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Sexing

Sexing is one of the eternal dilemmas for tarantula keepers. For many, it remains so. For others, with practice and access to a reasonable number of specimens, it becomes second nature. In general, there are two methods, examination of the inside of the discarded molt of the tarantula for the presence or absence of an organ known as a spermatheca, or examination of the outside, ventral (bottom) surface of the tarantula or molt for the presence or absence of bristles known as epiandrous fusillae.

Method 1 Internal Molt Spermatheca

We will start with the inside view, because for many, particularly the inexperienced, it can be most definitive, especially with medium to large specimens. To sex a tarantula in this way, the first thing needed is a discarded molt. The area to be examined is located at the anterior (towards the head) end of the opisthosoma (abdomen) near where it joins the carapace. In this general area will be found the anterior pair of book lungs with what is known as the epigastric furrow in between. From the outside of the molt, this appears as a ridge atop a groove. This is where sperm is received from the male. But from the inside of the molt, this groove, which the male does not possess, forms a tabular structure known as a spermatheca. It is the absence or presence of a spermatheca that we look for inside a molt. The presence indicates female, the absence indicates a male, or in the case of a smaller tarantula, an undeveloped or inconclusive situation. In sexing with this method, size definitely counts! Some images below will help to illustrate this process. Each photo may be clicked for an enlarged view. To the left in each picture pair will be the dorsal (inside) view of the molt, and on the right the ventral (outside) view of the same molt for comparative purposes, though in this stage our focus will be on the picture to the left of each pair.

This is the molt of a Chromatopelma cyanopubescens. Note the two prominent white markings to each side in the middle of the picture. These are book lungs. Above the right one, you will see the right anterior book lung, also white. The left is obscured by a rolled edge of the molt. Between these two anterior book lungs, you should see a dark, shiny trough with a pink mass at the bottom. This mass is a well developed spermatheca, indicating that we are looking at a female.

The molt above is from an Acanthoscurria geniculata. See the tab between the anterior book lungs? This is also a female.

This molt is from a Theraphosa blondi. Do you see the tabular spermatheca between the anterior book lungs? No? Good. You've just diagnosed a male.

Method 2 External Epiandrous Fusillae

This time we are going to make another pass, focusing on the right picture of each pair, the outside of the molt. 

Notice that where on this photo of a female to the left we see a spermatheca, on the photo to the right, one can see a pronounced bump of the epigastric furrow and uterus externus.

The bump on this female on the photo to the right is not as obvious. However, note the short 'velvet' appearance of the bristles on this right photo and the female above, plus the appearance of a slightly 'curled' appearing lip at the line demarcating the bottom of the patch of bristles. Subtle to be sure, but an important clue.

Now for the male. Please notice several things on the photo to the right. First, there is no bump at all. Second, whereas the triangle formed in the photos of the females are quite pronounced, the so called 'triangle' of this male is quite different. In addition to the short, 'velveteen' hairs of the female, longer hairs, different in length from the surrounding hairs can also be seen. Having a beard myself and being male, I like to compare the look of the male triangle to the triangular portion of a man's beard right below his lower lip. While the surrounding beard lays relatively flat, the triangle beneath a man's lip tends to be more sparse, and the hairs stand up to some extent rather than lay completely flat (especially if the person is biting their lower lip) giving an appearance much like what we are looking for in a male tarantula. Bristles of this nature in the triangular area of a male tarantula are known as epiandrous fusillae. These structures assist the male in making a sperm web.

Don't see the epiandrous fusillae in the above photo? Don't despair. It can be quite difficult as two dimensional photographs do not capture the level of detail that we are looking for well. Time for one more example.

Do you see a spermatheca on the left photo? No? Good. This is a male B. emilia. In the photo to the right, can you make out a row of longer hairs? Typically what we are looking for appears as a triangular or crescent shaped patch of bristles that are of a different length than surrounding bristles. Bristles of the females tend to be of equal length, more similar to crushed velvet than the patch of upright bristles in a male that can resemble the triangle of a man's beard. Notice also that this male has no bump corresponding to a spermatheca, since it has no spermatheca. 

You may also notice in some cases that males are generally slimmer in build (provided they are not power fed), may be more active even prior to maturity, and often have thinner chelicerae than a female of the same size. As you are learning, you may put several of these cues or clues together to come to a confident diagnoses. An additional indicator is the angle of the book lungs, those of males tending to be straight across while those of the female may slant a bit downward in the center, but this is likely a result of deformation of the area due to the aforementioned prominence of a bump, and possibly the curvature of greater girth. It has also been noted that this bump only becomes sclerotized (hardened) in mature females (hint, hint, hint). And as you become confident of your diagnoses, you will have LIVING tarantulas upon which to look for or identify the absence of epiandrous fusillae. Being able to do so will allow you to sex your animals, especially the larger ones, in the field or in a pet store, in the absence of molts, and with a high degree of confidence. 

A further resource recommended for information on the epiandrous fusillae method of sexing is 'The spinnerets and epiandrous glands of spiders' by B.J. Marples, Journal of the Linnean Society, 46, 310, pg. 209. This publication mentions epiandrous fusillae, and is the publication that brought them to the attention of Rick West, and subsequently Mark Hart. Unfortunately, to some extent, due to the limitations of photos, it appears that each interested party must, in a way, rediscover them for themselves. 

Of use for assistance in identifying spermatheca, associated structures, or lack thereof is 'Sex Determination of Immature Theraphosid Spiders From Their Cast Skins' by Kathleen and John Hancock, ISBN 0 9515324 0 5.