1. Introduction to Spider Eggs

The world of spider eggs is fascinating indeed. They may be incidental to the more central and practical concerns about the identification of adult spiders, but it is worth stepping back now and then to appreciate their elegance and oddness in the scheme of life. Biologically speaking, spider eggs are containers for offspring – above all, they nurture the young developing inside them. Adding richness to this, the activity of spiders in producing eggs and tending them, and all the other things spiders do and things that happen to them as they engage in this activity, can influence the diversity and distribution of spiders. In contrast to the limited appreciation of spider eggs, spiders themselves are popular with biologists studying the world’s diversity – about 48,716 species are known and many more await description.

Spiders reproduce sexually and lay eggs. A clutch may comprise a single fertilized egg or a couple of hundred or more, depending on the species. In the family Linyphiidae, which is probably the largest in terms of species, the group is small because of the brief associations between the sexes and the resulting small number of eggs per year. Where there is more sex and mating, there is more diversity. When fertilization is internal, the eggs are more often able to survive in the absence of environmental moisture, and they add additional layers to those discussed above. The 1,500 species of wolf spiders in North America, at least some of which are among the relatively few that plot locations for their egg sacs, lay clutches of typically more than 100 eggs. The average spider clutches containing one to several eggs in families with the same general abundance, lifestyle, and distribution are smaller but, with an average of 117 eggs, many pirate spider clutches have at least 120 eggs. There are many more individuals produced in a clutch than ever complete a female spider’s maturation cycle. Since spiders lay so many more eggs than could possibly mature, most eggs never accomplish this. Could most spider eggs possibly be disposable?

2. Types of Spider Eggs

Spiders are fascinating creatures. They are often referred to as the hunters and arguably one of the most important creatures on the planet. Spiders have always been the underdogs of the animal kingdom; they are interesting creatures. Even their look, with those long legs and scary fangs, makes them interesting, if not a little unsettling. Recently, we were having a conversation about how spiders reproduce, and one thing that seemed to come up a lot was about spider eggs and the different shapes they came in. To ensure that everyone understands, we will give a basic overview of spider eggs.

There are three basic shapes that normal, everyday spider eggs come in. The first type of egg is the spherical egg. They are mostly just typical round eggs. These eggs usually only come in this shape and have no distinct characteristics other than their shape. Cylindrical eggs are longer than spherical eggs; they are more oval in shape and look like peanuts. The third type of egg is the oval-shaped egg. These eggs have more of a pointy end. The tips of these eggs are often the ones buried in the silk of the egg sacs. It is believed that these different shapes are an adaptive evolution of the type of lifestyle each family of spider has evolved to live. One theory is that ovular spider egg sacs may have a lower survival rate in the wild than capsules or ballooning egg sacs do. That’s because ovular spider egg sacs are not as easily attached to substrates as other egg sacs are.

2.1. Spherical Eggs

Given that the natural world is full of an almost infinite variety of shapes, one might wonder why eggs are not always spherical. Coming in a suite of shapes—long, round, oblong—some spiders produce spherical egg sacs, or eggs, that more commonly occur in terrestrial animal eggs. Although not all studies come to the same conclusions, studies examining this question have found general agreement that reproductive strategies seem to profoundly influence egg shape. One possible reason for this is that the eggs that are not spherical—and any potential advantages that they might convey—can be produced and exploited.

True spherical eggs are quite rare, however, and soft and just under 1 mm in size. While other araneoid spider families also produce spherical or near-spherical egg sacs, these species often recycle silk from other egg sacs and the adult retreat. Mechanistically, it seems unlikely that spiders in the family Araneidae can do so, because their digestive systems are rather unlike those of their relatives and store and digest food in unique structures nearby the midgut. Evidence shows that spherical eggs, including those in the Araneidae, are well adapted to their terrestrial environment, as is evidenced by how widespread they are—and how likely they are to retain the population strategies of the species-carrying females. In nature’s laboratory, round has evolved several times independently for a number of species, although the Araneidae happens to be one of the oldest wrappings in this evolutionary gift box. Additionally, roundness facilitates the practical constraints of the animal eggs that nature makes.

2.2. Oval Eggs

Spiders rely on diverse reproductive strategies, but they all start with spider eggs—lots of them. The process of egg-laying, or oviposition, depends on the reproductive capacity and investment of a female, a topic that is often discussed in tandem with spider silk. This interrelation between eggs and silk leads to an entire extra domain of spider research: egg morphology. Let’s familiarize ourselves with the variety of spider eggs and investigate the biological realities that shaped their incredible diversity.

