[Book Excerpt] Basic Spider Anatomy

The following article, excerpted from the PCT Field Guide for the Management of Urban Spiders, describes the various parts of a spider’s anatomy, focusing on those elements used for identification purposes. Almost all of the anatomy described here will be external as there is little that a pest professional would be using on the internal anatomy of a spider for identification. Some information about internal structures is mentioned where it helps explain the external parts that are used for identification.

Cephalothorax Region. Insects have three main body parts — head, thorax and abdomen. Spiders have two main body parts; the head and thorax are fused together in what is known as the cephalothorax; the second body part is the abdomen. The cephalothorax is the head end of the spider and is the location for the eyes. It also is where the legs attach, the mouthparts (the chelicerae and fangs) start the feeding process, and, internally, where the venom glands are located (in mygalomorphs in the chelicerae; in the common spiders in the cephalothorax itself). The top or dorsal surface of the cephalothorax is called the carapace. The bottom or ventral surface is called the sternum.

Eyes. The eyes of the spider are unique among arthropods in that they are more similar to human eyes rather than to insects, which have compound eyes made up of dozens to hundreds of facets. Whereas insects see and have to interpret a mosaic made up of many images, spiders that have good vision see more of an image like we do. However, many spiders have simple eyes more likely used merely to distinguish light from dark. Most spiders have eight eyes; some like the recluse and spitting spiders have six; in Europe some have four; two-eyed spiders run around the Southern California deserts; and cave spiders have no eyes. However, most of the spiders you will be trying to identify will have eight eyes with a few having just six. Be forewarned, though, that very often a pair of eyes can be very small and very close together so they appear to be only one eye, which will throw off attempts to identify it.

Figure 1: Jumping spiders are easy to identify by the large middle eyes in the anterior (bottom) row.

The pattern of eyes is very useful in identifying some spiders to the family level. For example, only the family known as jumping spiders has a large set of anterior eyes in the middle of the front of the carapace with smaller eyes located behind (Figure 1, right). Jumping spider vision is so well developed that it rivals human eyes in the ability to distinguish objects. Wolf spiders have a row of four small eyes across the front but have two large “headlight” eyes above this row which allows for keen vision. With the well-separated eyes placed around their carapace, the wolf spider can see 360º around itself.

Legs. Without exception, all spiders have eight legs as long as they haven’t lost any to predators. Each leg is composed of seven segments (Figure 2, below). Nearest the body a small segment, called the coxa, connects the cephalothorax to the rest of the leg. The next segment, even smaller and called the trochanter, is usually only visible if you bend the rest of the leg away from the body. From here, the leg segment names become more familiar: femur, patella, tibia. The two end segments are the metatarsus and the tarsus.

At the end of the tarsus are claws, which the spider uses for running or grabbing the web as it moves through the silk strands (Figure 3, below). A spider can have either two or three claws. If two claws, the spider usually also has tufts of hair at the end of the tarsus and/or along its ventral length. Two-clawed spiders usually are hunters that run around the ground or in vegetation. If three claws are present, usually the legs have less hair and the spider is usually a web spinner, although some are still hunters. One interesting aspect of spider legs is that they only have muscles for contracting the legs. To extend them, they use a hydraulic system where they push fluid into a leg to get it to move away from the body.

Spider legs are usually covered with thick spines and/or fine hairs. These are all useful to keep the spider informed about its environment, either approaching prey or predators. Some very fine hairs detect wind movements and are very flimsy. For the advanced arachnologists, the claws, pattern of spines, pattern of hairs, etc., are very useful for identification purposes. However, all these features require the use of a good quality microscope and much practice.

Pedipalps. The pedipalps are on the front of the spider’s body in front of the first pair of legs and, typically, look like miniature legs. They are often called “feelers” by the general public, and their function is, indeed, mostly sensory as well as prey handling. Although the pedipalps look like legs, if you examine them closely you will see that they have six segments and not seven as do legs.

