Anatomy Of A Tarantula

Getting Into Tarantula Dissection And Preservation

Most people who ask about tarantula anatomy have either just lost a pet and want to understand what they had, or they are a biology student who needs to do a proper dissection for a class. I've done both. I've spent over a decade keeping tarantulas and doing post-mortem exams on them when the cause of death wasn't obvious, so I'm going to walk you through the actual anatomy in a way that matters if you're trying to look at one of these things under a microscope instead of just reading a textbook diagram. A tarantula's body is divided into two main tagmata: the cephalothorax and the abdomen. This isn't unique to tarantulas. Almost all arachnids share this body plan. What makes tarantulas interesting anatomically is the size and complexity of some of their internal organs relative to other spiders. They have a much more developed circulatory system than you might expect for something people call a primitive arachnid. Their hearts are visible from the outside when properly prepared, and their fangs operate on a mechanism that's worth understanding before you ever attempt a dissection. The cephalothorax houses the brain, the heart, the main digestive glands, and the attachment points for all eight legs. The abdomen contains the book lungs, the spinnerets, the reproductive organs, and the majority of the digestive tract. Between these two sections is a narrow pedicel, which is actually a flexible waist-like structure that allows the abdomen to move independently. This matters because if you're trying to extract internal organs intact, cutting through the pedicel without proper support will tear everything apart.

External Structures You Need To Know

Before you cut anything open, you need to understand what you're looking at on the outside. The chelicerae are the fang-bearing structures at the front. In tarantulas, these point downward and strike with a vertical motion, unlike most spiders which have fangs that close sideways. The molars on the inside of the chelicerae are used for grinding prey. You'll see a pair of pedipalps just behind the chelicerae that look like little legs but aren't. In males, the tips of these are modified into bulbi for sperm transfer. If you're looking at a dead male and see bulbous structures at the end of the pedipalps, that's your sex determination right there. Males are also easy to identify by the tibial apophyses, which are small hook-like projections on the front legs that females lack entirely. The legs themselves have five segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. Some people count seven segments because the tarsus and pretarsus (the claw assembly at the end) are sometimes grouped separately. The claws at the end of each leg are a key identification feature. Myogenic species like Grammostola have claws designed for digging, while arboreal species like Poecilotheria have more hooks for clinging to bark. This is something I learned the hard way after trying to extract a specimen from substrate and spending forty-five minutes picking dirt out from under the tarsal claws with fine forceps. Spinnerets are the organ clusters at the rear of the abdomen where silk comes out. Tarantulas typically have three pairs. The anterior median spinnerets are the smallest and produce the sticky capture spiral silk. The posterior median spinnerets are where the strong dragline silk comes from. The lateral spinnerets handle the sheet silk and egg sac construction. This division of labor is important if you're trying to preserve silk glands for study, because each gland type connects to a different spinneret cluster.

Internal Anatomy And What It Looks Like When Fresh

The digestive system runs from the mouth through the pharynx, then the esophagus, which passes through the brain ring and connects to the midgut. The midgut in tarantulas is massively enlarged compared to other spiders because these animals are opportunistic feeders that can consume prey larger than themselves. The stomach has internal filters called gastric teeth that sort particulate matter from liquid nutrients. The hepatopancreas, or digestive gland, wraps around much of the midgut and serves dual functions: it produces digestive enzymes and stores nutrients. This is why tarantulas can survive months without eating. The hepatopancreas acts as an energy reserve. The circulatory system is open, meaning hemolymph bathes the organs directly rather than staying contained in vessels. The heart is a tubular structure running along the dorsal side of the abdomen with paired ostia that act as valves. When you open a fresh specimen from the back, you'll see this pulsing tube near the surface. It's pale and semi-transparent. The hemolymph itself is blue-green because it uses hemocyanin for oxygen transport instead of hemoglobin. This is standard across arthropods but easy to miss if you're expecting red blood like in vertebrates. The excretory system consists of coxal glands at the base of each leg and malpighian tubules connected to the hindgut. The malpighian tubules are thin and threadlike, extending from the junction between the midgut and hindgut. They remove nitrogenous waste from the hemolymph. In preserved specimens, these are extremely difficult to see unless you're working with something substantial enough to make a whole mount slide. I usually skip them unless I'm doing a formal anatomy presentation because they don't add much diagnostic value for basic identification purposes.

