Carpenter (1899, 1902-1904) has lately endeavoured to show an exact numerical correspondence in segmentation between the Hexapoda, the Crustacea, the Arachnida, and the most primitive of the Diplopoda.
SPIDERS, the common English name of Arachnida of the order Araneae, resembling the Pedipalpi in many structural points, but differing from them as well as from all other Arachnida in retaining short abdominal appendages known from their silk-manipulating function as spinnerets or spinning mamillae, with which are associated silk glands.
It is probably owing to the possession of such glands and the varied purposes for which the silk is used that spiders as a group far surpass the other orders of Arachnida, with the possible exception of the Acari (mites and ticks), in diversity of form and of size, in numbers of genera and species, in extent of geographical distribution, and in adaptation to varied habitats.
The phenomena, known as "protective resemblance," or similarity to inanimate objects or vegetation, and the kindred phenomenon of "mimicry," or beneficial likeness to certain protected species of animals, are common in the group. In these particulars, considered in their entirety, spiders show a marked contrast to other Arachnida, such as the scorpions, pedipalps, book-scorpions and so-called harvest spiders, which by comparison are remarkably uniform, within the limits of the orders, in structure, habits and other respects.
The possession of silk-glands has also profoundly influenced the geographical distribution of spiders and has enabled them to cross arms of the sea and establish themselves on isolated oceanic islands which most of the orders of Arachnida are unable to reach.