Spiders (Insects)

For many, spiders are a cause of fear and a source of revulsion. Even to entomologists, spiders have often been thought of as a mere annoyance, filling nets and pitfall traps meant for insect quarry. It is therefore surprising to learn that spiders have held a prominent role in traditional cultures for centuries. Indeed, the terms “arachnid” and “arachnology” come from Greek mythology: A young maiden, Arachne, dared to challenge the great goddess Athena to a spinning contest. Athena wove a remarkable tapestry, yet that spun by Arachne was far superior in its intricacy and perfection. Infuriated that a mere mortal could spin such a masterpiece, Athena turned Arachne into a spider, condemned to a life of perpetual spinning. In native American culture, it was a spider that brought fire to the Cherokee people; the Navajo told of Spider Woman (Na ashje’ii’ Asdzaa), who came from the “first world” and taught the women how to weave; according to Pueblo legend, Spider Woman was at the core of creation; and the Sioux Indians use the “dream catcher,” spun by a spider, to capture the good dreams of life. Spiders are also prominent in African culture, as illustrated in the well-known stories of “Anansi, the Trickster.” Rather more recently, Scottish legend tells of King Robert the Bruce, whose observations of a spider inspired him to persevere, going on to conquer the English. The reasoning behind some of the tales can be obscure. For example, in the far-flung Micronesian island of Palau, the ability of women to perform natural childbirth is attributed to a spider. With such a diversity of lore, it is clear that spiders have been held in high regard across a global spectrum of cultures and for a very long time.
Spiders, like insects, belong to the phylum Arthropoda, but they are in the class Arachnida, which includes the orders Acari (ticks and mites), Scorpiones, Pseudoscorpiones, Opiliones (harvestmen or daddy-long-legs), and several less common orders. Arachnids are only distantly related to the other major terrestrial arthropod group, the insects, and represent a separate evolutionary transition from marine to terrestrial living, because their closest living relatives are thought to be the marine horseshoe crabs (xiphosurans) and sea spiders (pycnogonids). Together, these marine and terrestrial orders are called the chelicerate orders based on the structure of their mouth-parts, in contrast to the orders to which insects and crustaceans belong. Recent molecular and morphological evidence points to the Amblypygi, or tailless whip scorpions, as the group sharing the most recent common ancestor with spiders, with the other arachnid orders more distantly related. Spiders can easily be distinguished from other arachnids by their lack of visible segmentation and the marked constriction between the prosoma and the opisthosoma, dividing the body into cephalothorax and abdomen, respectively.
Although much less diverse than insects in habits and morphology, spiders, which are in the order Araneae, nonetheless occupy nearly all terrestrial environments and can be found wherever there are other terrestrial arthropods to prey upon. Research into spider biology, particularly the diversity of silks, webs, and venoms, together with the associated ecology and behaviors, has increased greatly in recent decades. Moreover, phylogenetic advances are beginning to provide the context for comparisons between spider taxa and between spiders, other arachnids, and other terrestrial arthropods.

EXTERNAL ANATOMY

The body of all arachnids is divided into an anterior prosoma and posterior opisthosoma, and in spiders these major divisions are referred to as the cephalothorax and abdomen (Fig. 1) . These are connected by a narrow stalk, the pedicel. The cephalothorax is covered dorsally by the carapace and ventrally by the sternum and labium. Unlike in insects and most other arachnids, the segmentation of the spider body is not visible externally (except in two primitive lineages) and the cuticle is relatively soft, particularly on the abdomen.
At the front end of the carapace are the simple eyes, or ocelli, usually in four pairs, but in some groups one or more pairs may be reduced or absent. The eight simple eyes are usually arranged in two rows of four, though each of these rows may be curved in such a way that individual eye pairs seem to form their own rows, and in some species one, two, three, or even all four pairs of eyes may be lost. The anterior eye row is closer to the chelicerae, whereas the

