sized arachnids.

An Argiope egg sac is typcially smooth on the outside surface, and somewhat papery.  The exterior of this egg sac, however, seemed very fluffy in comparison.

After a little further sleuthing, it appears these spiderlings may in fact be the offspring of the European Garden Spider (Araneus diadematus), that is known to frequent this area.

These spiderlings are presumably Araneus diadematus

The appearance of the egg sac, with it’s more cottony appearing exterior, the egg sac location, and the color and pattern of the spiderlings all seem to fit much better with Araneus sp., than Argiope sp.  It’s difficult to know for certain though, as the female responsible for producing the egg sac would not have survived the winter.

These spiderlings will gradually disperse from the cluster over the next few days

Next time I’ll have to pay closer attention during the fall months to see if I can find any Araneus species spiders associated with their egg sacs, and watch closely for the spiderlings the following spring.

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[1] Tolbert, W. W.  1977.  Aerial Dispersal Behavior of Two Orb Weaving Spiders.  Psyche 84:12-27