Spider Biology FAQs
- What is the function and origin of silk glands and spinnerets in spiders?
- Why don't spiders get stuck to their webs like the insects that they catch?
- Why don't orb weavers and other spiders fall off their webs?
- Why can some spiders climb slippery surfaces such as glass or run across ceilings?
- Do spiders sleep?
What is the function and origin of silk glands and spinnerets in spiders?
The development of spinnerets and silk represents a major evolutionary shift that has defined the biological and ecological uniqueness of spiders within the arachnids. Silk glands produce the silk that the spider uses for a variety of purposes. The spinnerets are the special organs that the spider uses to extract and manipulate the silk as is it is produced from the silk glands.
Spiders evolved from ancestors that had limbs on the abdomen, as did arthropods like crustaceans such as crayfish. In fact, one of their few living marine relatives, Limulus, the so-called "king crabs", has retained abdominal limbs, which have been lost or greatly modified in terrestrial spiders and other arachnids. The spiders' spinnerets are almost certainly derived from these ancestral abdominal limbs. In the basal (lowest) segments of spiders' limbs are small excretory glands - the coxal glands - that secrete and excrete waste body fluids. It seems that the silk glands may represent highly modified excretory glands that now manufacture silk instead of waste products, just as the spinnerets represent highly modified limbs.
It is possible that an intermediate stage in this process could have been the production of a secretion that included pheromone (scent) chemicals put out by the spider as a primitive "signal line" by which a spider could find its way back to its retreat burrow. This role was then taken over by the production of silk. The silk then became useful not only as a safety line, but also for prey capture, manufacturing egg sacs and a host of other activities.
[Modified from text by Dr Mike Gray - Principal Research Scientist (Spiders)]
Reference: Foelix, R.F.1996. Biology of Spiders. Oxford Thieme.
Why don't spiders get stuck to their webs like the insects that they catch?
If you look at an orb-weaving spider in its web, you'll notice that the body is held slightly clear of the web, especially when the spider is moving about. The spider has only minimal (but vital) body contact with its web via the claws and bristles at the tip of each leg. Compared to its prey, which crashes or blunders into the web, the spider has only a tiny portion of its surface area in contact with a very small amount of silk at any time. This is obviously an important factor when moving on a sticky web - the less contact the better.
Another important factor is that not all silk lines in a sticky web are sticky. For example, the central part of an orb web (where the spider sits) is made of dry silk, as are the spokes supporting the sticky spiral line, which the spider can use when moving around its web. It's only when the spider makes a quick, direct charge across the sticky spiral to capture prey that it may cause some disruption to the web - but it never gets stuck.
Spiders also spend a lot of time grooming their legs. The spider draws the ends of its legs through its jaws to clean them of debris, which may include silk fragments. This is a very important maintenance activity that contributes to efficient function of the claws and bristles. As well as cleaning them, some secretions from the mouthparts may help make the leg tips less susceptible to sticking.
Why don't orb weavers and other spiders fall off their webs?
Most web-building spiders have three claws on their tarsi (feet) - two combed main claws and a smooth central hook. The web silk is only grasped by the hook, and is pushed against serrated bristles, which snag the silk and hold it. When the hook is released by a special muscle, the elastic silk simply springs away from the hook.
Why can some spiders climb slippery surfaces such as glass or run across ceilings?
Many hunting spiders possess dense hair tufts called scopulae under the claws of their tarsi (feet). These scopulae allow many spiders to walk on smooth vertical surfaces, across ceilings and even window panes. Each individual scopula hair splits into thousands of tiny extensions known as end feet. These end feet increase the number of contact points of the tarsi with the surface, creating great adhesion. This is similar to the adhesion forces at work in vertebrates such as skinks and geckos, which can also walk on ceilings with ease. The scopulae can be erected or laid flat by hydraulic pressure through changes in the pressure of the hemolymph (blood supply).
Do spiders sleep?
It really depends on how you define "sleep". All animals have some sort of 'circadian' rhythm - a daily activity/inactivity pattern. Some are active during the day - diurnal - others are active at night time - nocturnal/crepuscular. The periods of inactivity are characterised by withdrawal (to a shelter perhaps) and a drop in metabolic rate.
This applies to spiders as well, although no studies have been done to measure the period of time spent in such a state or at what times different species do it. It seems that spiders with good eyesight that rely on vision to capture prey may tend to be more active in daylight hours, whereas others that rely on snares/webs could be active at other times, but this is not necessarily the case for all species.
In cold climates, spiders "overwinter", which means that they have a kind of hibernation period. Overwintering involves a drop in metabolic rate, where the spiders bring their legs into their body and remain huddled in a shelter during the coldest months of the year.
This ability to shut down for a long period of time indicates that they might be able to do it for shorter periods in their everyday cycle, which could be seen as a form of sleep or rest.
Information from Foelix, R.F. 1996. Biology of Spiders. Oxford Thieme. Arachnology section, Australian Museum
