March 14 is Save a Spider Day, and this year, Curator Invertebrates Phil Sirvid is answering some spidery questions from the public and Te Papa kaimahi. ️
How big are New Zealand’s spiders?
Most of our spiders aren’t very big at all, with most only a few millimetres long when fully grown. However, there are exceptions! Our largest spider by leg span is the Nelson cave spider (Spelungula cavernicola) and females may reach 15 cm across. Long legs like this are a common feature in cave-adapted invertebrates. If you meet something nasty in the dark, it’s better to do it at leg’s length rather than near the main body. Our heaviest spiders are the chunky-looking tunnelwebs and trapdoor spiders.
Are there really eyebrow spiders and do I need to worry about them?
For anyone feeling a bit squeamish about this, I have good news and bad news.
The good news is that there are no eyebrow spiders.
The bad news is that there are mites that are right at home in the hair follicles of our eyebrows and eyelashes. These microscopically small animals look a bit like tapered sausages with legs at the front end, and they consume skin cells and oils. About half of adults have them although most would never know they are there.
Why do spiders need all those legs?
I could say spiders, like other arachnids, have eight legs because they evolved that way, but that’s not a very helpful answer. However, I can say that having that having spare legs is useful. While it seems obvious that losing a leg to a predator is better for a spider than losing its life, there wouldn’t be much point if the spider couldn’t function properly.
Some studies suggest that spiders that lose a leg or two aren’t affected too badly, although the more legs lost, the harder things get. Spiders can regrow legs whenever they moult to grow. However, the new leg will never be as big as the original, and if a leg is lost after the spider reaches adulthood (and no further moulting), it’s gone forever.
Why don’t spiders fly?
Many spiders DO fly, but they don’t need wings to do it. Instead, young spiderlings often use a process called ballooning to take to the skies and disperse to new areas. After climbing clear of the ground, the spider points its rear end skyward and plays out some silk. Even a light breeze can carry the spider aloft. But there’s more than meets the eye here because spiders can balloon even on windless days.
It turns out the spiders are sensitive to electric fields, including in the atmosphere. When these fields are strong enough, it can cause spiders to initiate ballooning behaviour, and natural electrostatic forces can keep a spider airborne. Spiders have been recorded at altitudes of 4km and can travel long distances by ballooning. Most summers we get reports of species such as golden orbweb spiders (Trichonephila edulis) that have ballooned across the Tasman Sea as very much smaller spiderlings. As they make webs around a metre across, they are very hard to miss!
What do spiders need all those eyes for?
Most spiders have eight eyes (although some have six or fewer). Despite having so many eyes, many spiders, particularly web-building spiders don’t see very well at all. They can see rapid movement, light and dark and not much else. When it comes to prey capture, the vibrations of prey interacting with the web will tell them far more than their eyes will.
While most spiders don’t see very well, the number and positioning of eyes does give them a larger field of vision than we have. However, some hunting spiders have pretty good eyesight. This is particularly true of the jumping spiders (family Salticidae). Not only are they capable of recognizing shapes and colours, but the positioning of their eyes means their field of view is almost 360 degrees.
What is the difference between spider silk and silkworm silk?
Silkworm silk is coarser and weaker than spider silk, but it does have one big advantage: It’s much easier to harvest. You can throw some mulberry leaves into a basket of silkworms and gather up their silken cocoons. Spiders are not nearly as amenable for large-scale rearing as they are often aggressively territorial. Throw a bunch of spiders into a basket and you are likely to end up with one well-fed spider!
However, people have harvested spider silk and made clothing, but it’s not very cost-effective. A golden spider silk cape was made from the silk over a million spiders harvested by a team 80 people over five years. There is a considerable amount of research into synthesising spider silk.
Are there any cool symbiotic relationships involving spiders?
There are many, but let’s recap what symbiotic relationships are. They are interactions between two different kinds of organism in close physical proximity. That can include mutualism where both species get some benefit, commensalism where one benefits without affecting the other, and parasitism where one gains at the expense of the other.
Let’s start with that last one. If you find a classic cart-wheel shaped web of an orbweaver spider, you may see some tiny, silvery looking spiders on the outskirts. These are dewdrop spiders (Argyrodes antipodiana) and they will steal small prey items from the much bigger owner of the web. This is called kleptoparasitism.
Commensalism can be as simple as a spider spinning its web in a tree. The tree isn’t affected by this at all, but the spider has a nice place to set up shop. But sometimes spiders and plants team up for mutual benefit. Some crab spiders (family Thomisidae) live in carnivorous pitcher plants. The spiders catch flies and feed the pitcher plant by dropping corpses into its digestive fluids. The spider’s bodily wastes also help fertilize the plant.
Not only does the plant attract plenty of flies for the spider to eat, but whenever danger threatens, the spider dive into the digestive fluids to take cover. The spider can store air trapped against its body and doesn’t stay long enough to risk becoming a meal itself. When danger passes, it can get climb out thanks to a strand of silk it leaves behind when it takes the plunge.
What can you do for spiders in your backyard?
The first thing you can do is return spiders to your backyard when they wander indoors and you’re not happy to see them there. Place a jar over the spider, slip a piece of card under both the spider and the jar’s mouth and then turn the jar right side up. The spider is safely contained and ready for release outside.
Otherwise, the best thing you can do is simply let them get on with their lives. They’re really not interested in bothering you, whether you feel bothered or not, and they have their part to play in keeping the balance of nature.
Thanks. That was great. Life is full of mystery….