The clasper is erected by powerful adductor muscles as well as by vascular dilatation and is used as an organ of intromission. The gland secretes simultaneously from all its papillae a fluid which washes sperm down the clasper-tube into the female's cloaca. A centre for clasper erection exists in the cord between spinal nerves 48 to 50. In a spinal male tapping on the anal region causes a reflex erection of the claspers. Adrenaline apparently lowers the stimulus threshold for clasper erection.
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Michael H. Friedman (1935) studied this question.