The Respiratory System

Tarantulas have two types of respiratory structures. Most have both a pair of book lungs in the abdomen and a set of tracheae that branch toward the front of the body. The book lungs are located on the ventral side of the abdomen, and when you do a ventral dissection, you'll find them as paired, leaf-like stacks of tissue that resemble pages in a book, which is exactly what they're named after. Each page is a thin membrane where gas exchange happens. The number of book lung pairs varies by species, and this is one of those taxonomy details that matters more than people think. Some genera have only one functional pair, and that gap is used in classification. Tracheae in tarantulas are less prominent than in web-building spiders but still present. They branch from openings called spiracles, usually located near the pedicel and at the tip of the abdomen. The tracheal system delivers oxygen directly to tissues, while the book lungs rely on hemolymph circulation. This dual system is efficient enough that tarantulas don't have the high metabolic rates of active hunting spiders, which explains their famously slow movement and low food requirements. If you're preserving a specimen and need to identify the species based on respiratory structures, focus on the book lung placement and spiracle positioning rather than the tracheal branches, which collapse almost immediately after death.

Reproductive Anatomy

This is where things get complicated quickly. Females have two ovaries in the abdomen, each connecting to an oviduct that opens into a common genital atrium. The atrium sits on the ventral side between the third and fourth leg pairs. Sperm enters through the epigynum, which is an external sclerotized structure that varies dramatically between species. The epigynum is actually one of the primary features taxonomists use for species-level identification, and it's surprisingly intricate even within closely related species. What looks identical externally between two closely related Grammostola species can have completely different epigynal structures. I spent three weeks arguing with a colleague over whether we had one species or two until we did a critical preparation of the epigynum and found the difference. Without proper staining and dissection, it was invisible. Males have testes in the abdomen, each connecting to a spermatheca via a vas deferens. The modified palps, or bulbi, store and transfer sperm during mating. The structure of the bulbus is highly species-specific and, like the female epigynum, is a taxonomic key feature. If you find a male and want to know what species it is, the palpal organs are often more reliable than overall coloration, which can fade significantly after death. I've seen multiple cases where preserved males were misidentified based on body color alone, and the palpal dissection corrected it every time.

How To Actually Dissect One

The method depends on whether you want a quick internal exam or a proper preserved specimen. For a quick exam, place the tarantula dorsal-side up on a dissection tray and pin the cephalothorax and abdomen using fine insect pins. Make a single midline incision along the dorsal abdomen with sharp scissors, being careful not to cut too deep and damage the organs beneath. The book lungs sit just under the cuticle, so you'll expose them immediately. From there, you can gently remove the cuticle with forceps and observe the hepatopancreas, heart, and gut. For a permanent preparation, clearing and staining is the standard approach. You need to fix the specimen first, ideally in 70 percent ethanol, then transfer it to a potassium hydroxide solution to clear the soft tissues. This usually takes anywhere from twelve to forty-eight hours depending on the size of the specimen. After clearing, you stain with alizarin red for bones and sclerotized structures and alcian blue for cartilage and soft connective tissue. Tarantulas don't have bones, but the chelicerae, leg joints, and epigynal structures stain well with alizarin. Once stained, you dehydrate through a graded ethanol series and mount in Canada balsam or a synthetic resin medium. A fully cleared and stained adult tarantula takes about a week from start to finish if you're working with something under five centimeters in body length. Larger specimens take longer because the clearing step is diffusion-limited.