FIGURE 1 External anatomy of a spider. (A-B) Ventral view of abdomen, showing (A) four pairs of topic lungs, and (B) two pairs of topic lungs. (C) Ventral view of prosoma. (D) Ventral view of female cribellate spider, showing cribellum. (E) Ventral view of female ecribellate spider, showing colulus. (F) Lateral view of leg. (G) Tarsus of leg of three-clawed spider. (H) Tarsus of leg of two-clawed spider and claw tuft. (I) Dorsal view of prosoma (cephalothorax) of jumping spider. (J-L) Anterior view of carapace and chelicerae of (J) jumping spider, (K) tangle-web spider, and (L) orb-web spider. ale, anterior lateral eyes; als, anterior lateral spinnerets; ame, anterior median eyes; ple, posterior lateral eyes; pls, posterior lateral spinnerets; pme, posterior median eyes; pms, posterior median spinnerets.
posterior eye row is farther back on the cephalothorax. Each eye row consists of median and lateral eye pairs, so that each pair can be identified both by row and by position within that row. For example, the anterior median eyes (AMEs) are the central pair, closest to the che-licerae. Below the front margin of the ocular area is the clypeus, from which the two chelicerae extend downward. Each chelicera consists of a stout basal section, from the outer corner of which articulates a narrower distal section, the fang. Behind the mouth ventrally is a second pair of mouthparts, leg-like in appearance, the pedipalps. In mature males, these are modified for secondary sperm transfer and appear swollen, facilitating recognition of the spider’s sex.
Extending laterally from the ventral cephalothorax are four pairs of legs, each consisting of seven segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The legs are generally covered with hairs and often have a great diversity of spines, bristles, and scales. These outgrowths serve a variety of functions, including mechanical and chemical sensory functions (see below). Each leg terminates in two or three tarsal claws. In three-clawed spiders, there are usually two larger paired claws and a smaller unpaired median claw, whereas in two-clawed spiders, the median claw is often replaced by a tuft of dense, stiff hairs, called the claw tuft.
A spider’s abdomen is carried behind the cephalothorax. The abdomen may be globose or elongate in appearance and is sometimes covered in hairs or scales similar to those found on the legs. The respiratory topic lungs (one or two pairs) open at the anterior end of the abdominal venter. They can be seen externally as patches of hairless cuticle on the venter of the spider, adjacent to the genital opening. In mature female spiders, the genital area is found nestled between the topic lungs, the form varying from a simple slit or pair of holes to a complex sclerotized copulatory plate, the epigynum.
At the posterior end of the abdomen are the spinnerets—usually in three pairs, the anterior, posterior, and median pairs—and the anal tubercle. The median spinneret pair is often obscured by the larger anterior and posterior pairs, and the entire complex of spinnerets may be surrounded by a sclerotized . Each spinneret pair has its own complement of silk spigots that extrude silks for specific functions. In addition, many spiders have a cribellum, or spinning plate, found adjacent to the standard spinneret complement that they use to produce an ultrafine looped silk for prey capture. In others, the cribellum has been reduced to a small vestigial lobe, called the colulus, or lost completely.
The sex organs of male spiders are the palps (Fig. 2A-2G). The cym-bium, or modified tarsus of the mature male palp, is hollowed out to contain the copulatory organ. The basal appendage of the cymbium is called the paracymbium. The bulb of the male’s palp opens through a spine-shaped apparatus, which ends in a fine tube, the embolus. The male palp is generally less complex in primitive spiders (haplogynae) relative to more advanced spiders (entelegynae). More complex palps have hard sclerotized parts (sclerites, which include the conductor, embolus, tegu-lum, etc.) and soft parts (hematodochae), both of which can carry prominences, or apophyses. At rest, most structures are folded away, with the delicate sclerites protected. During mating, the hematodochae are filled using hydraulic pressure and the sclerites extended, as the male inserts his palp into the female epigynum. The intromittent portion of the palp must navigate through the complex ducts of the female to achieve sperm transfer, while other sclerotized projections on the palp help to lever the embolus into position during copulation. The actual copulation has been referred to as a “lock and key” mechanism because of the often complementary species-specific form of the genital structures.
The female genitalia (Fig. 2H-2N) of primitive spiders comprise a ventral fold within which a hidden opening leads into a single duct, or egg tube. This tube serves for sperm insemination and egg laying. In more advanced spiders (entelegynae), there are two openings, the egg tube and the genital tube, both hidden by an external structure, the epigynum. The epigynum is sclerotized and becomes visible upon maturity. The complex structures on the female epigy-num guide the male palpal bulb through a convoluted set of internal ducts into the female’s genital tube, which leads to the sperm pocket where the sperm is stored.

INTERNAL ANATOMY

The central nervous system of the spider is located in the cephalotho-rax and consists of two main ganglia, the larger subesophageal ganglion and the smaller, more anterior supraesophageal ganglion, sometimes

FIGURE 2 Spider genitalia. (A-G) Comparison of male palpal structures showing the change in complexity in different lineages of spiders (see Fig. 5 for major lineages). Lineages represented are (A) Eurypelma sp. (Mygalomorphae), (B) Atypoides sp. (Mygalomorphae), (C) Kukulcania sp. (Haplogynae), (D) Hololena sp. (RTA clade), (E) Theridion spirale (Orbicularia), (F) Araneus gigas (Orbicularia), (G) Araneus gemma (Orbicularia). Abbreviations of structures: c, cym-bium; co, conductor; e, embolus, p, paracymbium; r, retrolateral tib-ial apophysis. (H) Diagram of epigynum, ventral view. Abbreviations of structures: d, coiled ducts; f, fertilization duct; g, genital opening; s, seminal receptacle. [Reprinted, with permission, from Foelix (1996), Fig. 135, © Oxford University Press.] (I-N) Comparison of female genital structures. (I) Antrodiatus sp. (Mygalomorphae), (J) Kukulcania sp. (Haplogynae) showing simple slit opening of seminal receptacles (no epigynum), (K) Hololena sp. (RTA clade) showing sclerotized epigynum, (L) Theridion spirale (Orbicularia), (M) Araneus gigas (Orbicularia), (N) Araneus sp. (Orbicularia) showing elaborate sclero-tized epigynum and scape(s). Panels A, E, F, L, and M, adapted from Comstock, J. H. (1912). “The Spider topic.” Doubleday, Page, Garden City, NY. Panels B, C, G, I, J, and N, photographs (microscopic, auto-montage) taken by the authors. Panels D and K drawn by the authors.
referred to as the “brain” of the spider. The two ganglia are divided horizontally by the esophagus, and nerves radiate from both, forming the peripheral nervous system. The supraesophageal ganglion connects to the cheliceral and optic nerves, while the subesophageal ganglion connects to the peripheral nerves of the palps, legs, and abdomen (Fig. 3 ).

FIGURE 3 Internal anatomy of a female spider. [Adapted from Comstock, J. H. (1912). "The Spider topic."]. Ao, aorta; BL, topic lung; co, cardiac ostia of the heart; CN, cheliceral nerve; (P), pharynx (behind the esophageal ganglion); (E), esophagus (behind the esopha-geal ganglion); HG, hindgut; Mi, midgut; MD, midgut diverticula; MT, Malpighian tubules; Oc, ocular area; ON, optic nerves; Ov, ovaries; SG, silk glands; SS, sucking stomach; Sup. EG, supraesophageal ganglion; Sub. EG, subesophageal ganglion; SP, stercoral pocket; VG, venom gland.
Paired venom glands occupy the upper portion of the chelicera in all spiders except the Uloboridae and, in many spiders, extend well into the cephalothorax about midway between the eyes and the supraesophageal ganglion. The digestive tract consists of a pharynx, esophagus, sucking stomach, and the beginning of the midgut in the cephalothorax, with the rest of the midgut, hindgut, and anus located in the abdomen.
The major respiratory organs are in the lower abdomen and are called “topic lungs” because they resemble stacked sheets of paper. More primitive spiders have two pairs of topic lungs, others have one pair and may also have a set of tubular tracheae. Within spiders, there has been a sequence of replacement of topic lungs by tracheae, apparently in response to problems of circulation, with more active spiders having variably elaborate tracheal systems (Table I). The heart is a tube-like organ, suspended by muscles and ligaments along the dorsal

