General Information: The members of these three orders bear some resemblance to each other and to the solifugids. However, in the case of palpigrades with respect to the other two orders, resemblance is superficial (see below).
These orders are not familiar to many because of their size (although uropygids are large, schizomids and palpigrades are very small), their secretive habits (individuals are typically found in caves, the leaf litter and under rocks and vegetative matter), and their being restricted to the warmer regions of the Earth.
For information about each order, please scroll down or use these links:

Uropygids are tropical and subtropical arachnids. As in the closely related schizomids, amblypygids, and solifugids, the first pair of what are walking legs in other arachnid orders are elongated and thin. These legs act as sensory organs, somewhat like antennae or "feelers". The very robust appendages immediately anterior to these sensory legs (and just posterior to the chelicerae), are called the pedipalps and are used to crush and hold prey. They are conspicuous in the photos below.
Uropygids possess glands near the rear of their abdomen. When they are disturbed, they lift the rear of the abdomen and direct a spray of formic and acetic acids towards their enemy. This spray has the smell of vinegar (since it contains acetic acid) and thus the common name "vinegarone". Note that the name "uropygid" literally means "tail rump" and refers to the tail-like telson on the end of the opisthosoma.

Photos of Uropygi ("Whip Scorpions")

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Schizomida

Literally "split or cleaved middle", schizomids are sometimes placed with the uropygids. Their most obvious difference from uropygids (above) is that the thickened, plate-like exoskeleton that covers the anterior portion of the animal's body (known as the prosoma or cephalothorax) is divided into two separate plates. This can be noticed in the individual shown below. Schizomids are generally smaller than uropygids (for a better comparison click on the thumbnail photos to obtain larger versions of the schizomid and the uropygid).
Notice the same pattern of the three hindmost legs being used for walking, with the pair just in front of these being modified as a "feeler". The pedipalps (just anterior to the sensory legs) are well developed although they are not as conspicuous in the pictured schizomids as are those of the uropygids shown above.

Stenochrus portoricensis on lava. From LaPalma, Canary Islands © Copyright 2001 by Hans Henderickx Hi Res: 664 X 444 pixels, 52 KB

Stenochrus portoricensis from LaPalma, Canary Islands © Copyright 2001 by Hans Henderickx

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Palpigradi

Palpigrades are very small arachnids (less than 3 mm in length) that are found beneath stones and in caves. For this reason, little is known of their habits.
Although at first glance they look very much like uropygids and schizomids, a closer look shows striking differences. Unlike the other two orders, in palpigrades the pedipalps are not modified into highly conspicuous crushing appendages (the huge anterior appendages seen on the uropygids at the top of this page). Instead, they are used as walking legs. The next pair of appendages (what would be the first pair of walking legs in spiders and scorpions) are legs that have been modified into "feelers", as is the case in uropygids, schizomids, amblypygids, and solifugids.

modified March 11, 2006