Parotoid macroglands in toad (Rhinella jimi): Their structure and functioning in passive defence

ABSTRACT

When toads (Rhinella) are threatened they inflate their lungs and tilt the body towards the predator, exposing their parotoid macroglands. Venom discharge, however, needs a mechanical pressure onto the parotoids exerted by the bite of the predator. The structure of Rhinella jimi parotoids was described before and after manual compression onto the macroglands mimicking a predator attack. Parotoids are formed by honeycomb-like collagenous alveoli. Each alveolus contains a syncytial gland enveloped by a myoepithelium and is provided with a duct surrounded by differentiated glands. The epithelium lining the duct is very thick and practically obstructs the ductal lumen, leaving only a narrow slit in the centre. After mechanical compression the venom is expelled as a thin jet and the venom glands are entirely emptied. The force applied by a bite of a potential predator may increase alveolar pressure, forcing the venom to be expelled as a thin jet through the narrow ductal slit. We suggest that the mechanism for venom discharge within all bufonids is possibly similar to that described herein for Rhinella jimi and that parotoids should be considered as cutaneous organs separate from the rest of the skin specially evolved for an efficient passive defence.