Silken treasures: A gallery of spider egg sacs

Spiders are truly magnificent: from the animated personalities of jumping spiders, to the stealthy hunting prowess of wolf spiders, they inspire us, and can sometimes create unwanted fear. We often forget, however, that spiders are more than predators with two body parts and eight legs. Female spiders also produce wonderful structures in which to protect their eggs. Many years ago, when I was rearing wolf spiders as part of my PhD, I recall observing a female making her egg sac. She carefully and slowly prepared a disc of silk, laid her eggs, carefully wove the top disk, and “glued” the whole structure together, in under an hour. It was a beautiful process. Maximizing fitness is the ultimate evolutionary goal, and spiders have clearly worked out a system to help ensure their young can develop under the protection of silk.

Female spiders do different things with their egg sacs: species within the genus Agriope construct their egg sac later in the summer, and after the female dies, the egg sac overwinters. Spring will bring the hatching, and young spiderlings will emerge as warmer weather hits. Their egg sacs are stunning, as depicted here:

Argiope aurantia egg sac. Photo by J. Lapp, reproduced here, with permission.

Wolf spiders mothers are really quite extraordinary. In addition to the care and time in constructing an egg sac, females then attach the egg sac to their spinnerets, and carry around the package for a period of time. This is a remarkable feat, as the egg sacs often have a mass equivalent to the female’s body mass. They will typically carry egg sacs for a week or two before the young hatch, and then the spiderlings will catch a ride on the female’s abdomen for a while, after hatching.

An Arctic wolf spider carrying her egg sac (photo by C.Buddle)

You can also find other spiders that carry their egg sacs in different ways. In the family Pisauridae, for example, females carry egg sacs by their chelicerae (i.e., their ‘mouth’).

A Pisauridae, carrying her egg sac. Photo by M.Larrivee, reproduced here with permission.

In the following photo of Scytodes thoracica, you can find a female with her egg sac tucked underneath her body:

Scytodes thoracic female with eggs. Photo by Matt Bertone, reproduced here with permission.

Many species of spiders build egg cases that are thick, have a heavy ‘weave’, and can be spiny, spikey and presumably quite unpleasant for any potential egg sac predators. The case in point are these brown widow egg cases:

Brown widow spider egg cases. Photo by S. McCann, reproduced here with permission.

In other cases, the females do not build much of a true egg sac, but the individual eggs themselves are only loosely held together, and you can see the individual eggs clearly. This is rather obvious in critters like the cellar spider:

Cellar spider (Pholcus phalangioides) with eggs. Photo by S. McCann, reproduced here with permission.

Sometimes you can get very lucky, and see the spiderlings emerge within the egg sac before they eventually make their departure. In this photo, the jumping spider adult is just off to the side, and is keeping an eye on the spiderlings protected under a thin layer of silk:

Spiderlings protected under silk. Photo by S. McCann reproduced here with permission.

Probably my favourite of all egg sacs belong to some species in the family Gnaphosidae (the “ground” spiders). I have found that their eggs sacs are much more commonly encountered than the spiders themselves. Flipping over rocks or logs can reveal a pink saucer-like egg sac. Yes, PINK. I have no idea why the egg sacs are pink (but there are some thoughts here), or how the spiders achieve such coloration in their silk, but nevertheless, if you find one of these natural Pepto-Bismol – coloured treasures, you know that Gnaphosids are close by.

Gnaphosidae with egg case. Photo by S. McCann, reproduced here with permission.

The egg sacs of spiders are often overlooked, or not recognized as spider egg sacs. They are silken treasures: their construction is unparalleled, and they both confuse us and amaze us. Keep you eye out and you will surely see some of these brilliant products of evolution.

Can you see the egg sac? (can you see the spider?). Photo by S. McCann, reproduced here with permission.

(and please share your stories of spider egg sacs, below!)