20 



Poisonous Arthropods 



THE SCORPIONIDA, OR TRUE SCORPIONS 



The true scorpions are widely distributed throughout warm coun- 

 tries and everywhere bear an evil reputation. According to Comstock 

 (191 2), 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 chelas. They may be distinguished from 

 all other Arachnids by the fact that the dis- 

 tinctly 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 woxind. 



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 colimmar; and apparently of three different types of cells. 



I. 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 

 hematoxylin, lies close to the base of the cell. 



A true scorpion. 

 Comstock. 



After 



