Prey capture and feeding
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Spiders have an amazing array of prey catching strategies ranging from simple ambushing to the use of complex silk snares.
Most mygalomorph spiders are ambushing hunters. Many leap from burrow entrances in ground, log or tree trunk retreats to capture passing prey. Some burrow dwellers lurk behind trapdoors but others, like tarantulas and funnel-webs, will forage on the surface in the vicinity of the burrow at night. A few make sheet or curtain-like webs at their burrow entrances which can impede both prey and predators. Others have silk or twig trip-lines radiating out from the burrow entrance to alert spiders to prey walking nearby. Yet others are vagrants living in leaf litter and using vibration and touch to sense and ambush prey.
The ancestors of today's araneomorph spiders used cribellate (wool-like) catching silk, probably in some form of a simple sheet web, to capture their prey. These webs are still common today. They are made by primitive cribellate spiders like the Tasmanian Cave Spider, Hickmania troglodytes, and many modern spiders like the striped sheet web spiders (Therlinya spp). make such webs. The common Black House or Window Spiders progressively thicken their sheet webs with several silk layers - the shawl-like webs. Most of these webs are built out from a crevice retreat in a soil bank or tree trunk. By contrast, Hammock Web spiders sit on the rock or wood substrate shielded by their hammock-like sheet web. These webs are effective for capturing walking and jumping prey but will also entangle flying prey like moths and flies.
Insects provide the vast majority of spiders' food and many web-based prey catching strategies evolved in response to this plentitude. These included 'space webs', 3-D webs with a maze of threads that delay the prey long enough for the spider to enswathe it in silk or bite it; knockdown webs combining a maze of lines above with a silk sheet below - the maze of 'knockdown' threads stop flying or jumping prey which fall onto the sheet below and also help keep the sheet clear of debris; orb webs, with large, planar catching surfaces that are sticky, strong and stretchy and virtually invisible to flying insects; sticky 'gum-footed' webs like that of the Redback Spider that catch walking prey; and many others. Many of these web builders use silk enswathing and wrapping to subdue and 'package' their meal for immediate or later consumption.
Despite its great success as an insect trap, the orb web has undergone some interesting specialisations. For example, moths are a very abundant food source. The body and wings of moths are covered in scale-like hairs that can be easily shed, and this often allows them to struggle free of a silk trap. However, some orb web weavers have evolved long, ladder-like orb webs. In such elongate webs, moths lose so many scale hairs while struggling to get free that they become stuck before they can roll out of the web.
Even more specialised prey capture strategies have evolved in other descendants of orb weaver lineages. These involve simplification and modification of the orb web and highly specialised web handling behaviour. Two notable examples are the Bolas Spiders (Ordgarius), another moth specialist, and Net -casting Spiders (Deinopis).
Hunters without webs
There are many other araneomorph spiders that no longer build snare webs. Such spiders also have a surprising range of prey catching strategies. Many are ambush hunters like the flower or crab spiders (Thomisidae). These spiders have quite good eyesight. They sit in the open, on foliage (Sidymella spp.), flowers (Diaea spp.) or bark (Stephanopis spp.). Their body colours usually merge well with their background (flower frequenting species are capable of adjusting their body colour to suit), concealing them from both predators and prey. Using sight, vibration and touch senses they target insects and other prey alighting or walking nearby - like flies, butterflies and bees, some of which may be considerably larger than the spider. Their strong, spiny front legs allow them to grasp and hold the prey while it is bitten. A tropical Thomisid species, Phrynarachne decipiens, has gone about things a little differently. Its body colour and shape resembles a drop of bird dung. However, as well as looking like a 'turd' the spider also secretes a chemical scent that makes it smell like one! Flies and butterflies that feed on dung (yes, some butterflies do this) are attracted to these 'dung' spiders, which ambush and eat them.
Many other litter, bark and foliage hunting spiders use lie-in-wait ambush as well as active wandering strategies to catch their prey. These include groups like the ground spiders (Gnaphosidae), huntsman spiders (Sparassidae), sac spiders (Clubionidae) and sun spiders (Corinnidae). None of these groups have good vision and they depend more upon vibration and touch cues to sense and ambush any insects or other arthropods encountered - such senses are obvious attributes for night hunters (all except the corinnids). Water spiders (Pisauridae) hunt along stream and pond banks, their legs extended onto the water film sensing vibrations caused by fallen insects or small fish and tadpoles.
Some of these hunters, like white-tail spiders (Lamponidae) and assassin spiders (Mimetidae) are spider hunters that will readily invade the webs of other spiders. The remakable archaeid spiders are also spider hunters in leaf litter and moss. These small, bizarre looking spiders have extremely long, pointy heads and elongate, slender jaws. They use the long jaws and fangs to spear their spider prey.
Large eyed spiders like the wolf spiders (Lycosidae), jumping spiders (Salticidae) and lynx spiders(Oxyopidae), visually hunt and ambush prey in ground litter, on bark and and foliage. Probably the most remarkable of the visual hunters are the jumping spiders of the genus Portia. They use sight, disguise and a remarkable set of stalking strategies to capture spider prey, including other jumping spiders.
Spiders immobilise their prey in two ways - by biting and injecting paralysing venom, and by silk swathing and wrapping. Most hunting spiders simply grab and hold their prey in the pedipalps and front legs, while biting it. Many web builders use bands of swathing silk to throw over or wrap around the entangled prey, often before biting it, although larger web builders tend to bite first. Securely silk wrapped prey is sometimes stored in the web to be eaten later. Spider venoms affect the nervous systems of arthropod prey and interfere with nerve-muscle impulse transmission, resulting in paralysis. Venom also helps with the chemical break down of prey tissues. When feeding the spider regurgitates enzyme rich stomach fluids over and into its prey. This external digestion by venom and stomach chemicals, often aided by the grinding, masticating action of the fangs and toothed jaw bases and maxillae, reduces the prey's body and tissues to a chitinous soup. The liquid is sucked up through the spider's tube-like mouth, aided by the action of the pumping stomach, leaving the hard parts behind. Spiders like flower spiders (Thomisidae) inject digestive fluids into the bitten prey and suck out its liquefied internal tissues, leaving an almost intact body husk behind.