Spider egg invader!

The crunchy ground on a cold February afternoon was holding everything captive except for the chickadees and squirrels. My hike was purposeful. I was keeping up a good heart rate, rhythmically breathing in icy air and exhaling vapor. At the top of a hill, I stopped to take in the view. I could see the parkway east crossing a valley of skeletal trees. The sky was a cloaked gray. Meh, winter…nothing exceptional in the landscape to see, not even snow. As I allowed my pulse to return to its normal rhythm, I focused in on a skinny snag with its bark hanging loosely along the trunk. I peeled back a chunk at eye level which revealed an egg sac of some sort.

I assumed it was a spider sac since it had a white, silky, oval shape, about one inch by a half inch. In the winter, spiders, like most other animals, have to survive the cold. In this case, baby spiders were surviving as eggs within the protection of a silken cocoon. I thought about how many layers I was wearing to stay warm – at least two. This egg sac had its own layers that kind of did the same thing. There could be a hundred eggs in there all clumped together, its own mass working to retain any heat from the sun. There are layers of silk that act as a waterproof barrier and help maintain an internal environment so that the eggs/spiderlings don’t freeze, overheat, or desiccate. The silk also protects the eggs from opportunistic scavengers that may not want to deal with getting through all that mess. And lastly, being under the bark provided a bit of camouflage and an additional barrier to the elements. Seems cozy, doesn’t it? The spiderlings will hatch out of their individual eggs and remain in the comfort of the sac until the rising temperature compels them to tear through the silk and disperse. As I carefully placed the piece of bark back against the tree, I thought, maybe I should take it home and find out what kind of spiders emerged in spring since I was really just guessing that this was even a spider sac. Plus, being that it’s generally too cold in February to find spiders I thought it would be something cool to tide me over…the “What egg sac is this?” mystery! I love mysteries!

My entertainment center does not have TV or stereo components. Instead, it’s a heated terrarium station which houses tarantulas, scorpions, and lizards. I put the bark with the mysterious egg sac into a lidded container and put it in the cabinet thinking the heat would speed up the development process and I wouldn’t have to wait until April/May to see what crawled out. I checked it every other day. I would occasionally spritz the container with water because it seemed like the right thing to do although the silk is waterproof and the eggs inside have their own micro climate. After a few weeks I expected to hold the container to eye level and see the silhouettes of tiny little spiders inside, but that never happened. I assumed I had not provided a proper environment or had disrupted it too much so I pulled out the sac to see if I could figure out what was happening.

Instead of spiders, I found a dead, adult mantisfly which puts this story on a whole different level – the “choose your own adventure” of nature takes a dark turn.

Mantisflies prey on spider eggs. They are specialists in that regard, so I knew that this was indeed a spider egg sac and these eggs were unfortunately doomed early on.

This is the third time I’ve encountered mantisflies in my collection, but each encounter was with a different developmental stage of this insect. I didn’t put it all together until recently.

Summer of 2020, I caught a spider near my compost bin that I identified to be in the Phrurotimpus genus, a type of ground dwelling spider. It was an adult female and after a few weeks in captivity, she created an egg sac. It was a pinkish, papery disc about 5 mm wide “glued” to the side of the container. The female died a week after creating the sac. While trying to remove the sac into a smaller vial, I accidentally opened it. It popped like two thin wafers sealed together. Inside was a live, tiny, fat larva with stubby legs laying next to a dried lump of what I assumed were spider egg remnants. I kept the larva, but it did not survive. My guess is it, too, needs the protective layers of the spider’s sac for the perfect microclimate.

I took some notes on the above situation but never looked into it until something else crossed my microscope six months later. A 2021 February hike proved more productive when I walked right up to a small spider crawling very slowly on the snow covered path. It was a Ghost Spider in the Anyphaenidae family. I thought it was kind of crazy to find this spider in the open, in the snow, in February. It may have escaped being a chickadee’s dinner and dropped from a tree to escape.

At home, I put the spider under the scope to see if it was a mature female. To do this, you have to use CO2 to anesthetize the spider so you can flip it on its back to see its belly. There would be a dark, sclerotized structure (the epigynum) present if it were a female. If the epigynum wasn’t there, it could be an immature female or a male. I saw no epigynum…but when I zoomed closer, I noticed something insanely tiny attached to the spider’s pedicel (the “waist” of a spider).

I was able to remove the tiny thing with some very fine forceps. The spider was unharmed and recovered from anesthesia normally. But what the heck was this thing I just pulled off of it? I had seen larvae attached to spiders before but nothing like this. It had six stubby legs indicating an insect of some sort and was heavily segmented, not legless and bloated like a maggot or grub. I messaged arachnological expert Sarah Rose asking if she’d ever seen anything like this. Yep! She said it was a MANTISFLY LARVA.

BOOM – I dove into research mode and what I dug up is absolutely fascinating!

Mantisflies are a family of insects that fall under the order Neuroptera (antlions and lacewings). They look like little praying mantises but they’re not. They have raptorial front legs and big googly eyes to hunt other insects. It’s almost as if they’ve stolen the praying mantis body design except they have beautiful, long, clear wings. All mantisflies are predatory. As adults, they eat a variety of bugs. If we unpack the Mantisfly family, we’ll find the Mantispinae subfamily. What is unique about this subfamily is that the larvae of Mantispinae strictly eat spider eggs. These insects have co-evolved with spiders in such a way, that the larvae don’t care about any other food option. They are experts at finding and devouring spider eggs!

