Fine structure of the stinger aculeus in Euscorpius

A scorpion's last metasomal segment (telson) consists of a bulbous base that contains two venom glands and a curved tip (aculeus) where two venom ducts open to the outside. These two openings lie laterally just before the very tip of the aculeus; to see both of them at the same time, the stinger has to be looked at “tail-on” from the dorsal side. The two venom ducts have a distinct cuticular lining, which can be recognized in a transparent exuvia as long tubes (1 mm) extending from the distal pores back to the venom glands. Whereas the proximal bulb has many long sensory hairs on its surface, the distal aculeus is very smooth but contains small pits with tiny club-shaped hairs. These are probably contact chemoreceptors. The advantage of such sunken sensory hairs is certainly that the stinger can penetrate into prey (or foe) but can still perceive mechanical or chemical stimuli. Additionally, the aculeus bears several slit sensilla and numerous fine pores of unknown function. The aculeus is thus not only a well-adapted injection device but also contains sensory structures, which provide information on mechanical and chemical input.