Harvestmen are also known as Daddy Long-legs, or Granddaddy long-legs, they look like spiders by having eight long spindly legs and are classified with them in the Class Arachnida, but that is where the similarity ends, as most have only two eyes (some cave-dwellers have none), are not venomous, do not produce silk, but most obviously, the body is not divided into two segments (cephalothorax and abdomen). Due to these and other features Harvestmen have been split from spiders at the Order level, with spiders placed into Araneae and harvestmen into Opiliones.
Like spiders, harvestmen go back a very long way to over 400 million years and have not changed substantially over that time, thus indicating a highly successful animal well adapted for living in the harsh mini-world environment of invertebrates. This success may also be due to their omnivorous diet where they will consume, vegetation, fungi, live and dead animals, etc. Unlike spiders that need to slurp up the liquid parts of invertebrates, harvestmen have different mouth parts and can also tear-off and eat small chunks of more solid food and so have a substantial survival advantage.
As can be seen, most harvestmen have a pair of exceptionally long legs, these are used like antennae to feel around its world in search of food and danger, as the pair of eyes on top of its head could not produce anything like clear vision. The legs if grabbed by a predator can be shed and will continue to twitch permitting a hasty retreat on the part of the harvestmen. Also the long gangly legs would keep the vital body way above ground level where most invertebrate predators hunt. So although a relatively simple physical shape, it has nevertheless a highly effective means of locating food and of defence.
Worldwide there are many thousands of Opiliones species, with a large number undescribed, these are further divided into several families with many genera, so I am not even going to try placing the above into any grouping. Locally, their body size is about the same as a small match head 3-4 mm (1/8") and very dark (although there are species elsewhere that are considerably larger). They tend not to keep still either, so trying to photograph them can be very frustrating as little black blobs on long-legs are not particularly informative.
The Harvestmen in the sandy heathland around Esperance will seek dry cover amongst leaf-litter, under loose bark, logs, rocks, etc. They are nocturnal and reasonably common, although usually overlooked.