17 September 2008

Spider and Scorpion Overview

Spider and Scorpion Overview

These and others are generally referred to as arachnids from their Class Arachnida and include amongst others, the Infraorders of Mygalomorphae (Primitive Spiders) and Araneomorphae (Modern Spiders) of which I shall concentrate.
With few notable exceptions, spiders are very difficult to identify to species level and most even to genus level. They usually require microscopic inspection of dead animals, not to mention published descriptions, which in turn mean access to a very good and up to date scientific library. This needs not only considerable expertise, but also considerable resources that I for one do not possess. So in most cases I shall only attempt to take identity to subfamily level, occasionally to genus, and rarely if a highly distinctive arachnid, to species level based on visual observation of live animals.

Unlike the reptiles, frogs and mammals where most local species are known and recorded (usually because there are not many of them), with invertebrates it is a very different story. The number of invertebrate species is staggering, with many yet to be described, so to attempt to give equal attention to these is not realistic for a casual observer like myself. But hopefully I will have encountered most of the more common species and been tempted to photograph them.

Spider Infraorders

Mygalomorphae the Primitive Spiders, are a group of quite distinctive burrowing spiders, or who construct and live within a silken tube like the Tree Funnel-web Spiders of NSW and Qld. (funnel-web spiders do not occur in WA, although several similar looking ones do, but these are all burrowing species). Mygalomorphs are usually medium to large hairy spiders of various dull shades of brown, grey and black. Fortunately for most arachnophobes the larger female spiders seldom if ever leave their burrows, so most of this group that are seen, tend to be the much smaller males who wander around at night in search of a mate. Very few Australian mygalomorph species are able to climb, so the vast majority are to be found on the ground.

Many of these primitive male spiders often pass without notice, just appearing like a small to medium dark colored spider, but on closer observation the differences to modern spiders soon become apparent. For a start they can be very aggressive if interfered with, so as the venom toxicity of many is unknown it is best to leave them alone. However they are usually very cooperative as photographic subjects with a tendency to freeze for extended periods when a light is shone on them, thus enabling you to take as many photos as you wish.

The most obvious difference between these spiders and modern ones is the position of the fangs. Modern spider species work like a pair of pincers coming together across their mouth parts, but with primitive spiders they point downwards, which mean they must rear up to strike down on their prey. Modern spiders with their pincer type fangs are far more adaptable and can capture prey from a web, wander around to ambush or run down, so they don't need to lie in wait down a hole but have developed many ingenious ways of capturing prey animals.

The Araneomorphae or Modern Spiders as stated are extremely diverse, with web builders, those that hunt without a web by ambush, pursuit or deception, some even hunt underwater, however some live in burrows and may even build a trap door (some wolf spiders), but they usually leave them to hunt and unlike primitive spiders, generally do not stay in them for extended periods.

The majority of modern spiders climb quite well, but a few do remain on the ground and actively hunt at night. Size-wise there is not a lot of difference between them, although the primitive spiders are generally much bulkier. Modern spider bites tend to be far less toxic to mammals than primitive species, although the Red-back Spider and others are rightly described as dangerous; nevertheless the funnel-web spider species are regarded as extremely dangerous.

The spider's eyes and their arrangement are an important identification aid and where possible their details should be noted if possible. Some modern spiders have two or more large eyes with accompanying good vision, these tend to be the ones that seek out their prey, whereas primitive spiders only have small eyes (often closely grouped), so their eyesight is poor. This limited vision presents a problem for the much smaller males when trying to mate. To combat this, many primitive male spiders have a spur or a notch on their front legs to prop up the female's fangs to keep her from treating him as her next meal. It often works, but not always.

With scorpions, I know of only two species in the Esperance region, so there is not the need for additional explanation here.