In most spiders, the pedipalps have no use whatsoever for identification purposes. As the spiders go through various molts, the pedipalp looks like a small leg. In the females, there are no drastic changes throughout her life. However, in the males, this structure develops into the reproductive organ (Figure 4, above). When the male enters the molt before maturity (called the “penultimate” molt), the tip of the pedipalp becomes swollen. At this point, no structures are present on the pedipalp, thus useful only to identify that the spider as a male with one more molt to go before becoming fully mature. Once the penultimate male molts, the swollen bulb acquires various structures that are important in helping him align with the female’s genital openings and transfer sperm for mating. These structures are usually very consistent in form within a species but are very different among species. Because many spiders have poor vision, this is the mechanism that allows them to prevent mating with the wrong species in nature. These differences are also exploited by voyeuristic arachnologists who use the variation to separate species. Although it is most proper to call these palps, many people refer to them as “boxing gloves.” Using the genitalia to separate species is beyond the scope of this article but the non-arachnologist can use them to identify a mature male.

Chelicerae. The chelicerae are the movable mouthparts in the front of the spider, the tip of which houses the fangs. Just behind them are the mouthparts through which the spider intakes its nutrients.

The chelicerae are very useful for identifying the major groups of spiders. The mygalomorphs have parallel chelicerae such that they have to raise the front of their body upwards to swing the chelicerae outward, and unfurl the fangs in order to accomplish a downward strike into the prey (Figure 5a). The common spiders have opposing chelicerae that are more like pliers (Figure 5b). In order to envenomate prey, they spread their chelicerae sideways, unfurl the fangs and bring them together like pincers. The hunting spiders typically have teeth bordering the furrow in which the fang rests (similar to a knife blade in a knife when it is sheathed). These teeth help the spider grab and hold on to its prey while the venom takes effect. Because web spinners typically use silk to wrap their prey, they usually lack these teeth.

Abdominal Region. The abdomen is the second and more posterior main body portion of a spider. In many arachnids (e.g., scorpions, daddylonglegs), the abdomen is composed of segments, but almost all the spiders encountered by the pest professional will have unsegmented abdomens. However, abdominal segmentation occurs in very primitive, “missing link” spiders in Indochina, and one can find a few American mygalomorphs which have remnants of their segmented evolutionary past. Many spiders have stripes or chevrons running cross-wise over the dorsal surface of the abdomen; these are additional indications of the segmented past of their ancestors.

Book Lungs. The respiratory organs of spiders are called book lungs and are located on the underside of the abdomen toward the cephalothorax. They are usually lighter in color than the rest of the abdomen. They have slits where air enters, and the lungs contain sheets of tissue for oxygen exchange within the body. Because several sheets are piled on top of each other, they resemble the pages of a book and, hence, the name. This can be a diagnostic identifier because mygalomorphs have two pairs of book lungs (Figure 6), and common spiders have one pair (or in the smaller and more advanced common spiders, they have lost the book lungs and replaced them with a system of conduits called trachea, similar to insects).

Spinnerets. At the posteriormost portion of the spider’s body, tube-like or conical structures can be found poking out (Figure 7, below). These are the spinnerets and are the organs through which silk is emitted. The ancestral spider had four pairs of spinnerets, but one pair of them has become vestigial or disappeared completely in the common spiders. Most mygalomorphs have lost yet another pair so they only have two pairs of spinnerets. Most spinnerets are small, but some spiders have spinnerets that can be about ¹⁄³ the length of their bodies. The length or shape of the spinnerets is diagnostic for some spider families or genera.

The surface of each spinneret is covered with spigots which correspond to specific silk glands inside the body. The different silk glands have different purposes and will be covered in the next paragraph. The placement of the spigots are specific to certain spinnerets; for example, in orb-weavers, the spigots that exude the main dragline silk are typically on the front spinnerets and the spigots that exude the sticky silk are on the hind spinnerets because as the spider moves through its web, it needs to be laying down the dragline silk first before it can cover it with sticky droplets for prey capture.