	TABLE I
Relative Compositions of topic Lungs vs. Tracheae in Different
	Groups of Spiders
Second	Third abdominal	Example
abdominal	segment
segment
Topic lungs	Topic lungs	Mesothelae,
		Orthognatha
Topic lungs	Short tube tracheae	Filistata
Topic lungs	Long tube tracheae	Dysdera
Topic lungs	Long entapophyseal tracheae	Argyroneta
Topic lungs	Short tube tracheae and short	Araneidae,
	entapophyseal tracheae	Lycosidae
Topic lungs	Short tube tracheae and long	Cryphoeca
	entapophyseal tracheae
Topic lungs	No respiratory organs	Pholcidae
Sieve tracheae	Long tube tracheae	Caponiidae
Sieve tracheae	No respiratory organs	Symphytognathidae
Tube tracheae	Long tube tracheae	Telema

Note: The more primitive groups (Mesothelae, Orthognatha) retain topic lungs on both second and third abdominal segments. More advanced groups show loss of topic lungs and/or replacement by tracheae.
midline of the abdomen, with multiple ostia that serve as valves to keep the blood flowing in one direction. The heart pumps the hemol-ymph forward through the aorta into the cephalothorax. Silk glands are numerous and may fill up to a third of the volume of the abdomen. In more advanced spiders they can have varied functions (Table II ). The gonads consist of paired, coiled, tubular testes in males and paired ovaries in which the follicles may appear grape-like in females.

	TABLE II
Silk Gland Types, Silk Uses, and Location of Spigots within
	Spinnerets
Gland type	Silk uses	Spigot locations
Major ampullate glands	Dragline, web frame	Anterior
Minor ampullate glands	Dragline reinforcement	Median
Aciniform glands	Swathing silk, sperm web,	Median,
	egg sac outer wall	posterior
Cylindrical (or	Cocoon silk	Median,
tubuliform) glands		posterior
Aggregate glands	Glue for sticky spiral	Posterior
Flagelliform glands	Core of sticky spiral	Posterior
Piriform glands	Attachment disc silk	Anterior

PHYSIOLOGY

Feeding and Venoms

Upon capture, a spider sinks its fangs into the body of its prey. At this point, the spider must paralyze, or otherwise restrict the movement, of the prey rapidly before it can eat it. Nearly all spiders use venom to incapacitate their prey. When envenomating prey, muscles surrounding the venom glands contract, forcing venom out of the glands and through ducts that carry the venom to the tips of the fangs and into the prey. Spider venoms are toxic cocktails of polypeptides and proteolytic enzymes that are quite effective for paralyzing the spider’s (usually insect) prey. Because most of the polypeptides have evolved to act on nervous systems of arthropods, which use glutamate as a neurotransmitter, most spider venoms have little effect on vertebrates, but there are exceptions. Some venoms, like those of the widow spiders (Latrodectus), contain components that broadly affect vertebrate nervous systems. In humans, a black widow bite, which may go unnoticed when it happens, can result in several days of pain, muscle spasms, abdominal cramping, and weakness. More serious symptoms may include respiratory difficulty and hypertension. Deaths are rare, however, and antivenins are available in emergency situations. Other species, such as the brown recluse spider (Loxosceles reclusa) , produce venoms that cause tissue death (necrotism) at the site of the wound. While these may not cause systemic effects like the black widow toxins, the wounds produced may take weeks or months to heal, and infection is a serious risk.

Digestion and Excretion

Spider digestion begins outside its body. Once the spider has disabled its prey with venom, silk, or both, it extrudes digestive enzymes into the prey and then, using negative pressure from its sucking stomach, reingests the soup of digestive enzymes and partially digested food. This is repeated until all of the prey’s soft tissue has been consumed. Once the liquefied food has passed through the sucking stomach, it enters the midgut where nutrient absorption takes place, with secretory cells producing digestive enzymes and resorptive cells absorbing food into vacuoles.
Waste is concentrated in the cytoplasm as the nearly insoluble products guanine, adenine, hypoxanthine, and uric acid. These products are collected via the Malpighian tubules and moved into the stercoral pocket, which empties through the hindgut and anus. Other excretory tissues in spiders include the coxal glands, which appear to be involved in water balance, and the large nephrocyte cells that concentrate metabolites.

Circulation and Respiration

Although spiders have well-defined blood vessels, they lack capillaries and have few veins; thus, their circulatory system is basically open. The heart is suspended in the pericardial sinus, and blood enters the heart through paired slits called ostia, which open when the heart is at rest. The heart primarily pumps hemolymph from the abdomen forward through the aorta into the cephalothorax, supplying oxygen to the central nervous system and the skeletal muscles. Once depleted of oxygen, the fluid passes into two sinuses, which lead to the base of the abdomen where the fluid is reoxygenated by the topic lungs and (if present) by tubular tracheae before pressure pulls it through pulmonary veins back into the pericardial sinus.
Unlike most insects, spider hemolymph has an oxygen-carrying pigment, hemocyanin. Hemocyanin is structurally similar to hemoglobin, but instead of iron it uses copper as its oxygen-binding metal, which can make spider hemolymph appear bluish green. Compared to hemoglobin, hemocyanin is less efficient (~5%) in oxygen transportation and is not concentrated in specialized cells.

NEUROBIOLOGY

The detection of touch and sound, mediated by vibrations, is the primary sense of spiders, although other senses may be well developed in some groups. These senses are discussed in detail in a 1985 topic by F. G. Barth.

Touch and Sound

Spiders are notably hairy, and most of the various hairs on their bodies function as mechanoreceptors, sensing movement and vibrations from both the spider’s substrate (which may be the web, the ground, or vegetation on which the spider is situated) and the surrounding air. They also serve as touch receptors. The many stout hairs on the legs, cephalothorax, and abdomen, as well as the finer, more upright trichobothria, found only on the legs, are triply innervated and are involved in the localization and identification of potential prey. Additional mechanoreceptors are the slit sensilla and a variety of other proprioceptors, which respond to stresses in the exoskeleton caused by external vibration or by the spider’s own movements. The slit sensilla are found all over the exoskeleton, but may be arranged in groups to form “lyriform organs.” Together with the trichobothria, the slit sensilla/lyriform organs may be the functional equivalent of spider “ears.” Other proprioceptors include internal ganglia at leg joints and hairs that respond to joint movement.

