The term "daddy long legs" actually refers to two different arthropods: the daddy long legs spider and the harvestman--the latter of which is an arachnid but is not a spider. The two are superficially similar, but in reality are very different. Much confusion arises from the common practice of calling them both by the same name.
Harvestmen
Daddy long legs, as the term applies to harvestmen, are identified easily by their extra-long legs. While the body is a single oval segment 2 to 10 millimeters in length, the flexible appendages on these arachnids extend up to 50 millimeters. About two times a month, the harvestman molts (outgrows) its old skin, taking almost 20 minutes to pull the long legs out of their old coverings. Harvestmen tend to be gray or brown with chevron-type stripes. This helps them blend in with their surrounding environment--under rocks, logs and piles of leaves.
They do not have the apparatus for spinning silk, so unless they are trapped by another spider's weaving, they will not be found in a web. Because of this, the harvestman is an arachnid, but not technically a spider.
Spiders
Another type of daddy long legs is actually a spider. These daddy long legs spiders are virtually indistinguishable from harvestmen from a distance, but upon close inspection, you can see that the body has two parts rather than being a single piece. All eight legs are attached to the front body piece. Also, rather than bearing chevron-shaped stripes, this spider has a stripe along its belly and, unlike the harvestman, it can and does spin elaborate webs.
Behavior
You also can distinguish between harvestmen and daddy long legs spiders by observing their behavior.
If you happen upon a harvestman at meal time, you will observe the spider cleaning its orifice by dragging a leg through it periodically. If threatened, harvestmen release a foul-smelling chemical that is meant to discourage predators.
The daddy long legs spider, on the other hand, begins to turn rapidly in its web, rotating quickly until it becomes a mere blur. This is a very effective defense mechanism, making the spider difficult to see.
Diet
The two types of daddy long legs differ also in their diets. Without venom, fangs or a web, the harvestmen have no way to trap and restrain live food. Therefore, they are opportunistic feeders, eating decaying plant and animal matter. In captivity, they survive on crusts of bread and butter, and the occasional bit of fatty meat.
With its ability to spin webs, the daddy long legs spider can trap its prey, but it seems to prefer hiding in the webs of other spiders. To launch an invasion, the daddy long legs spider approaches the web, causing the silk strands to tremble and mimicking the resistance of ensnared prey. When the web owner approaches to investigate, the daddy long legs spider eats the host.
The Legs
If you handle a daddy long legs of either variety, there is a strong possibility that one of its legs will fall off. It is one trait they have in common and may have developed as a way to escape from possible enemies. Unfortunately, this means of escaping predation comes at a high cost: The legs also function as an extension of the senses, enhancing sight, taste, touch and hearing.
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