As mentioned previously, most spiders have six spinnerets with one pair disappearing. However, in some genera, the fourth pair of spinnerets has evolved into a flat plate with many spigots. This plate is called a cribellum (Figure 8, above). Spiders that have a cribellum also have a row or two rows of special curved hairs on the metatarsus of the fourth leg, which is brushed over the cribellum to pull out the silk. These curved hairs are called the calamistrum. Creatures with these structures are called cribellate spiders. Their silk is not sticky like other spiders but instead look like puffy skeins of yarn that one would buy in a craft store. Although cribellate silk is not sticky, it acts like Velcro®, where the hooks and spines of insect legs get tangled up in the puffs of silk. This may be useful in identification because cribellate silk looks more like cotton candy and has a bluish tint to it. In comparison, the silk of non-cribellate spiders that use drag lines (like the black widow or the orbweavers) looks more like thin fishing line which glistens in the sun. One very frustrating aspect for the beginning arachnologist is that because the cribellum and calamistrum are used to spin silk, male cribellate spiders lose these structures upon maturity because, at that point, they abandon their webs to run around looking for females. In order to identify mature male cribellate spiders, one must first learn what the females look like and then extrapolate the other features of the spiders to the males.

Silk Glands. The evolution of silk in spiders is a topic that is exhaustively researched. Silk production is one of the evolutionary advances that have allowed spiders to become so successful in such a great diversity of habitats. Most spiders have four types of silk glands and each silk has a specific purpose. Some silk is used for making structural webs, another gland only produces attachment discs so the spider can attach other silk to the substrate. The egg sac is covered with its own special silk. Additional silk glands are found in the orbweavers and combfoot spiders where they produce sticky globules of silk that are laid down over the dragline silk in forming a web that filters the environment for flying prey.

Reproductive System. Just to the rear of the book lungs in the common spiders (or the anteriormost pair in the mygalomorphs) is a line that runs across the body. This is called the epigastric furrow. Just forward of this furrow between the book lungs is where the reproductive organs of spiders are found. In the male, these are of no diagnostic importance because once the male matures, he squeezes sperm out of an opening that is attached to the epigastric furrow and suctions up the sperm into his palp. However, this is where the female’s reproductive organs lie and is the target for the male’s palp during mating. In the more simplified spiders, very little indication of female genitalia occurs here; however, in the common spiders, the genitalia is usually a hardened plate called an epigynum (Figure 9, right). This structure consists of openings into which the male inserts structures of his palp and internal ductwork that carries sperm into the spermathecae where it is stored until the female extrudes eggs into an egg sac, fertilizing the eggs as they are passed out of the body. Just like the male palp, the female epigynum is typically very consistent in its appearance, and, therefore, is a diagnostic feature for arachnologists to certify a species’ identity. However, this often requires the advanced skill of dissecting the area under a high quality microscope. For the pest professional, awareness of the epigynum may be just sufficiently useful to be able to identify a spider as a mature female. In some spiders, the epigynum is darkened and very identifiable; however, in other species, there may only be a small opening which may not be visible except under high magnification.

Conclusion. To properly identify spiders, pest management professionals need to have an understanding of basic spider anatomy. By familiarizing themselves with the key body parts of these ubiquitous pests, they will be better prepared to identify spiders in the field and answer customer questions about these common home invaders.
 

Stoy Hedges is editorial director of the 10th Edition of the Mallis Handbook of Pest Control and director of technical services at Terminix, Memphis, Tenn. Richard Vetter recently retired from a research position at the University of California at Riverside, where he developed a reputation as one of the country’s leading arachnologists. Hedges can be reached via e-mail at shedges@giemedia.com while Vetter can be reached via e-mail at rvetter@giemedia.com.