Taste and Smell

Chemoreceptive hairs are localized mainly on the tarsi of the front legs, and in the palps, although some chemoreception may take place at the mouthparts. The hairs involved in chemoreception resemble tactile hairs, but are open-ended and S-shaped. Spiders respond to sex pheromones of conspecifics, and it is also likely that they use chemoreception to identify potential prey, enemies, and environmental change.

Sight

The “simple” eyes, or ocelli, have a single cuticular lens. The back of the eye contains the retina, which consists of visual cells (including the light-sensitive rhabdomeres) and pigment cells. The main eyes (always the AMEs) consist of a lens, vitreous body, and retina (which contains the visual cells), while the secondary eyes may also have a light-reflecting surface, the tapetum, behind the retina, which causes the well-known eye shine of spiders at night. The visual acuity of spiders depends on the shape of the lens and the number of rhab-domeres. For most species vision is poorly developed. Most web-building species rely almost exclusively on touch, with vision used to detect light and dark and (in a few species) direction of polarized light. However, vision is well developed in jumping spiders, with the AMEs having high visual acuity (acting like the fovea of the human eye), the remaining eyes having lower acuity but a broader field of vision and functioning for peripheral vision. In addition, jumping spiders have been shown to be sensitive to ultraviolet light. Some species show striking color dimorphism in ultraviolet light, suggesting a role in intraspecific sexual communication.

SILKS AND SPIDER WEBS

Spiders are unique among arthropods in their use of silks at all stages of their lives. Silks are produced in the abdomen in specialized silk glands, each of which yields a different kind of silk. The general structure of a silk gland is a tail area that secretes the liquid silk proteins into a sac-like lumen, or storage area. The lumen empties into a duct leading out to the spinneret spigots. The ducts are important in silk production because their tapered shape helps to orient the molecules relative to the axis of the thread to maximize strength, whereas the cells surrounding the duct draw off water from the oriented protein and turn them into solid silk fibers. The ducts of the different silk glands terminate at specific spigots in the spinnerets (Table II).
The various silks produced by spiders show different combinations of remarkable material properties, including high tensile strength, extensibility reminiscent of rubber, and resistance to decay. These properties have led to research in the pursuit of silk genes and techniques for spinning the translated products of these genes; genetically engineered spider silk may soon be found in parachutes, bulletproof vests, car bumpers, artificial ligaments, etc. At the molecular level, spider silks are a family of proteins made up primarily of a subset of amino acids (alanine, glycine, serine, proline, glutamine, and tyrosine make up over 75% of the composition of characterized spider silks) arranged in a highly repetitive manner. The smallest repeating units of two to six amino acids are strung together into larger ” tandem repeats.” This arrangement is thought to form secondary structures that determine the kind of silk produced. The overall structure can be compared to a composite material with stiff, stress-resistant crystals interspersed in an extensible, energy-absorbing matrix.
The most obvious use of spider silk is in web construction, but it is employed for many other purposes in the life of a spider, including the following.

Ballooning

Spiders are capable of producing silk as soon as they emerge from the egg, and two behaviors, the manufacture of a brood web and ballooning, are characteristic of newly hatched juveniles, or spiderlings. A brood web is made by some spiderlings upon emergence and for the earliest instars can serve as a communal nest for the young to catch prey together. Ballooning generally occurs once dispersal in the spiderlings has been initiated by developmental or environmental cues. The spiderlings let out a silk thread, which produces enough drag to catch the wind and carry the spider off, sometimes for great distances.
Nearly all spiders trail a dragline behind them during locomotion. The dragline is extruded from the ampullate glands as the spider walks along or moves in its web, stopping occasionally to anchor the line with an attachment disc secreted by the piriform glands. The jumping spiders (Salticidae) may make seemingly reckless jumps, but are almost invariably anchored with a dragline, so a missed leap is seldom fatal. Dragline diameters vary from several hundred nanometers to several micrometers, depending on the size of the animal being supported. Draglines are stronger than any other known natural fiber, with the added ability to stretch 15-30% beyond their original length before breaking.

Egg Sacs

All spiders use silk to protect their eggs. The silk covering of the egg mass may range from a few strands to a thick covering with multiple silks. The resulting silken cocoon can take on a huge variety of forms depending on the spider and the habitat.

Sperm Webs

Silk is also used to make the male sperm web. Shortly after the final molt to maturity, a male spider makes a small web (sometimes just one or a few strands of silk), upon which he deposits sperm from the abdomen. He then places the tip of his palp into the sperm, which is drawn through the palp’s opening into the sperm duct, where it is stored.

Capture Webs

The most remarkable use of spider silk is in the construction of snares for catching prey. In webs, silks can function both directly as a mechanical trap, to stop or slow potential prey, and indirectly as an extension of the spider’s sensory apparatus, alerting it to the trapped prey. A number of spiders can produce sticky capture webs, which come in two forms. The “hackled” or woolly silk of cribellate spiders is sticky (akin to Velcro) because of its fine fibers. The silk is formed by rapid combing (by the calamistrum on the rear leg) of a silk produced from the cribellum, a field of fine openings in front of the spinnerets. In contrast, the web silk of ecribellate spiders usually has sticky globules, secreted by the aggregate glands, arranged along its length.
The type of web that spiders produce is often characteristic of a family, with forms including the two-dimensional orb, tangle (cobweb), sheet, and funnel webs (Fig. 4). Each of these webs has its variants as well; for example, orb webs can be oriented either vertically or horizontally or may be reduced, sometimes to a single section of a complete orb. The impressive body of literature on the form, function, and evolution of spider webs was reviewed by Eberhard in 1990.
Despite the fact that all webs serve the same basic function and despite the common characterization of spiders as generalist predators, the form of the silk of certain webs can be specialized for capture of a small subset of potential prey types. For example, many comb-footed spiders (family Theridiidae) make tangle webs with viscid threads extended to the substrate below, the last centimeter or two of which are coated with a sticky substance (” gum-foot lines ” ) and serve to snare cursorial prey. More specialized is the bolas spider,

