Determining Sex of Theraphosidae with Exuviae

This is a guide for those who would like to learn the basics of determining the sex of Theraphosidae (tarantulas) via exuviae and how to properly process an exuvia for the best results. This guide will also go over the importance of knowing the structure of spermatheca as well as determining the presence of male accessory organs that may be confused as spermatheca.

Spermatheca Structure and Accessory Organs

Before attempting to determine the sex of a tarantula via the exuvia, it is very important to know the species of the specimen and the unique structure of spermatheca that belongs to that species. This will prevent confusion or the possibility of mistaking male accessory organs as spermatheca. While not the case for all species, there are species that possess larger or more prominent accessory organs, which can make determining the sex difficult if one does not know which structure to look for.

It is also important to know the structure of the spermatheca to confirm which species the specimen is. While not always the sole indicator nor exempt from a degree of variability, the structure of the spermatheca could be used to identify the species of the specimen being worked with. This can be an important method to use when species are very similar to each other visually or have similar or overlapping natural ranges.

Below are a few clear examples of male accessory organs, both of which are Tliltocatl albopilosus. Note that the structure of the male accessory organs are vastly different from the spermatheca structure of Tliltocatl albopilosus.

To provide an example of the spermatheca structure of Tliltocatl albopilosus, the below images are of female specimens.

Preparing an Exuvia

After determining what the specimen’s reproductive organ structure should be like according to the species, the next step is to obtain an exuvia, or molt, from the specimen. It is ideal to frequently check a specimen’s enclosure for a molt when premolt behavior such as a darkening abdomen is observed. It is best to work with exuvia that have been freshly cast off as they are less fragile and more pliable. The cast off exuvia can be taken from the specimen’s enclosure as soon as the specimen is completely free from the old skin.

To prepare the exuvia prior to using a microscope or other tools to process the exuvia, it will likely require further softening, even if it is freshly cast off. To soften the exuvia, it is recommended to use a deli cup or other container filled with enough water for the molt to properly submerge. As Theraphosidae exuviae are hydrophobic and thus do not soak up water effectively, a few drops or light drizzle of dish soap such as Dawn should be added to the water prior to adding the exuvia to the water. This will allow the water to penetrate the exuvia and soften it. Allow the exuvia to rest in the water/dish soap solution for roughly 10-15 minutes to assure that the exuvia becomes adequately pliable.

When the exuvia has rested, use a pair of tweezers to gently scoop it out of the water/dish soap solution. It is best to lay the molt on a paper towel to remove excess water. As the excess water is wicked up by the paper towel, processing the exuvia can begin. To start, remove the carapace by gently pulling it back and removing the attached portion of the abdomen with it.

The following process is for specimen exuviae that are 3” diagonal leg span (DLS) or larger that will be examined under a microscope.

After the exuvia is laid atop of the paper towel, use a pair of sharp scissors to cut through the cephalothorax of the exuvia, located above the last pair of legs before the abdomen. With the top portion of the cephalothorax removed, proceed to cut off the legs at the base of the joint. What is left is the area of the abdomen processed enough to fit on a microscope slide.

Determining Sex From Exuvia

Using a pair of sewing pins, needles, toothpicks, or other similar items, it is time to check the exuvia for spermatheca or the lack of. It is recommended to use items like this to gently pry open the abdomen portion of the exuvia as the skin is thin and easily torn if blunter items are used. Make sure that the abdomen of the exuvia is not twisted and is facing the correct way. The hairs of the abdomen should not be facing up and the two sets of book lungs at either side of the inside of the abdomen should be white, indicating that the inside of the abdomen is being observed.

After the exuvia has been properly prepared and the inside of the abdomen has been spread to display the correct anatomy, it is recommended to use a source of light to illuminate underneath the abdomen. This better shows any structures that may be present, especially if the spermatheca is not sclerotised (hardened and darkened). When a spermatheca is sclerotised, it is the indicator of a mature female and will be much darker than immature or developing spermatheca. It may be difficult to observe immature or developing spermatheca without a microscope or proper lighting from beneath the abdomen of the exuvia.

The area of interest for viewing structures within the abdomen of the molt is between the anterior (top pair) set of book lungs. This pair of book lungs is located closest to the cephalothorax of the molt. Spermatheca will be visible with uterus externus for most species, with the exception of species such as Encyocratella olivacea, which lack spermatheca but still possess uterus externus for determination of sex via exuvia.

Using a microscope or other means of magnification is recommended to provide a clearer image of any structures present. Magnification will vary depending on the size of the exuvia being worked with. Adults may require anywhere from 2x-40x and juveniles to slings may require up to 80x magnification.

Using the set of tools mentioned previously, gently manipulate the area between the anterior set of book lungs. If spermatheca and uterus externus are present, the structures will move independently when disturbed by the sewing pins or choice set of tools. These structures will resemble what is commonly described as a “flap” and should be able to be flipped back and forth. Male accessory organs will not move independently and will be fused to the exuvia.

Spermatheca Examples

The following references are of female specimens of varying species. Each will be labeled with the corresponding species.