Tuesday, May 22, 2007

Prowlers of the night

They are not great climbers, don't build webs and hunt on the ground at night.

These are the ground dwelling spiders and there are hundreds of species spanning numerous families and dozens of genera. But to those who try to avoid spiders at any cost, or are simply not that interested, they tend to look much the same. Even to those who do take an interest, they are often confused with other spiders or lumped together into a handful of species. Their coloration usually is lack lustre, so few attract much attention. They are not highly venomous so are not sensational, and as they blend into their surroundings and seldom enter houses, are not regarded as pests. However, they are very common and several species would be living in most gardens.

Size-wise they can be very small and easily overlooked, or quite large with a ferocious appearance, but most are not frightening, generally they have long legs with a body size to 2 cm in length.

There are 25 genera in the family Lycosidae, with around 130 Australian species, so just calling them Wolf Spiders is probably the easiest way to go. They are identified from other families by the arrangement of their eyes that are in three rows of 4, 2, 2 with one pair much larger than the others. Some species dig shallow burrows and others climb low foliage, but all are mobile hunters, only using their nest as a daytime retreat.

Wolf Spider Hogna immansueta

These ground spiders should not be confused with the ones that live in silk lined holes (often with a trap door) and jump out to catch passing prey. These are primitive spiders and known as mygalomorphs from the main spider division in the suborder Mygalomorphae, which separates these from the modern spiders Araneomorphae. Some of these spiders are highly venomous and should be treated with the greatest of respect. One of the obvious differences between these two groups are the position of the fangs. The primitive ones have downward pointing fangs and must strike down on their prey. Whereas the modern spider fangs are designed to close on the prey like a pair of pincers, this permits these spiders to be more versatile in the way they hunt, of which running down their prey or lying in ambush are just a couple that ground spiders specialise.

A spider often confused with the Wolf Spider is the Nursery-web Spider from the family Pisauridae. Many have similar coloration to the Wolf Spider, but their eye arrangement is the main feature that separates them. Their eyes are arranged in two rows of 4 with the top row strongly recurved. The size of the eyes are similar, or the top row slightly larger. Another difference (can be seen in the photo below) is the female carries her egg-sac directly beneath her body, whilst the wolf spider trails it behind her.

Spiders or arachnids go back a very long way, possibly to when animals first emerged from the sea over 400 million years ago. The first fossils of the primitive mygaloph spiders date from the pre dinosaur period around 230 million years ago and the first modern araneomorph spider fossils from the Jurassic around 175 million years. The modern spiders with their various physical advantages then evolved into the many (and now more numerous and diverse) species we have today.

Below is another Pisauridae species, this one is commonly found on low-lying clumps of twigs waiting for insects to come within range. This species is much smaller than most wolf spiders, but can move quite quickly to catch its dinner. Like the wolf spider, these nursery-web spiders also reflect a torch light when shone in their eyes. So just scanning around with your torch held just below your eyes, you can pick them out from over 10 metres away, which is an effective method to judge their population size and preferred habitat.

Nursery-web Spider from the family Pisauridae

Modern ground dwelling araneomorph spiders, come from several families. They are hunters, do not construct a web, but some will construct a small daytime or nursery retreat and all protect their eggs in a structure made from silk web material produced from their spinnerets. Because they do not catch their prey in a web, they must restrict themselves to prey they can physically overcome, which generally would be smaller invertebrates, although with some aquatic spiders, vertebrates are also taken.

The same pisaurid species as above, but this one pretending to be an octopus. :)

Yet another Pisauridae species that is still a nursery-web spider, but is more commonly known as a Fishing Spider from the genus Dolomedes and is one of about a dozen species. This one is distinctively colored being dark brown with cream stripes. They move around easily on the surface film of still water, but can dive under to hunt small fish, tadpoles and aquatic insects. They will do the same to avoid predators. Body length is around 2 cm, making them among the largest of this group.

Fishing Spider a Dolomedes species

Although a Fishing Spider, they do not need water to survive. In my area the swamps usually dry completely during summer, it is then this spider may be found further afield. The one below I discovered in my outside bathroom, where she had decided to build her nursery-web on the ground between an old washing machine and a piece of corrugated iron.

Below is part of her nursery-web, here the spiderlings (100 plus) have hatched and after a week had ballooned off on the wind to make a life for themselves. However mum stayed with them until this period.

Below is one of the reasons mum stays with the kids, a Daddy-long-legs (Pholcidae species), is intent on gobbling up a few. This it did for a couple of days until mum noticed it, where upon it ended up a snack for her.

Another ground spider below is from the family Miturgidae and the genus Miturga (my thanks to the WA Museum for identification), it is part of a large group, some Australia wide. They build silken retreats to moult or produce egg-sacs. Some also construct a lose web framework amongst twigs near the ground.

Miturga species

It is interesting to note the legs on most of these ground spiders. They are usually strong and longish and used for running down and holding prey, but their size and arrangement is interesting. With most spiders at least one of more pairs of legs are longer than the others, plus they are usually arranged so some face forward and the others backwards. You can see with these spiders, that when at rest their legs are about the same length and arranged evenly like spokes in a wheel.

The spider below is an Argoctenus species from the family Zoridae (my thanks to the WA Museum for identification) and has just caught a moth. Spiders in this family generally hunt amongst low vegetation or leaf-litter. It is another species that looks like some members of Lycosidae the wolf spiders, except the legs tend to be more slender.

Zoridae species

Yet another ground dwelling spider, but this one very small, the body length would be no more than 5 mm, but it is extremely fast. I have no idea to which this species belongs, but I am not alone here, as there are so many ground dwelling spiders you need an expert to tell the difference between families, let alone between genera and species. So next time you see a similar shaped spider on your veranda, have a closer look. You may not be able to positively identify it, but now you should see there are different species, which make up a part of your fascinating garden eco-system.


Gaye from the Hunter said...

A fascinating review of ground-dwelling spiders, Jack.

I was unaware that the spiders that skim the water surface can actually dive. Of course, it makes perfect sense, but they must be a complex creature in structure to allow this.

I don't think I have ever seen the Nursery spiders, and hope that I can eventually observe this interesting family of spiders for long enough to determine that they belong to this family.

I think the only spiders I've seen carrying their egg-sacs under their body are black or dark spiders that I casually refer to at home as 'house spiders', although I have seen them in bushes too.

Spiders are fantastic creatures to observe.

Your photography is excellent.


Esperance Blog said...

Hi Gaye,
Thanks for the interest and comments. The nursery-web spiders are more common around me than the wolf spiders. So maybe they don't get on together :) or perhaps the habitat has something to do with it. I think there is far more that we don't know about spiders than we do.