Can Tarantulas Lay Unfertilized Eggs? (Phantom Egg Sacs)

Many invertebrates and reptiles will naturally lay phantom eggs. These eggs are generally not fertilized, but there are cases where this is possible.

Understanding how this works for tarantulas will make keeping a tarantula less stressful.

Tarantulas can lay eggs even if they haven’t mated. These eggs are unfertilized and are sometimes called phantom egg sacs. Unfertilized eggs will never grow into baby tarantulas. However, female tarantulas can save sperm for several months until they find a safe area to lay fertile eggs. Therefore, even if a tarantula hasn’t mated recently, the female may still lay fertilized eggs.

Can Tarantulas Lay Unfertilized Eggs

In this article, you’ll learn all there is to know about unfertilized tarantula egg sacs and tarantula egg fertilization, which is essential knowledge if you’re considering breeding tarantulas.

Even if you don’t intend to breed the female tarantula, knowing some basics will reduce your stress levels and allow you to care for your tarantula better.

Table of Contents

Can Female Tarantulas lay Eggs without Mating?

Female tarantulas can lay phantom egg sacs that contain unfertilized eggs. They do not lay empty egg sacs, so they will always hold eggs, but they’re not usually fertile. Even with a phantom egg sac, the female tarantula may initially react to it like it’s fertilized, then discard it or eat it afterward.

Phantom egg sacs are a relatively common sight for tarantula keepers. Adult females will lay them periodically, but this doesn’t mean the eggs are fertile. She does not need to mate with a male to produce eggs.

There’s no harm in removing an unfertilized egg sac from the enclosure during your regular cleaning process. While a female tarantula might treat the egg sac like it’s fertilized for a time, she will eventually discard it or eat it.

Females sometimes eat unfertilized egg sacs to regain some of the lost energy spent making the sac and laying the eggs.

Read More: How Often Should You Clean Your Tarantula Enclosure

A female spider defending her egg sac
Egg sacs always contain eggs, even if they are sometimes unfertilized.

Can Female Tarantulas lay Fertile Eggs without Mating?

Female tarantulas can not self-fertilize eggs but can mate with a male and store the sperm for later use. Therefore, a female tarantula can lay a fertile egg sac several months after mating. 

Tarantulas and spiders can store sperm for later use, meaning that wild-caught tarantulas could have mated in the wild before being captured and may lay fertilized eggs in captivity even if it looks like they haven’t mated.

Captive-bred tarantulas only produce fertile eggs if they breed in captivity due to intentional breeding or by accident if kept in a communal enclosure.

While a female tarantula can lay fertile eggs without you seeing her mate, this is unlikely unless she was wild-caught as an adult.

How does the Mating Process work for Tarantulas?

When a male adult tarantula is ready to mate, he will build a sperm web in preparation. He will then transfer sperm from the web to his palpal bulbs at the end of his pedipalps which he uses to put the sperm into the female.

Tarantulas will perform a courtship ritual first before mating. The male will tap or “drum” his legs or pedipalps on a female’s webs to announce his presence and intention. This ritual determines if the female is interested. If she is, the female will return the “drumming” to signal he can move closer.

The male will use the hooks on his first pair of legs to lift the female to insert his sperm-loaded pedipalps into her genital opening. Lifting her like this also helps keep her fangs away from him, giving him time to escape after mating.

Since many females attack the male during and after mating, mating is a high-risk activity for male tarantulas.

Read More: Are Female Tarantulas Larger than Males?

Pternochilius Mirinus Tarantulas Mating
You can clearly see the size difference between these male and female Pternochilius Mirinus Tarantulas

How long can a Female Tarantula store sperm?

Female tarantulas and true spiders can store sperm for up to six months after successfully mating with a male, or until their next molt if it occurs sooner.

Research into female tarantulas’ ability to store sperm is limited, but we know that female true spiders can do it. In addition, we now know that some spiders can even hold the sperm of multiple males internally until they are ready to fertilize their eggs.

It’s unknown if female tarantulas can store the sperm of multiple males, but it is clear tarantulas can store sperm for several months from at least one male.

How do I know if a Tarantula Egg Sac is Fertilized or Unfertilized?

Fertilized and unfertilized tarantula egg sacs look the same at the start, but the egg sac will feel more dense and dry over time if it is unfertilized. The female will also lose interest in it. If the sac is fertile, then she will continue to guard it.

Female tarantulas will guard egg sacs, even if they are unfertilized. However, the female will eventually abandon or eat any unfertilized egg sacs. The general rule is that if the female tarantula has been guarding the eggs for at least one month, it’s safe to assume that the eggs are fertilized.