FIGURE 4 Web structures. (A) Typical orb web (Araneidae). (B) Orb web of species of Tetragnathidae. (C) Cribellate orb web (Uloboridae). (D) Tangle web of black widow (Theridiidae). (E) Tangle web of Steatoda sp. (Theridiidae). (F) Sheet web of Frontinella sp. (Linyphiidae). (G) Funnel web of Agelenopsis sp. (Agelenidae). (H) Web of basilica spider, Mecynogea lemniscata (Araneidae). (I) Web of labyrinth spider, Metepeira labyrinthea (Araneidae).
an orb weaver whose “web” consists of a single strand of silk with a sticky droplet (bolas) at the end, which it uses to catch moths. In addition to these differences in silk form between web types, silks often have different functions within webs. For example, silk from the flageUiform glands makes up the core fibers of the capture spiral in orb webs and has properties quite different from those of the silks that form the web frame and radial elements. The remarkable property of this capture-spiral silk is its ability to stretch; it may more than double its length while absorbing the kinetic energy of a flying insect. In addition, the capture spiral recovers tension slowly, which prevents prey items from being flung back out of the web.

Web Decorations

One of the most conspicuous features of the webs of some orb spinners is the presence of a stabilimentum, a prominent silk line, cross, and/or spiral, at the center of the orb. The function of the stabilimentum is not clear, though it may serve to camouflage the spider, startle predators, or protect the integrity of the web from accidental damage.

RELATIONSHIPS AND TAXONOMY

Understanding of the relationships among different spider groups and between spiders and other arthropods has increased dramatically in the past two decades. Some portions of the spider tree remain unresolved, and additional morphological and molecular study will be needed to settle these uncertainties. Statements of phylogenetic relatedness between spider groups are based on a review of morphological data by Coddington and Levi in 1991, and all groups mentioned can be found on the summary cladogram (Fig. 5 ), unless otherwise noted.
Spiders are divided into three suborders, the Mesothelae, Mygalomorphae, and Araneomophae, and 106 families. The Mesothelae (1 family, Liphistiidae, 2 genera, 40 species) are considered the most primitive of all living spiders, based on their external visible segmentation and location and number of spinnerets (all four pairs present, without a cribellum). The Mygalomorphae, recognized by the articulation of the chelicerae parallel to the body (paraxial), are also considered primitive; they are generally stout-bodied and include the trap-door spiders and the impressively large tarantulas (family Theraphosidae) among their 15 families. Araneomorphs, the “irue” spiders, represent over 90% of spider diversity. They can be distinguished from the more primitive spiders by the sideways (diaxial) articulation of their chelicerae. Paraxial and diaxial cheliceral orientations are also referred to as orthognath and labidognath, respectively. The most basal lineage of the Araneomorphae is the Paleocribellatae, consisting of one family, the Hypochilidae, in which the body plan is a mosaic of primitive and derived characters.
The rest of the araneomorphs belong to one of two clades, the hap-logynae and the entelegynae, distinguished on the basis of the complexity of the female genitalia (Fig. 2). In the haplogynae, the smaller group, the external female genitalia consist of a simple opening, or gonopore, tucked between the topic lungs, with a single duct serving both the copulatory and the fertilization functions. Two well-known haplogynae families are the spindly legged Pholcidae (or daddy-longlegs spiders), common around and inside houses, and the Sicariidae, which may also occur in and around houses, and include the much-feared brown recluse spiders (L. reclusa), whose venom may cause necrotic wounds in humans. In the entelegynae, the female geni-talia are more elaborate, with separate fertilization and copulatory ducts and sclerotized epigynum. The most diverse and well-known entelegynae spider families belong to the RTA clade, so named because of a retrolateral tibial apophysis on the male’s palp, and the orbicularia, which includes the orb-web builders and their relatives. The RTA clade is a huge, ecologically diverse group of spiders and includes about one-third of all described spider species. Many are web builders, for example the funnel-web spiders (Agelenidae). However, some of the most successful lineages have shifted to non-web-building hunting strategies, including the jumping spiders (Salticidae), in which the median tarsal claw is replaced by a tuft of tarsal hairs that facilitates a cursorial lifestyle. Salticidae also have greatly enlarged AMEs, giving them the best vision of all spiders. They use this vision to track prey and carry out unique jumping attacks with great accuracy and in mate choice when faced with flamboyant courtship displays. The crab spiders (Thomisidae) are another family of two-clawed spiders with an interesting variation on the typical spider sit-and-wait game. Thomisids hide in the petals of

FIGURE 5 Phylogenetic relationships of major spider lineages, with exemplar families.
flowers, where their bright colors often disguise them, and use their raptorial forelegs to capture visiting pollinators. Wolf spiders and their relatives (Lycosoidea) are also cursorial hunters, though they retain all three tarsal claws.
The Orbicularia includes some of the best known web spinners. In particular, the orb web is often considered to be the “classic” spiderweb—a two-dimensional sphere consisting of a frame, with radii projecting outward, and a spiral of sticky silk wrapped surrounding the center. There are three families primarily responsible for these orb webs, the cribellate Uloboridae and the Araneidae and Tetragnathidae, which have lost the cribellum. The orb design, once thought to be the “pinnacle” of web evolution, is now thought to have served as a point of departure for some successful groups (the “higher araneoids”) that make webs of quite different designs. The comb-footed spiders (Theridiidae) make seemingly disorganized cobwebs, often with viscid gum-foot lines as mentioned above. On the other hand, “bowl-and-doily” or money spiders (Linyphiidae) make dome-like sheets from which they hang on the lower surface; the viscid silk in these spiders dries up after being produced and serves to cement together the different layers of the sheet.

