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Keeper guide

Sexing your tarantula

The moult tells you for certain. Real exuvium photographs below show exactly where to look - then record the answer on your specimen.

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Moult sexing is certain

Reading the exuvium is the only 100% method. Ventral guessing from above is maybe 80% - the moult is definitive.

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Look between the book lungs

Both sexes show their structures in the same place: the epigastric furrow area between the front pair of book lungs.

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Size matters

Very small slings are hard to read even under a loupe. If you cannot see anything, wait a moult or two.

Step 1

The view you need

Flatten the dried exuvium abdomen-up so you are looking at the INSIDE. The four pale slits are the book lungs - the front pair frame the area where everything happens. The curved line between them is the epigastric furrow.

Ventral view of a tarantula exuvium showing the book lungs and epigastric furrow

On this view, check two things

  • 1 Above the furrow (towards the head): dark, rigid structures = female spermathecae
  • 2 Below the furrow (towards the rear): a tiny bristle patch = male fusillae

Photo: tarantula exuvium, ventral view (Uwe Kils, Wikimedia Commons, CC BY-SA 3.0).

Step 2

Female: spermathecae

Dark, rigid, chitinous structures sitting above the epigastric furrow, between the front book lungs. This is where she stores sperm - it holds its shape out of the moult and looks almost machine-made.

Female tarantula moult with the spermatheca highlighted between the book lungs

Annotated female exuvium - the spermatheca is highlighted where the arrow points, between the anterior book lungs (Oneillbeck, Wikimedia Commons, CC0).

Acanthoscurria geniculata spermathecae under a microscope

A. geniculata spermathecae under magnification - note the paired structure (Viki, Wikimedia Commons, CC BY-SA 3.0).

Shapes vary by family - all of these mean female

Diagram comparing fused and paired spermathecae between two Tliltocatl species

Scientific diagram: Tliltocatl vagans (fused) vs T. kahlenbergi (paired) (Micha L. Rieser, after Rudloff 2008, Wikimedia Commons, CC0).

  • • New World terrestrials (Brachypelma, Grammostola, Aphonopelma): two separate fingers, or fused into one arch
  • • Old World terrestrials (Hysterocrates, Pelinobius): long, paired, often curved outward
  • • Avicularia-type: two soft rounded lobes close to the furrow

Step 3

Male: epiandrous fusillae

A small patch of tiny dark bristles just below the furrow, between the front book lungs. Males use them to spin the sperm web - females never have them. No freely-licensed macro photo exists yet, so this is a simplified schematic of what to hunt for:

epigastric furrow FUSILLAE - bristle patch BELOW the furrow schematic - zoom any real exuvium photo to spot the individual hairs
  • • To the naked eye it looks like a blurred dark smudge - a loupe or phone macro photo resolves it into individual hairs
  • • Easier to spot on exuviae of 4cm+ DLS specimens
  • • If you see fusillae, it is male. Done - nothing else to check
  • • No fusillae AND no spermathecae? Immature specimen - recheck next moult

Mature males make all of this unnecessary - tibial hooks and bulbous pedipalps are visible from across the room.

More female examples

Spermathecae across species

Same structure, different shapes - once you have seen one, you will recognise them all.

Grammostola rosea spermathecae, fused arch shape

Grammostola rosea - the classic fused arch (Viki, Wikimedia Commons, CC BY-SA 3.0).

Acanthoscurria geniculata paired spermathecae

Acanthoscurria geniculata - clearly separated pair (Viki, Wikimedia Commons, CC BY-SA 3.0).

Check your own exuvium photo

Upload or drop a photo of your moult's epigastric area. It stays on your device - nothing is uploaded to any server. Hover to magnify and compare against the photos above.

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Drop your exuvium photo here

or

Tip: photograph the INSIDE of the abdomen, flattened, with light behind it - the book lungs and furrow should all be in frame.

Your verdict

Create a free account to record sexing results straight onto your specimens.

Common mistakes

  • x Reading the moult right side up - structures are on the INSIDE (abdomen-up view)
  • x Confusing book lung edges or the furrow itself for spermathecae - spermathecae are free-standing structures, not part of the fold
  • x Declaring "female" because you cannot see fusillae - small males have faint patches; absence of one thing is not proof of the other
  • x Trusting ventral markings over the moult - ventral patterns mislead constantly
  • x Throwing the exuvium away before checking - every moult is a free, definitive sexing opportunity

Ventral sexing (the live guess)

With the spider standing, photograph between the front book lungs:

  • • Males may show a dark smudge between the furrow and the book lungs - the fusillae through the skin
  • • Females tend to have a wider, straighter furrow with clean skin around it

Treat ventral reading as a hint, never an answer - accuracy is roughly 80% at best, worse on juveniles. A moult confirms; ventral only suggests.