Problems People Run Into

The most common issue is collapsing the abdomen during dissection. Tarantula abdomens are soft and pressurized internally. When you cut into one, the hemolymph and internal contents can ooze out in a way that ruins the organ arrangement. The workaround is to first freeze the specimen at minus twenty degrees Celsius for a few hours before dissection. This firms up the tissues and reduces fluid loss significantly. I started doing this after losing my first three attempts to abdominal collapse, and it changed everything for specimen quality. Another frequent problem is misidentifying the hepatopancreas as disease or parasites. The hepatopancreas is large, lobed, and pale yellow to cream-colored. When a tarantula is freshly molted and hasn't fed yet, the hepatopancreas can look shriveled and odd. Beginners often mistake this for a pathological condition. It's not. It's just a dormant digestive gland waiting for the next meal. If the animal is otherwise normal and you're seeing these structures, don't panic. The biggest limitation of tarantula anatomy work is that many internal features change dramatically depending on the feeding state. A fully engorged specimen is nearly impossible to dissect cleanly because the distended midgut pushes everything else out of position. If you can choose when to examine a specimen, wait until it has emptied its gut or is in the process of digestion but not at peak fullness. This usually means two to four weeks after feeding for most species at room temperature. I once tried to dissect a freshly fed Brachypelma and ended up with a soupy mess that took me six hours to even partially clean up. Never again.

There's also the issue of scale. Most tarantula anatomy resources are written for species that are twenty centimeters or more in leg span. If you're working with a small species like a Poeciloides or a young juvenile of any species, standard dissection techniques don't translate well. The organs are proportionally smaller and more fragile. You need finer tools and lower magnification objectives to actually see anything. A dissection microscope at forty to sixty times magnification is the practical minimum for sub-three-centimeter specimens. Below that, you're mostly guessing at organ identity unless you have a reference specimen to compare against.

What You'll Miss In Textbooks

Textbooks show tarantulas as static diagrams. In reality, the internal organs shift position constantly based on posture, digestion, and molting stage. A tarantula that has recently molted has almost no internal organ definition because the old exoskeleton was holding everything in place and the new one hasn't expanded enough yet. The hemolymph pressure distribution is completely different during this phase, and organs appear displaced or compressed in ways that look pathological but are normal. I've had people email me panicked photos of what they thought was a ruptured organ, and it was just a recently molted spider lying on its back with its abdomen inflated from water absorption. Another thing textbooks gloss over is the variation within a single species across different geographic populations. The abdominal segmentation patterns, the size of the gastric ceca, and even the relative size of the book lungs can differ between populations of the same species. This isn't always taxonomically significant, but if you're doing comparative anatomy, you need to account for it. I found this out when I compared two Chiloepialis specimens from different regions and initially thought I had two different species until I checked the original descriptions and realized the variation was intraspecific. The color of internal organs is another thing that disappears almost entirely in preserved specimens. Live tarantulas have distinct coloration in their hepatopancreas, heart, and reproductive organs. After fixation and especially after clearing, almost everything becomes translucent or pale white. If you're photographing internal anatomy for a guide or presentation, you need to do it quickly after dissection or use staining protocols to restore contrast. Clearing alone will make your specimens look like ghost outlines, which is informative but not very visually useful without additional preparation.

Where To Find Reference Materials

The most comprehensive single resource for tarantula internal anatomy is the work by Klaus Hebscher and colleagues, which includes detailed dissection photographs and illustrations across multiple families. It's paywalled through academic databases but worth the effort if you're doing serious work. For free resources, the Journal of Arachnology has occasional anatomy papers, and some university arachnology labs post dissection guides online. The tarantula hobbyist community has been building decent reference material over the last decade, particularly for species-level identification through genital anatomy, but this is scattered across forums and personal websites rather than centralized anywhere. If you want to see actual dissection procedures, there are a handful of YouTube channels run by arachnologists and skilled hobbyists who document their work. The quality varies enormously, but a few are genuinely accurate and show the dissection at enough magnification to follow along. Just be aware that most YouTube content prioritizes entertainment over accuracy, so verify any procedure you see against a peer-reviewed source before you attempt it on a specimen you care about. The bottom line is that tarantula anatomy is more complex than most people expect and more frustrating to work with than it should be. The specimens are fragile, the organs shift position depending on state, and the published references don't always match what you actually see under the scope. But if you take the time to learn the variations and build your own reference collection, you'll end up with skills that cover far more than just tarantulas. The dissection and preparation techniques are transferable to any arachnid, and once you understand tarantula anatomy, looking at a scorpion or a whip scorpion feels almost familiar by comparison.