COURTSHIP, REPRODUCTIVE BEHAVIOR, AND GROWTH

Courtship

The first step in courtship involves maturation of the male spider, at which stage the palps have become modified and swollen for sperm storage and transfer. Once the palps are charged with sperm, the male sets out to find a receptive female. However, spiders are assiduous predators, and the male must overcome this propensity of the female if he is to mate. There is a huge array of courtship strategies that allow the male to approach. Most spiders employ some kind of vibratory communication during courtship. Among web-building spiders, the male often locates himself on the edge of the female’s web and gently plucks. It often takes hours, even days, until the female becomes receptive. Wolf spiders (Lycosidae), although they do not have webs, make use of vibrational cues during courtship. Vibrations generated from stridulating organs are usually transmitted through the leaf litter. Visual cues, although used by wolf spiders, are most complex among jumping spiders (Salticidae). Male jumping spiders generally communicate courtship by performing a variably elaborate dance for the female, waving their often brightly colored legs and body to show off iridescent plumes. If successful, the male can extend his forelegs to touch the female before climbing on top of her.
In a number of species the female must adopt a state of complete immobility before the male can initiate copulation. In other species, the male waits until the final molt and can mate with the female immediately. In certain species the female is very much larger than the male and appears hardly to notice that the male is either approaching or copulating with her. Other males secure the female by wrapping her in silk prior to copulation. Species in the genus Tetragnatha have an unusual way of mating whereby the cheliceral fang of the female becomes wedged against a dorsal notch on the cheliceral surface of the male. The male then locks the female in place by closing his fangs over hers.

Copulation

Copulation involves the injection of sperm from the male’s palps into the seminal receptacles of the female, with the palps being inserted alternately into the epigynum. In some species, the palp breaks off and seals the epigynum. Once copulation has been completed, the male must escape from the female before her brief period of receptivity ends. If the male fails to escape in time, he can be caught and consumed by the female, although more commonly he escapes to mate again. However, the life span of a mature male is generally short, and many do not eat at all. Female spiders, on the other hand, are able to store sperm and so can produce fertile eggs long after copulation. Accordingly, females may survive for some time before egg laying is complete, and in many species they survive longer, to care for their offspring.

Egg Sacs

Some time after mating, the female will deposit an egg sac. The eggs are always laid within a cocoon of silk. However, like courtship, there is a diverse array of egg sac types and behaviors that spiders use to protect their eggs. In most spiders, the female spins a layer of silk into which she deposits her eggs. She then covers the eggs with more silk. The covering may be scant, as in the daddy-long-legs spiders (Pholcidae). In other spiders the sac is very thick, with multiple silks, soft inside but tough and water resistant outside, as in many spiders of the family Theridiidae (e.g., Latrodectus). Egg sacs are also produced in a variety of colors, from white to yellow, green, or black, in pastel or vivid shades. Textures may vary from papery smooth to tufted and furry and shapes from flat to round or angular, with connections to the substrate ranging from a tight fixture to loosely suspended or pendulous. These colors and shades generally match the substrate on which the egg sac is laid, serving to camouflage the developing eggs as protection against predation and parasitism.

Parental Care

Parental care varies tremendously among spiders. Some females abandon their egg sac immediately after it has been laid although, even in these species, the female selects specific sites for deposition of the egg sac. For example, it can be plastered on a twig (e.g., Tetragnatha), placed under a leaf or stone, wrapped in a leaf, or suspended in the web or retreat, either with a stalk (e.g., Argyrodes) or without. However, extended parental care has been documented in a number of spiders and has broad implications with regard to the origin of social behavior.

EGG SAC DEFENSE

A number of female spiders guard their eggs closely until (or after) hatching. Such guarding is common among jumping, crab, sac, and ground spiders, and a variety of others, and can be critical for egg survival in, for example, the green lynx spider Peucetia viridans (Oxyopidae) and the Hawaiian happy face spider Theridion grallator (Theridiidae). In several groups of spiders, the females carry their eggs with them wherever they go. In particular, wolf spiders (Lycosidae) carry their egg sac attached to their spinnerets; fishing (Pisauridae) and giant crab (Heteropodidae) spiders carry their egg sac under the sternum; daddy-long-legs spiders (Pholcidae) carry their egg sac held in the chelicerae.

CARE OF SPIDERLINGS

Maternal care in spiders, when present, often terminates after hatching, and the young disperse. However, female wolf spiders carry their young on their abdomen once they have hatched from the egg sac; fishing spiders build a “nursery web,” a large tent-like structure in which the spiderlings live while the mother stands guard. Among a few comb-footed and other spiders, care of the emergent spiderlings can be developed to an extraordinary degree. This is true of the communal comb-footed spiders Theridion saxatile, T. sisyphium, T. grallator, and Anelosimus studio-sus. Females of the latter two species defend the egg sac aggressively and then capture prey for, and even feed, the young, which are unable to capture prey on their own. Providing the young with food appears to be the primary function of brood care once the spiderlings emerge from the egg sac. In addition to simply securing prey, a mother may feed her offspring by regurgitation or by laying ” trophic eggs. ” In some species she may even feed herself to her offspring, a process known as “matriphagy,” and in at least one species, Amaurobius ferox, the mother is known to expedite this process herself.

Dispersal

One of the most intriguing aspects of spiders is their dispersal. When spiderlings hatch, they are generally aggregated. However, on the first day with suitable wind speeds, they will frequently move up to the highest point they can find (e.g., the tip of a grass stalk) and let out silk which catches in the wind. As the spider lets out more silk, the pull of the wind on the silk becomes sufficient to allow the spiderlings to become airborne. Spiderlings can travel tremendous distances by ballooning and are frequently the dominant component of aerial plankton, although the family groups represented vary with locality: Linyphiidae comprise much of the fauna above land areas that have been examined, whereas Tetragnathidae dominate over oceanic areas. Because of their capacity to balloon, spiders are often the first to colonize unoccupied landmasses, whether cleared land or new islands in the middle of the ocean.

Growth

Spiders, like insects, have a rigid exoskeleton and must molt to grow, with three to nine instars (stages between molts) before reaching maturity. Spiders do not metamorphose: A first-instar spiderling looks similar to, though smaller than, an adult. Most spiders live for about a year, though mygalomorphs may live for 30 years.

SOCIALITY

Although most spiders are solitary and highly intolerant of others, there are several species that exhibit some form of social behavior, ranging from aggregations at a certain life stage to prolonged maternal care and even quasisociality or true sociality. Considerable controversy surrounds the origins of quasisocial and true social behavior in spiders, discussed in topics of a topic by Choe and Crespi published in 1997. It may have evolved through coloniality and the development of aggregations around an abundant resource. Alternatively, it may have arisen through extension of brood care into later instars. Many studies have examined the biology of social theridiids, including the formation of colonies and sex ratios in true social species.

