Cantharides owe their value to the presence of a peculiar chemical principle, to which the name cantharidin has been given.
It is most abundant in large full-grown insects, while in very young specimens no cantharidin at all has been found.
Of cantharidin has been obtained from different samples; and it has been ascertained that the elytra or wing-sheaths of the insect, which alone are used in pharmacy, contain more of the active principle than the soft parts taken together; but apparently cantharidin is most abundant in the eggs and generative organs.
Cantharidin constitutes from z to r% of cantharides.
The external action of cantharides or cantharidin is extremely characteristic. When it is applied to the skin there are no obvious consequences for some hours.
It is very rich in cantharidin, yielding fully twice as much as ordinary cantharides.
Those which act on the skin: The best known of these is cantharides (Spanish fly), the active principle of which is a colourless crystalline body - cantharidin - which is extremely irritating.