Newly hatched spiderlings from spider eggs
If the eggs don’t hatch after a while, the female tarantula will abandon or eat the egg sac.

Should I remove a Phantom Tarantula Egg Sac?

While a female tarantula might eat a phantom egg sac, they don’t always. Removing a phantom egg sac is perfectly safe as long as the tarantula is not guarding it at the time and it’s not in a position that would be dangerous to remove.

If the egg sac isn’t causing any problems, it is better just to leave it alone until the female realizes it is unfertilized and eats it or discards it herself.

If the tarantula has her egg sac inside her burrow, it will be harder to remove it. Poking around in the burrow could cause it to collapse, which places the tarantulas’ life at risk. If the egg sac is unfertilized, the female tarantula will move it out of her burrow and discard it eventually.

Not every tarantula lives in a burrow, so it might not be a problem for you. Burrowing tarantulas are called fossorial tarantulas and can be difficult to look after.

You can learn more about fossorial (burrowing) tarantulas in our Fossorial Tarantula Guide

A female tarantula will be very defensive of any egg sac she still thinks is fertilized. Trying to remove a phantom tarantula egg sac when the female is still guarding it also puts her life in danger.

First, she should be trapped in a capture cup to protect you and her. Then, it may be possible to bait her away from the egg sac with food before quickly removing it.

How to safely remove a phantom egg sac

  1. Make sure the egg sac really is unfertilized
  2. Wait until the mother tarantula has stopped defending the egg sac
  3. Use bait to lure the mother away from the egg sac
  4. Use a capture cup to safely isolate the mother tarantula
  5. Remove the egg sac using gloves or tongs and dispose of them safely
  6. If the egg sac is in a burrow, be extra careful because it could collapse easily
Tarantula Unfertilized Phantom Eggs Guide Infographic
Tarantula Unfertilized Phantom Eggs Infographic

Do all Phantom Tarantula Egg Sacs contain Eggs?

All phantom tarantula egg sacs contain eggs. There are no phantom egg sacs that don’t have any eggs inside. There is no guarantee these eggs will be fertile, and in most cases, they will be unfertilized.

Tarantula egg sacs can contain hundreds or thousands of eggs, even if it’s a phantom egg sac. If the eggs are unfertilized, they will never develop. Unfertilized eggs will rot and turn black instead.


Do Tarantulas Know Their Egg Sacs are Unfertilized?

While the egg sac is new, the female tarantula will guard it just like a fertilized egg sac. Once she realizes the egg sac is not viable, the female will dispose of the sac by removing it from the den, or by eating it.

Phantom egg sacs contain unfertilized tarantula eggs. They are perfectly safe and a normal part of the tarantula reproduction process, they are made from silk and eggs.

Although the egg sac itself isn’t dangerous, the mother tarantula will defend the egg sac vigrously until she realizes it’s unfertilized, so you should treat them just like they are fertilized and assume the female will be more agitated than usual.

Read More: Are Tarantulas Herbivores or Carnivores?

Panamian Gold Fossorial Tarantula
Tarantulas like this Panamian Gold are fossorial, which means they live in burrows, making it difficult to remove their eggs.

Are Phantom Egg Sacs Dangerous?

Phantom egg sacs don’t cause any harm unless you leave them in there for a long time. If the mother tarantula is still guarding it, it’s best to leave it in the enclosure.

If you leave the unfertilized eggs for too long, they may cause mold or provide a breeding ground for bacteria, which can hurt your tarantula.

Eventually, the female tarantula will dispose of the egg sac by herself, either by eating it or removing it from her nest.

Once this happens, the female will be less aggressive and you can remove it when you clean your enclosure next.

Read More: How to Spot and Prevent Mold in your Tarantula Enclosure

Summary – Unfertilized Eggs are Normal for Tarantulas

Adult female tarantulas and other spiders can lay eggs, even if they have not mated. Eggs laid in this manner will usually be unfertilized. This is sometimes known as a phantom egg sac.

Tarantulas and spiders can store sperm for months after successfully mating with a male. They can wait months before fertilizing any eggs, which means you don’t need to see the tarantula mate for them to be capable of producing fertilized eggs.

If you find a phantom egg sac in your tarantula enclosure, it’s best to leave it alone until your tarantula either eats it, or discards it. It may take a month or two before the female realizes the eggs are unfertilized and until that point she will defend it vigorously.

Written by:

Clare Godwin

Clare has always been fascinated by invertebrates, especially arachnids. Over the years, Clare has gained experience keeping various invert species including mantis, isopods, and arachnids.
Clare is passionate about educating people about keeping inverts as pets and enjoys helping people look after their spiders and tarantulas.