Colonial Spiders

Some spiders (e.g., several species in the families Araneidae, Uloboridae, and Tetragnathidae) live in colonies, usually around an abundant food source. The benefits from such behavior include sharing silk support lines for prey capture, sharing prey that is not overall in short supply, and communication.

Subsocial Spiders

In subsocial spiders, there is both maternal brood care beyond the first few developmental instars that is typical of most spider species and an extended phase of tolerance among young within the maternal nest. However, these species live solitarily as adults. True sociality in spiders is thought to have evolved via the “subsocial pathway” by a prolongation of an early tolerance phase without dispersal. The sub-social route to sociality entails decreasing the genetic variance within breeding groups as a result of families staying in proximity.

True Sociality

True sociality appears to have arisen independently multiple times in spiders. Social spiders, unlike eusocial insects, have no castes that are morphologically different or sterile and most individuals within colonies reproduce. In addition, social spiders show a female-biased sex ratio and high population turnover and inbreeding. Their breeding colonies are closed and new colonies are formed by splitting an existing colony, by a swarm of related females, or by single gravid females. High levels of inbreeding and relatedness among females bias the sex ratio toward the dispersing sex. So, unlike most spiders, males do not appear to disperse large distances in social species. Gene flow between established colonies is rare, and colonies show a high degree of genetic structure. It seems likely that sociality may be maintained by behavioral preadaptations that lead to tolerance and cooperation among colony members on the one hand and population structure on the other.

CAMOUFLAGE AND MIMICRY

Crypsis

Spiders have many enemies. In addition to other spiders, perhaps the most important predators are birds and spider-hunting wasps, both of which possess high visual acuity and color vision. Spiders can have specific adaptations for matching background colors (crypsis) such as flowers/leaves, grass/twigs, bark, underleaf surfaces, and the ground. In some spiders, the crypsis is extraordinarily close and can vary between individuals on different backgrounds. There are, for example, several species that are variable in color but always seem to match the bark on which they are living. Selection for crypsis in similar types of habitat has led to the repeated evolution of similar colorations in unrelated species. For example, spiders that live under leaves in the tropics are generally translucent yellow, often with dark leg joints that disrupt the outline of the legs. During the day these spiders lie flat against the underside of the leaf, thereby reducing shadows and becoming highly cryptic against the light transmitted through the leaf. Five unrelated spider species living in the same Hawaiian forests exhibit these adaptations for underleaf crypsis, as do at least four species from the forests of Panama. Within a single lineage, repeated evolution of similar cryptic coloration in different species has been found in a radiation of Tetragnatha spiders in Hawaii.

Disruptive Coloration

In some spiders the characteristic shape of the body is concealed, for example by bold, juxtaposed colors. These kinds of colors tend to be found where the propensity to wander over different backgrounds while searching for prey might preclude true crypsis. Crypsis per se may also be difficult in diurnal orb-spinning spiders. Some of these have developed mimetic resemblances to dead leaves and sticks but others have apparently adopted disruptive coloration.

Mimicry

Most studies of mimicry in spiders have been concerned with the imitation of ants, thought to be a form of Batesian mimicry. The spiders may gain some protection from predators through their resemblance to aggressive or unpalatable ants. The topic of ant mimicry has recently been reviewed by Cushing, in 1997. Spiders may also mimic a range of other organisms, alive or dead, and inanimate objects. For example, many Cyclosa spp. (Araneidae) build vertical “sticks” of prey remains within the web but leave a gap in the center, which is filled by the spider itself. Crab spiders (Thomisidae) mimic the color of flower heads very precisely and prey on pollinators that approach. Some spiders resemble bird droppings, which are attractive to insects. In the garden spider Argiope argentata, the visibility of both the contrastingly colored ventral and the UV-reflecting dorsal side of the opisthosoma may increase insect prey caught in the web.

Apostatic Coloration

At least some predators can develop search images, concentrating their search effort on more common forms of individuals of any species. This can result in polymorphism within a prey species. One of the best examples of this is found in Theridion grallator (Theridiidae) (Fig. 6). Within this species, there is a remarkable diversity of color forms, yet the frequency of color forms is similar in different populations, apparently maintained by bird predation.

FIGURE 6 Some representative color morphs of the Hawaiian happy face spider Theridion grallator.

HABITAT SELECTION

How do spiders select a site in which to live? Many studies have demonstrated that there are clear associations between spider abundance and the structural diversity of the habitat, climatic regime, and prey availability. Most spiders are considered to be sit-and-wait predators, spending much time in locating a suitable site in which to wait and remaining there until its quality deteriorates. The time they remain at a site depends on their investment at the site. For species that do not spin webs, the investment is only the time spent finding the site.
Ecribellate orb-web spiders can regain most of their resource investment at a site by ingesting the web before abandoning the site. Among other web-building spiders, however, the investment at a site cannot be regained; it can only be decreased through reduction in web size. Movement from a site may be dictated by disturbance or web destruction, microclimate change, growth of the spider relative to structural requirements for web construction, and/or prey capture success.
In selecting a site, it is often not clear what aspect of the environment it is to which the spider is responding. It is possible for hunting spiders to move directly to their feeding site, at least when they can perceive air- or substrate-borne vibrations. Web-building spiders are generally confined to their web for prey detection so that movement from a web site is generally not directed, and to select a site they must “sample” a location by building a web. However, the main vibration receptors of web spiders (trichobothria) are basically the same as those of wandering spiders. Accordingly, web-building spiders may also be capable of perceiving vibrations mediated by the air or the substrate.

