THE SCORPIONIDA, OR TRUE SCORPIONS
The true scorpions are widely distributed throughout warm countries and everywhere bear an evil reputation. According to Comstock (1912), about a score of species occur in the Southern United States. These are comparatively small forms but in the tropics members of this group may reach a length of seven or eight inches. They are pre-eminently predaceous forms, which lie hidden during the day and seek their prey by night.
The scorpions ([fig. 11]) possess large pedipalpi, terminated by strongly developed claws, or chelæ. They may be distinguished from all other Arachnids by the fact that the distinctly segmented abdomen is divided into a broad basal region of seven segments and a terminal, slender, tail-like division of five distinct segments.
The last segment of the abdomen, or telson, terminates in a ventrally-directed, sharp spine, and contains a pair of highly developed poison glands. These glands open by two small pores near the tip of the spine. Most of the species when running carry the tip of the abdomen bent upward over the back, and the prey, caught and held by the pedipalpi, is stung by inserting the spine of the telson and allowing it to remain for a time in the wound.
The glands themselves have been studied in Prionurus citrinus by Wilson (1904). He found that each gland is covered by a sheet of muscle on its mesal and dorsal aspects, which may be described as the compressor muscle. The muscle of each side is inserted by its edge along the ventral inner surface of the chitinous wall of the telson, close to the middle line, and by a broader insertion laterally. A layer of fine connective tissue completely envelops each gland and forms the basis upon which the secreting cells rest. The secreting epithelium is columnar; and apparently of three different types of cells.
1. The most numerous have the appearance of mucous cells, resembling the goblet cells of columnar mucous membranes. The nucleus, surrounded by a small quantity of protoplasm staining with hæmatoxylin, lies close to the base of the cell.
2. Cells present in considerable numbers, the peripheral portions of which are filled with very numerous fine granules, staining with acid dyes such as methyl orange.
3. Cells few in number, filled with very large granules, or irregular masses of a substance staining with hæmatoxylin.