Adult mantisflies can lay thousands of eggs that look like tiny capsules with stalks. The larvae have a few tactics to actually get inside those cozy spider egg sacs for room and board. Think about insects that go through complete metamorphosis – you have the egg, larva, pupa, adult, right? Well, Mantispinae have one-upped that. They go through HYPERmetamorphosis which is egg, highly agile larva (A), then fat, grub-like larva (B), then pupa (C), adult (D).

Depending on the type of mantisfly in that subfamily, the highly agile larvae (A) either locate and penetrate an egg sac (called an obligate penetrator) OR they will inconspicuously hitch a ride on a passing spider (called an obligate boarder). It’s usually one or the other.

Penetrators search for spider eggs sacs without getting onto a spider. As specialists, they have the right kind of chewing mouth parts to get through the protective silk that an ordinary insect, like a random scavenger, does not possess. Penetrators tend to attack the eggs of a variety of spider families, both ground dwelling and web building.

Boarders, on the other hand, wait around like ticks to grab onto a passing spider. Once attached, a boarder feeds on spider blood while it waits for the spider to start the egg laying process. When the spider begins to lay eggs, the larva sneaks into the sac before it’s even sealed! What if it boards a male spider, you ask? Welp, that’s an easy transfer during the mating process. Very clever. I told ya, these guys are experts!

Back to dead adult mantisfly that I found in the container. I was easily able to identity the green mantisfly because it is the only green mantisfly we have in the eastern US. Zeugomantispa minuta is its name, formerly known as Mantispa viridis. I learned that Z. minuta is an egg sac penetrator. This means that when the green mantisfly was an agile larva, it came across this egg sac and burrowed into it instead of hitching a ride on the mom spider.

Having identified this mantisfly gave me a clue that the agile larva (A) I found on the Ghost Spider could not have been Z. minuta because it was clearly a boarder and that’s all I can say with confidence since the larval specimen has been discarded. Literature suggests that it could’ve been one of three obligate boarding suspects.

I do not know what type of mantisfly was in the Phrurotypidae pink egg sac either. It was in the (B) stage of hypermetamorphosis. I guess that it was a boarder since the spider was in a container when she spun the egg sac. The larva must have been on her somewhere. How else would a mantisfly larva get in there? I went back and zoomed in on the photos I have of that spider and I didn’t see anything on her. I read that some mantisfly larva will hide in the spider’s book lungs, but will only hang out there until after the final molt. The mom spider was fully mature, so if this were the case, I would’ve found the larva on the spider’s pedicel. I’m just going to have to be okay with letting this go unsolved or I’m going to end up a retiring from spiders to become a MantisflyMentor….

Back to the original topic of this post before the detour…what egg sac is this? The most valuable piece of evidence for this case is that I have identified the adult mantisfly. There are multiple papers that tackle which host spiders are associated with this particular mantisfly. After reading Brushwein et al, It seems the most likely type of spider host would be a funnel web weaver in the Agelenidae family. And that makes sense since I see those guys a lot out in the woods in the summer. They usually have a web that funnels into a crack or crevice, like in loose bark or stones. So the egg sac mystery has a pretty solid lead, but will still remain a cold case.

Now that it’s late April, things are slowly awakening. On a hike along the same trail, I now see yellow forsythia, green garlic mustard, and red and white fuzzy tree buds. The ground is squishy from the rain and there are dozens of different birds announcing their territories in clear, crisp declarations. The spiders that have overwintered in egg sacs, and survived nature’s choose-your-own-adventure array of outcomes are emerging to dive into the food chain in more ways than one. This is all of the fun stuff, right? The heralding of spring is so very exciting, but we have to consider the seemingly barren (and seemingly boring) February days that have led up to now. Consider all of the strategic moves that go unseen to make spring actually happen. The winter landscape is hiding a much more colorful story than we think, even when it’s cold and gray!

Austin A.D. 1985. The function of spider egg sacs in relation to parasitoids and predators, with special reference to the Australian fauna. Journal of Natural History 19: 359-376.

Brushwein J.R., Culin J.D., Hoffman K.M. 1995. Development and reproductive behavior of Mantispa viridis Walker (Neuroptera: Mantispinae). J. Entomol. Sci. 30(1): 99-111.

Brushwein J.R., Hoffman K.M., Culin J.D. 1992. Spider (Araneae) taxa associated with Mantispa viridis (Neuroptera: Mantispidae). J. Arachnol. 20: 153-56.

Bugguide website. https://bugguide.net/node/view/4796. Accessed multiple times in April 2022.

Foelix, Rainer F. Spider Biology, third ed. New York, New York: Oxford University Press; 2011.

Hieber, Craig S. 1992. Spider cocoons and their suspension systems as barriers to generalist and specialist predators. Oecologia. 91: 530 – 535.

Hoffman, K.M. 2002. Family Mantispidae, 251-275. In: Penny, N.D. [ed.] , A guide to the lacewings (Neuroptera) of Costa Rica. Proc. Cal. Acad. Sci. 53: 161 – 457.

Redborg K.E. 1998. Biology of the Mantispidae. Annu. Rev. Entomol. 43: 175-194.

Reynoso-Velasco D., Contreras-Ramos A. 2008. Mantispidae (Neuroptera) of Mexico: distribution and key to genera. Ent. Soc. Am. 101: 703-712.