PREDATION

Prey Capture

For web-building spiders, prey are usually captured upon being trapped in the sticky threads. The spider will generally respond to the vibration of an insect caught in the threads by shaking the web, causing the insect to become more firmly stuck and allowing the spider to locate the position of the insect. The spider may then approach the insect, often doing so very rapidly, and wrap it in silk of various amounts depending on the size and strength of the insect. This approach and attack behavior varies, with some web-building species leaping upon and biting the prey even before it becomes firmly secured in the web. More specialized strategies have evolved in a number of web builders. For example, the ogre-faced spider (Deinopis) holds its cribellate web, orb-like in form, between the two front pairs of legs; it uses the web much like a net, hurling it over prey that come close. Comb-footed spiders may use their gum-foot lines to catch crawling prey; the unsuspecting prey may either dislodge the tensed lines, which springs them into the tangle of the web, or they get reeled in by the alerted spider.
For spiders that do not build webs, prey may be detected by air- or substrate-borne vibrational cues or, alternatively, at least in the case of jumping spiders, by visual cues. Cursorial spiders often jump on their prey without wrapping it in silk. However, certain groups have elaborate mechanisms for securing their prey. In particular, the spitting spiders (Scytodidae) have a high-domed cephalothorax, which contains venom glands connected to a posterior gland that secretes a sticky silk. The spider creeps up on its prey and, by rapid contractions of the muscles in the prosoma, ejects sticky silk and venom over the prey, which is thus immobilized. One species of long-jawed spider in Hawaii has long tarsal claws that it uses to impale insects directly from the air. Other specialized mechanisms of prey capture are discussed below.

Diet

Spiders are exclusively carnivorous, although they are usually regarded as generalist predators, taking prey as they are encountered. However, their diet is dictated by the habitat in which they select to live, and habitat specialization can greatly restrict the dietary repertoire of many species. Even within a given habitat, species may specialize on prey, either through choice or because they exhibit specialized predatory behaviors. For example, species in the genera Dipoena and Euryopis (Theridiidae) are specialized for feeding on ants, while those in Dysdera (Dysderidae) appear to specialize on isopods, and pirate spiders (Mimetidae) and some Argyrodes (Theridiidae) feed on other spiders.

Kleptoparasitism

Kleptoparasitic spiders steal food from the webs of other, unrelated and usually larger web-building, spiders, which are also potential predators. They may form groups of up to 50 individuals around the web of a “host” spider and glean small insects from the periphery of the host spider’s web, eat the host’s silk, steal food bundles previously wrapped and left in the web by the hosts, or even approach a feeding host spider and then feed, undetected, next to the host spider. The most diverse collection of obligate kleptoparasites is found in the spider genus Argyrodes (Theridiidae), which contains at least 200 species, about 100 kleptoparasitic.

Aggressive Mimicry

Aggressive mimicry is mimicry that enhances the access of spiders to their prey. Perhaps the most interesting case of aggressive mimicry is shown by certain jumping spiders that can mimic the vibratory signals of a fly by plucking with their legs and palps on the web of other spiders. This may attract the occupant to the jumping spider close enough to be attacked. The best-studied species in this regard is Portia fimbriata (Salticidae) from Queensland. This spider preys on web builders with poor eyesight, which allows the araneophagic Portia to drum out a pattern of signals on the web that imitates the vibrations of the normal prey of the web spider. In contrast to the vibrational mimicry of Portia, the bolas spider (Araneidae) emits odors that mimic the pheromone of female moths to attract male moths to the vicinity. As soon as a moth flies within range, the spider launches its sticky bolas on a silk thread; if successful, the bolas hits and sticks to the moth, and the spider reels in its prey.

DIVERSITY AND CONSERVATION

There are about 40,000 described species of spiders, though the actual number of species has been estimated at around 170,000. In common with insects, species diversity is highest in tropical regions, where knowledge of the biota is least. Also like other arthropod groups, spiders exhibit spectacular examples of adaptive radiation and local endemism in isolated situations, for example, the genus Habronattus (Salticidae) on the North American mountaintop “sky islands” and the genus Tetragnatha (Tetragnathidae) in the Hawaiian Islands. Studies of the Hawaiian Tetragnatha have shown how similar sets of distinct ecological types, or ecomorphs (” green ” which sits on leaves; “maroon,” which is mostly on moss; ” small brown, ” among twigs; and “large brown,” on tree bark) occur on each island, and have arisen independently through convergent evolution. These island studies show how biodiversity arises in communities over evolutionary time. However, in almost every ecosystem, the number and diversity of spiders make them important in the management of ecosystems, whether natural or disturbed, agricultural or urban.

Importance and Conservation in Agricultural Systems

Spiders have been known to be important agents of insect pest control for hundreds of years. It is also well known now that chemical pesticides kill not only the insect pests, but also their major predators, in particular spiders. A review by Riechert and Lockley in 1984 brought attention to spiders as potential agents of biological pest control. Manipulations of habitat structure have resulted in increased spider densities and concomitant decrease in insect pests. The effectiveness of spiders is often enhanced by the ability of the web to kill prey even in the absence of the spider and because many species practice wasteful killing (i.e., they kill more insects than they consume). Control of insect pests by spiders appears to be achieved most efficiently by using an assemblage of spiders (rather than specific species) and incorporating natural refuges in and around the crops. The particular methods that are best for enhancing the role of spiders in pest control in agriculture are: (i) conservation tillage and retention of stubble, preserving weeds and mulching; (ii) intercropping, provision of native vegetation within crops; and (iii) agroforestry, planting a combination of trees and food crops.

Importance and Conservation in Natural Ecosystems

We have little understanding of the precise role that spiders play in a given ecosystem. However, we do know that spiders are the dominant invertebrate predator in most terrestrial ecosystems and are also important as a food resource for other predators. Accordingly, they must inevitably play a key role in community dynamics. Studies on birds in natural areas have shown that spiders can comprise a significant proportion of the diet of many species, including a number of endangered birds. Spiders also have utilitarian value and have been used as model organisms for research in ecology, behavior, and communication. Moreover, there has been much recent attention on their potential for providing silk for materials science and supplying venom for medical and insecticide research.
Clearly, there is a huge need to augment taxonomic information on spiders, particularly in tropical areas, and to understand patterns of biodiversity. Pursuing this knowledge is imperative as we recognize the obvious importance of spiders in ecosystems, both as prey and as predators. Moreover, given the intrigue that spiders have long held in human society, such an undertaking can only lead to greater heights of fascination.