Orb-weaver spider

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Orb-weaver spider

Argiope catenulata
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Order:	Araneae
Suborder:	Araneomorphae
Superfamily:	Araneoidea
Family:	Araneidae Simon, 1895
Diversity
168 genera, 3,006 species

The typical orb-weaver spiders (family Araneidae) are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields and forests. Their common name is taken from the round shape of this typical web, and the taxon was formerly also referred to as the Orbiculariae.

Orb-weavers have eight similar eyes, hairy or spiny legs, and no stridulating organs. The Araneidae family is cosmopolitan, including many well-known large or brightly colored garden spiders. The 3,006 species in 168 genera worldwide make Araneidae the third-largest family of spiders known (behind Salticidae and Linyphiidae). The orb-weavers include over 10,000 species and make up about 25% of spider diversity.

However, orb-webs are also produced by members of other families. The large golden orb-weavers (Nephilidae) and the long-jawed orb weavers (Tetragnathidae) were formerly included in the Araneidae; they are indeed closely related to them, being part of superfamily Araneoidea. Their webs are similar to those of the typical orb-weavers, but tend to be less sophisticated and often have an irregular instead of a neat spiral arrangement of the prey-capturing threads. The cribellate or hackled orb-weavers (Uloboridae) belong to a distinct superfamily of the suborder Araneomorphae; their webs are often very sophisticated, but Uloboridae use neither venom to kill their prey, nor sticky threads in their web, and probably evolved the orb structure independently. Uloboridae are cribellate, and their threads can be recognized by the fuzzy and dull appearance, which captures prey by a velcro-like mechanism.

The orb-web

Generally, orb-weaving spiders are three-clawed builders of flat webs with sticky spiral capture silk. The building of a web is an engineering feat, begun when the spider floats a line on the wind to another surface. The spider secures the line and then drops another line from the center, making a "Y". The rest of the scaffolding follows with many radii of nonsticky silk being constructed before a final spiral of sticky capture silk. The third claw is used to walk on the nonsticky part of the web. Characteristically, the prey insect that blunders into the sticky lines is stunned by a quick bite, and then wrapped in silk. If the prey is a venomous insect, such as a wasp, wrapping may precede biting.

Many orb-weavers build a new web each day. Most orb-weavers tend to be active during the evening hours; they hide for most of the day. Generally, towards evening, the spider will consume the old web, rest for approximately an hour, then spin a new web in the same general location. Thus, the webs of orb-weavers are generally free of the accumulation of detritus common to other species, such as black widow spiders.

Some orb-weavers do not build webs at all. Members of the genera Mastophora in the Americas, Cladomelea in Africa, and Ordgarius in Australia produce sticky globules, which contain a pheromone analog. The globule is hung from a silken thread dangled by the spider from its front legs. The pheromone analog attracts male moths of only a few species. These get stuck on the globule and are reeled in to be eaten. Interestingly, both types of bolas spiders are highly camouflaged and difficult to locate.

The spiny orb-weaving spiders in the genera Gasteracantha and Micrathena look like plant seeds or thorns hanging in their orb-webs. Some species of Gasteracantha have very long, horn-like spines protruding from their abdomens.

One feature of the webs of some orb-weavers is the stabilimentum, a crisscross band of silk through the center of the web. It is found in a number of genera, but Argiope, which includes the common garden spider of Europe, as well as the yellow and banded garden spiders of North America, is a prime example. The band has been hypothesized to be a lure for prey, a marker to warn birds away from the web, and a camouflage for the spider when it sits in the center of the web. However, recent research suggests the stabilimentum actually decreases the visibility of the silk to insects, thus making it harder for prey to avoid the web. The orb-web consists of a frame and supporting radii overlaid with a sticky capture spiral, and the silks used by orb-weaver spiders have exceptional mechanical properties to withstand the impact of flying prey.

