Why are Tarantulas so Big? (Compared to other Spiders?)

Tarantulas are the largest spiders on Earth, and there’s an excellent reason for this. These large and incredible predators often have a bad reputation due to their size compared to other spiders. Even compared to other invertebrates, tarantulas are pretty big.

Tarantulas evolved at least 300 million years ago in the Carboniferous period when all arthropods such as insects and arachnids were getting bigger. Arthropods became so large for a time due to their being more oxygen in the atmosphere, and as the oxygen reduced, arthropods became smaller. Because their prey was large, tarantulas had to become larger-bodied to survive.

There are a lot of pros and cons to being a large tarantula in today’s world and an excellent reason why they’re still so big now. Tarantulas are incredible predators, and the reason why they’re so much larger than other invertebrates is equally astonishing.

Why Are Tarantulas So Big?

While all spiders share a similar ancestor, tarantulas belong to a primitive group called mygalomorph spiders. This means they have two sets of book lungs, a type of respiratory organ. However, they don’t have a respiratory trachea. Most other true spiders species have one set of book lungs and a respiratory trachea.

In short: Tarantulas are bigger because they can process more oxygen than most other spiders.

The Incredible Ancient History of Tarantulas

Tarantulas are an ancient group of spiders from the Theraphosidae family. Some fossils of this family have been found dating back to the Triassic period, around 251 million years ago. However, the first spider evolved about 400 million years ago, and they were among the first animals to live on land.

Most early spider fossils belong to the Mesothelae, which were primitive spiders with spinnerets underneath the middle of their abdomen. Spiders only evolved spinnerets at the end of their abdomen around 250 million years ago, which is what we still see today!

At least 300 million years ago, during the Carboniferous period, spiders became larger partly due to the enormous oxygen content in the atmosphere.

Why did High Oxygen Levels make Spiders so much bigger?

Different theories exist about why the high oxygen level 300 million years ago created such large arthropods.

One of the leading theories suggests that young arthropods had to grow larger quickly to avoid oxygen poisoning. Growing fast would make survival more likely in a high oxygen environment.

It’s possible that the high oxygen had a more significant effect on the young, and one study (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0022610) found juvenile stoneflies are much more sensitive to oxygen. They found that adult stoneflies could regular their oxygen intake while the young couldn’t do this, putting them in danger of death.

Another possibility is that arthropods benefited in some way from the incredibly high oxygen levels. This isn’t something you can do at home, though. Putting modern insects in a high oxygen environment will not make them grow to a large size.

There might be another potential reason why spiders were so giant back then, but these are all just theories! Spiders could have just grown larger because their prey was getting bigger, and it was an evolutionary advantage at the time.

Do any of the Ancient Spider species still exist today?

Some segmented mesothelid spiders do still exist today in eastern Asia. These spiders are very similar to their ancient ancestors and haven’t changed too much over millions of years. However, they are not tarantulas!

Spiders who are in these families are all segmented mesothelids:

  • Pyritaraneidae
  • Arthromygalidae
  • Liphistiidae

Most spiders within these families are extinct today, but some from the Liphistiidae family are still around. These are primarily tube-dwelling species that create relatively primitive trap doors and rarely explore the surface.

The Advantages of Tarantulas being Big

While tarantulas and spiders have shrunk since the Carboniferous period, tarantulas are still pretty giant compared to their prey. Being large provides tarantulas with a range of benefits. If their size was a disadvantage, tarantulas would have evolved to become much smaller than they are today.

Tarantulas come in all sizes, and while some might seem small compared to others, they’re still quite large compared to their prey!

  • Being larger than your prey makes it easier to take them down.
  • Their size allows for impressive threat posturing to scare off potential predators.
  • Larger tarantula paws are perfect for digging and making burrows.
  • Easier to grab prey with their appendages.
  • Larger tarantulas have a wider range of prey options, including small mammals and amphibians.

Disadvantages of being a Large Tarantula

While there are pros to being a big tarantula or spider, there can be problems with it too. Many of the largest true spiders in the world are burrowing species or don’t build webs at all. There are different good reasons why being a large spider or being a large tarantula is also, disadvantage.

  • Being larger means the spider is heavy, which makes it difficult to hang from webs as many true spiders do.
  • A large tarantula can be easily seen when they’re not inside their burrow, so they can be targeted by potential predators more.
  • Tarantulas often need to dig large burrows or make large hides that are more accessible to potential predators.

How Big can Tarantulas get?

There are many different tarantula species, and their size varies as well. Some species are known in the tarantula keeping hobby as “dwarf” species due to their smaller size, and others reach over ten inches. On average, tarantulas grow to reach somewhere in-between 4.5 – 11 inches.

For measuring tarantulas, we do this diagonally. This means they are measured from the front right leg to the rear left leg and not from the front left leg to the rear left leg. It’s the standard method for measuring them. All tarantula sizes are taken from one front leg the other rear leg on the opposite side of the body.

Smallest Tarantulas in the world

It’s impossible to get one as a pet, but the smallest tarantula is the Spruce-fir Moss (Microhexura montivaga). An endangered species with limited information available. They only grow to 4mm max making them one of the smallest tarantula species alive today.

For pet tarantulas, “dwarf” species will reach a three-inch maximum size. These include the Pumpkin Patch tarantula (Hapolopus sp. Colombia), Trinidad Dwarf tarantula (Cyriocosmus elegans) and also the Brazilian Blue Dwarf Beauty (Dolichothele diamantinensis).

Dwarf tarantulas are usually faster than larger species.

Biggest Tarantulas in the world

The largest tarantula on earth is the Goliath Birdeater tarantula (Theraphosa blondi). Their body length reaches 5.1 inches while their legs can grow to a massive 11 inches. They are also the largest spider in the world by mass at 175g or 6.2 oz. However, they don’t have the longest legs out of all spiders. This species can be kept as pets and are commonly seen in the tarantula keeping hobby.

Almost all tarantulas from the Theraphosa genus are giant. The Burgundy Goliath Birdeater (T. stirtmi) can grow to an equally impressive size with adults reaching around 10 – 11 inches in leg span.

Other large tarantula species include the Brazilian Salmon Pink Birdeater (Lasiodora parahybana), Grammostola anthracina, and the Colombian Giant tarantula (Megaphobema robustum). 

Are there any Spiders are big as Tarantulas?

Most true spiders are relatively light compared to tarantulas. While tarantulas are large and heavy, some true spiders can grow to enormous sizes too. The largest spider in terms of leg span is the Giant Huntsman spider, not a tarantula.

The Giant Huntsman (Heteropoda maxima) has a leg span that can reach up to 12 inches or 30cm in length. Only discovered in 2001, these spiders were seen living in a cave in Laos. They have the largest leg span out of all spiders currently discovered. Giant Huntsman spiders are not available as pets, however.

Other large spiders such as Camel Spiders (Solifugae) can grow to reach 6 inches in length and are available as pets. However, some large spiders are more venomous such as the Brazilian Wandering spider (Phoneutria fera), which are slightly under 6 inches.


Tarantulas are an ancient group of spiders that haven’t changed much over the millions of years they’ve existed on Earth. As a result, they’re larger than most other spiders since their ancestors were so large. We might not know why arthropods reached such an enormous size, but it’s clear that the large size is beneficial to tarantulas today.

Written by:


Stuart is the editor of SpiderAdvice.

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