The most “truest egg-like” eggs are oval. Out of all egg shapes, oval eggs find themselves in the ideal spot between egg size and the amount of space available for egg storage. This means clutches of oval eggs can make the most of a mother’s abdomen, offering an evolutionary explanation for the indeed oval form of egg sacs. Furthermore, the larger length of oval eggs fits in well. These eggs also seem to offer an ideal “uniformity” for offspring survival. In different ways, both small spheres and large cylinders are more vulnerable than ovals, as an interesting postulation suggests. Sedimentary surveys of egg shapes reproduce a sorting-out by habitat type. Oval eggs pop up nearly everywhere but are least represented in deserts. Oval eggs also appear in larger numbers than other oval-shaped hides, suggesting the immense diet played by spiders in supporting hatchling survival. In addition to appearing in deserts, balloons are used in the ovary of waterbound spiders. Whether they float as a species or simply enable more mobile archaeological enclosures remains unexplored. Both ecodispositions have been shaped by infectious disease. Warriors, like the semi-aquatic orb weavers, typically provide more tactile input and supplies than their desert-friendly cousins. When placed among foliage, few balloons can camouflage from predators while waiting for the aid of wind to send them sailing into adulthood alongside oviposition substrates. Burying and ground-nesting spiders, on the other hand, use smooth cylinder shapes to minimize tense friction with soil. Any clod that is able to come loose during downtime can function as a buoyant diving board for the coucar, a notion that the silk inlay used by ground nesters supports. Oval eggs reflect a primary combination of round and cylindrical traits in their handcrafted exploration of the egg landscape. The identifiable compromises embodied by oval eggs play an extensive role in spider motherhood. They can mix both small-sized round eggs crucial for preventing successful litters for non-calibrating localized poor resources. Given the importance of a long ovoid profile, the tucking of assorted fields towards the bulging section at a species-distinctive point discloses an unexpected link between the progression of the egg, as well as the beginning. It was most likely homoplastically obtained in the central araneoids where abundance presents more possibilities for a survival fade. Rather than laying many tiny eggs, most spiders prefer to make large ovals with a higher propensity for survival. However, others vary greatly in their use of pure ovals to define their own character. This diversity reflects the rise of diverse life histories and physical exclusivity levels first in pyratecology. Trophy bulges to house extra eggs for special exertions. Larger, higher spheres are targeted at swelling to manage dissimilar offspring fitness at any true company given a selective moment. Overall, the ovoid shape yields a truly proliferant plethora of resources for spiders seeking to live a good egg life.

2.3. Cylindrical Eggs

Most spider eggs are oval or roundish in shape. Roundish eggs are well adapted to endurance within the web, where they are often well protected by the folded leaf of a silken egg sac, and where they may be laid in smaller numbers or singly. However, some spider eggs have developed an elongate or cylindrical shape, and the way such elongate shapes may influence the ecology and behavior of the laying mothers is not known. Cylindrical eggs are reported in a diversity of spider taxa, particularly in oviparous species. The interspecific variation in egg size and volumes of cylindrical eggs from three oviparous spiders was significant, particularly given the species we have chosen to study, which all live in wetlands and could be affected by anaerobic conditions.

The environment of a few spiders may be ecologically distinctive enough that the slender form of the cylindrical eggs could potentially be of adaptive value for the nesting itself. Nesting ecologies with such slender-egg laying spiders include below water and temporary pond areas and swamps, inside the middle of vases where water is added, and below streams and brooks during periods of high water and drought. Mothers in some of our studied oviparous species build interesting egg nests in which the cylindrical eggs are woven with cobwebs that stick to the eggs within the web. Silken nests produced by the laying mothers above the water level could withstand many times the period of submersion of their submerged egg sac transformation. Cylindrical eggs have also been reported for some species in heavily wet and often flooded fields.

3. Spider Egg Development

The eggs, in the simplest and typical cases, are grouped as silk-wrapped buds on top of a hemispheric or conical web. When completion is reached, the spider wraps its eggs in a tight silk cocoon. In most cases, the spider dies and becomes a meal for its offspring. The inner wall of the cocoon is composed of a mix of protein and carbohydrates, combined with inorganic substances. This edible layer most probably serves as food for a newly hatched spider, and after becoming moistened and softened by a simple pumice gland present in the spider’s chelicera, it assists in gnawing the egg envelope. The external silk of the cocoon, which is usually thicker than the internal layer, is composed of fibroin. During the process of silk spinning, the parent female can sometimes spin characteristic openings across the length of the silk pipe. This pipe-like structure decreases the risk of desiccation while providing good gas exchange. It is usually covered with silk and spider feces to guarantee protection for the developing brood. Spiders that spin such a structure are recognized as possessing an advanced form of social behavior.

3.1. Egg Formation

Egg formation begins as the egg cell leaves the ovary, a small sac-like structure attached to the animal’s heart. The cytoplasm of the egg cell accumulates food stored in granules, precursors of the yolk needed to nourish the embryo. Prior to fertilization, the egg nucleus travels to the periphery of the cell and arrests further development at a stage called “metaphase I” in most spider species. Cells that provide nourishment for the developing egg are thought to be incorporated among interconnected cells in a novel variation of “germarium,” a specialized set of nurse cells found in many invertebrates. Cells then form a thin sheath around the egg cell, where third-hand precursors of the “horseshoe” were found. External cells differentiate into specialty cells to secrete a vitelline coat, a thin, hard, freeze-protected shell, and a string or pad of fibers holding the ovipositional setae. These features are unique to spiders. The hard shell is often as long as it is wide (spherical), which may help prevent the egg from being crushed while sitting in a silk egg sac, but this is an untested hypothesis. Surrounding the egg in some species is a layer of air separated from the shell by sheet-like proteins commonly called “bubble wrap.”