The orb web derived from substrate-bound web, likely an irregular ground web or brushed sheet web. The change from a ground web to an aerial web allowed for both horizontal and vertical orb webs that could capture flying prey. During the Cretaceous, a radiation of angiosperm plants and their insect pollinators occurred. Fossil evidence shows that the orb web was in existence at this time, which permitted a concurrent radiation of the spider predators along with their insect prey. The capacity of orb webs to absorb the impact of flying prey led orbicularian spiders to become the dominant predators of aerial insects in many ecosystems. Insects and spiders have comparable rates of diversification, suggesting they co-radiated, and the peak of this radiation occurred 100 Mya before the origin of angiosperms. Vollrath and Selden (2007) make the bold proposition that insect evolution was driven less by flowering plants than by spider predation – particularly through orb webs – as a major selective force.

Most arachnid webs are vertical and the spiders usually hang with their head downward. A few webs, such as those of orb-weaver in the genus Metepiera have the orb hidden within a tangled space of web. Some Metepiera are semisocial and live in communal webs. In Mexico, such communal webs have been cut out of trees or bushes and used for living fly paper. In 2009, workers at a Baltimore Wastewater Treatment Plant called for help to deal with an estimated 107 million orb-weaver spiders, living in a community that managed to spin a phenomenal web that covered some 4 acres of a building with spider densities in some areas reaching 35,176 spiders per cubic meter.

Natural history

Argiope lobata photographed in Conil de la Frontera (Southern Spain).

The oldest known true orb-weaver is Mesozygiella dunlopi, from the Lower Cretaceous. Several fossils provide direct evidence that the three major orb-weaving families, namely Araneidae, Tetragnathidae and Uloboridae, had evolved by this time, about 140 million years ago. They probably originated during the Jurassic (200 to 140 million years ago). Based on new molecular evidence in silk genes, all three families are very likely to have a common origin.

The two families, Deinopoidea and Araneoidea, have similar behavioral sequences and spinning apparatuses to produce architecturally similar webs. The Araneidae weave true viscid silk with an aqueous glue property, and the Deinopoidea use dry fibrils and sticky silk. The Deinopoidea (including the Uloboridae), have a cribellum – a flat, complex spinning plate from which the cribellate silk is released. They also have a calamistrum – an apparatus of bristles used to comb the cribellate silk from the cribellum. The Araneoidea, or the "ecribellate" spiders, do not have these two structures. The two families of orb-weaving spiders are morphologically very distinct, yet there is much similarity between their web form and web construction behavior. The cribellates retained the ancestral character, yet the cribellum was lost in the escribellates. The lack of a functional cribellum in araneoids is most likely synapomorphic. If the orb-weaver spiders are a monophyletic group, the fact that only some species in the group lost a feature adds to the controversy. The cribellates are split off as a separate taxon that retained the primitive feature, which makes the lineage paraphyletic and not synonymous with any real evolutionary lineage. The morphological and behavioral evidence surrounding orb webs led to the disagreement over a single origin or a dual origin. However, molecular analysis provides more support for a monophyletic origin.

Reproduction

In the cannibalistic and polyandrous orb-web spider Argiope bruennichi, the much smaller males are attacked during their first copulation and are cannibalized in up to 80% of the cases. All surviving males die after their second copulation, a pattern observed on other Argiope species. Whether a male survives his first copulation depends on the duration of the genital contact: males that jump off early (before 5 seconds) have a chance of surviving, while males that copulate longer (greater than 10 seconds) invariably die. Prolonged copulation, although associated with cannibalism, enhances sperm transfer and relative paternity. It was observed that when males mated with a non-sibling female the duration of their copulation was prolonged and consequently the males were cannibalized more frequently. When males mated with a sibling female they copulated briefly and thus were more likely to escape cannibalism. By escaping, their chance of mating again with a non-kin female likely would be increased. These observations suggest that males can adaptively adjust their investment based on the degree of genetic relatedness of the female in order to avoid inbreeding depression.

Systematics

Main article: List of Araneidae genera

The categorization into subfamilies and tribes follows Joel Hallan's Biology Catalog.

The Nephilidae were elevated to family status in 2006. Some researchers also consider the genera Leviellus, Parazygiella, Stroemiellus and Zygiella to reside in the family Zygiellidae.