Spiders Without Poison Glands
Uloborids Have Modified Their Prey Immobilization Technique
Oct 29, 2009
Typically, spiders do not have chewing mouthparts. They usually spin a web with sticky threads, sit on their webs waiting for prey to become entangled, then inject both a venom that subdues and kills the prey, and enzymes to digest the animal. The prey is wrapped in silk until digested at which time the spider drinks it. Instead of adding glue to their sticky threads, the uloborid spiders make rather primitive, highly teased web fibers to ensnare prey and cannot poison their prey because they have secondarily lost their venom glands.
A Different Silk
Among others, spiders of the worldwide orb weaver family Uloboridae produce a primitive silk that does not have glue droplets, but is teased to entangle the body parts of insects that fly into it. These spiders have specialized combs of hairs on the tarsi of their hind legs that sweep back and forth over an array of spinnerets, the cribellum, as the array produces up to two dozen or more strands of silk. This activity causes the fine silk threads to form cribellate silk, a sticky tangle of loops (much like Velcro loops), that ensnares and holds their prey until the spider can spin a tight cocoon that immobilizes the prey.
Although expensive to produce both energetically and temporally, cribellate silk does not dry out and lose its stickiness – a feature that means the silk does not have to be replaced as often if it is not damaged by prey tearing free from the web. Perhaps because of the expenses of producing cribellate silk, uloborids do not eat their webs each morning as many orb weaving spiders do. Rather, they repair damaged webs and often use an old web as support for a new one. The old webs do not serve to capture prey, but are useful for early predator detection as vibrations originating in old webs are transmitted directly to the spider sitting on the new one.
Cribellate silk loses its adhesiveness when wet and adheres to the wing scales of moths which are readily removed from the wings, allowing the moths to easily escape the web. Thus, cribellate silk is less appropriate for night hunting spiders than for day hunting spiders.
While both sexes of uloborid spiders produce cribelular silk when young, males lose this ability when they reach maturity as they spend most of their adult life searching for females. Males often mate with several females while females normally mate only once.
Prey Crushers
In addition to using cribellate silk, uloborid spiders do not have venom glands to kill their prey. These spiders wrap their living prey in many layers of silk – sometimes using up to 140 meters (450 feet) of it, and use the silk coffin to kill the prey by suffocation or crushing. Each strand of silk exerts a tiny amount of pressure on the prey, but the combined force exerted by the entire cocoon is quite large and legs may be bent out of shape or ripped off off, eyes crushed, and wings torn as the spider wraps the prey.
The title image of this article shows one leg of a fly next to the silken cocoon that crushed the rest of the fly. Note the legs partially sticking out of the cocoon. The entire fly is in the insert box labeled "a".
Digesting the Prey
Uloborids do not inject digestive enzymes into their prey like other spiders. After the prey is encapsulated, the spider slathers digestive enzymes all over the silk. Some of the silk is digested and almost all of the prey liquified in the process. The spider soon begins sucking up the fluids from the surface of the cocoon. What is not digested is simply cut out of the web and discarded.
Here are links to other articles describing unusual spider behaviors.
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