3.2. Incubation Period

Incubation periods for spider eggs are generally quite long, promoting huge brood sizes, but sometimes they can be quite brief. If you can think of a place in the world, there is likely a spider that lives there and lays eggs there. Everything about the duration of the incubation phase of spider eggs has to do with the ecology of the species involved. There is a species of wolf spider where the female utilizes her legs to push the eggs off of the abdomen, upon which the eggs land on the ground and then explode. With such a short period of incubation for the eggs, having the female carry them around is more difficult than just providing the general environment the eggs will need to successfully hatch. Most species of spiders lay eggs that will take an incubation period of several weeks to several months in order for the spiderlings to hatch. The big question is why the wide range in duration? The general explanation for the duration of any given species’ incubation phase can be explained by thinking about the physical environment in which the eggs have been laid and some basic life history traits that are particular to the spider that laid the eggs. If a species of spider can only expect a short time window during which the newly hatched spiderlings can find enough food and shelter to last throughout the immature stages and through the first winter, then those eggs should probably not last inside the egg for too long before hatching. The length of the period of incubation reflects the limitations placed on the offspring: where the eggs were laid and the life history trade-offs these spiders are making.

3.3. Hatching Process

3. The Hatching Process and Spiderlings

After sticking to the wall or column of the egg, the fully developed spiderlings await hatching. This stage is called “near hatching.” Some have already started to molt into the spiderling skin, and the rest are preparing to do so. The moment when the anterior part of the abdomen is nearly outside the egg is the actual hatching moment. The rest of the birth is simply a matter of the spiderling pulling the rest of its body out of the egg. This process takes a few minutes or less. The newly hatched spiderling is yellowish to cream-colored and looks somewhat flattened. They also seem a bit more plump than they actually are. This is because the spiderlings leave their eggs with a relatively large amount of yolk stored in the abdominal pedicel. This yolk turns into a whitish substance as it is moved around in the pedicel by the jumping motion of the newborn. After the inside of the egg sac has been cleaned, the tiny spiderlings will gather around the parental female. They sit in the octave position only during the coolest moments of the first weeks of their development. If the temperature rises, the spiderlings will disperse and find a cooler place to sit. This will continue until the octavation behavior vanishes after thirty to fifty days. Very cold temperatures and long days cause the spiderlings to suppress the growth of their webs. During the night, almost all spiderlings will sit spread out and do not use the mother to hide from potential predators.

4. Protective Measures of Spider Eggs

Spiders use their silk for a lot more than just wrapping up annoying insects. Among other things, they make protective silk sacs to hold their eggs. This is a pretty good idea since spiders are rather small and generally the parents cannot continue to guard their eggs. As with adults, different species use different strategies for egg protection. Bat and bird droppings, mud, and other materials are mixed in with the silk, helping to camouflage the eggs even further. The kids are protected from getting picked off, stepped on, and washed away. Since the female can lay anywhere from around one hundred to thousands of eggs at a time, taking a few moments to reinforce the sac is well worth anything she gets out of it. Plus, there is some evidence that silk may actually help protect the kids from nasty bacteria as well. A fair number of females even stand guard nearby, protecting the eggs from any would-be eaters during the time the clutch is being produced and from the minute the eggs are laid to when they hatch. Some mothers even come back to find their eggs buried in a different location. By the time the kids emerge from their eggs, the search party headed by Mom has already moved on, increasing the odds of the eggs surviving. Overall, spiders that invest time and energy in making a good protective egg sac have more young that manage to survive to develop than those who do not. It is likely both parental care and the fancier egg cases have evolved because of predators that would eat the eggs and young spiders.

5. Ecological Importance of Spider Eggs

Given the low number of eggs per reproductive event in most spiders, few families or species have relevant economic importance as producers of edible eggs. However, spider eggs, particularly in combination with silk, are a significant component of the food web. Many kinds of spiders store their eggs in silken sacs, which are often hidden and/or camouflaged on the mother’s body or attached to the substratum. In this way, spiders and their eggs can escape visual discovery and predation by other spiders and small predators. It has been observed that these eggs are often utilized by several animals as prey: mites, ants, and beetle larvae have been recorded eating the eggs, particularly where spiders are absent or in low numbers, showing that the eggs of predatory arthropods may be a crucial component of the so-called ‘detritus food web’, i.e., the food web that includes detritus and its small consumers and microfungi.

Because they are such an important energetic source for several consumers, spider egg clutches may have a central role in determining local biodiversity, both at the patch scale via predation of the clutch itself and at the landscape scale, influencing predator-prey (or detritus-consumer) relations. Consequently, an excessive reduction in the numbers of spiders or some spider species, caused by environmental disturbance or habitat loss, may in turn influence a number of depauperated but potentially delicate webs of relationships. Moreover, the eggs and their carrying silk are unusual in at least some aspects, if we think about the vulnerability of spider populations to environmental changes. Any shifts in the availability of such inorganic resources, such as pollution or extreme weather conditions, may in fact influence the spawning behavior and the survival of embryos within the eggs, which should maintain a negative rate of population growth.

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