CANTHARIDES, or Spanish Flies, the common blister-beetles (Cantharis vesicatoria) of European pharmacy. They are bright, iridescent, golden-green or bluish-coloured beetles (see Coleoptera), with the breast finely punctured and pubescent, head and thorax with a longitudinal channel, and elytra with two slightly elevated lines. The insect is from half-an-inch to an inch in length, and from one to two lines broad, the female being broader in the abdomen and altogether larger than the male. It is a native of the south of Europe, being found in Spain, France, Germany, Italy, Hungary and the south of Russia, and it is also obtained in Siberia. The Spanish fly is also occasionally found in the south of England. The insects feed upon ash, lilac, privet and jasmine leaves, and are found more rarely on elder, rose, apple and poplar trees. Their presence is made known by a powerful disagreeable odour, which penetrates to a considerable distance. They are collected for use at late evening or early morning, while in a dull bedewed condition, by shaking them off the trees or shrubs into cloths spread on the ground; and they are killed by dipping them into hot water or vinegar, or by exposing them for some time over the vapour of vinegar. They are then dried and put up for preservation in glass-stoppered bottles; and they require to be very carefully guarded against mites and various other minute insects, to the attacks of which they are peculiarly liable. It has been shown by means of spectroscopic observations that the green colour of the elytra, etc., is due to the presence of chlorophyll; and that the variations of the spectral bands are sufficient, after the lapse of many years, to indicate with some certainty the kind of leaves on which the insects were feeding shortly before they were killed.
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. From about one-fourth to rather more than one-half per cent, 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 to 1% of cantharides. It has the formula C10H14O4, and on hydrolysis is converted into cantharinic acid, C10H14O5. It crystallizes in colourless plates and is readily soluble in alcohol, ether, etc., but not in water. The British Pharmacopeia contains a large number of preparations of cantharides, but the only one needing special mention is the tincture, which is meant for internal administration; the small dose is noteworthy, five minims being probably the maximum for safety.
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. Thereafter the part becomes warm and painful, owing to marked local vascular dilatation. This is the typical rubefacient action. Soon afterwards there is an accumulation under the epidermis of a serum derived from the dilated blood-vessels. The numerous small blisters or vesicles thus derived coalesce, forming a large sac full of "blister-fluid." The drug is described as a counter-irritant, though the explanation of this action is very doubtful. Apparently there is an influence on the afferent nerves of the part which causes a reflex contraction - some authors say dilatation - of the vessels in the internal organs that are under the control of the same segment of the nervous system as that supplying the area of skin from which the exciting impulse comes. When applied in this fashion a certain quantity of the cantharides is absorbed.
Taken internally in any but minute doses, the drug causes the most severe gastro-intestinal irritation, the vomited and evacuated matters containing blood, and the patient suffering agonizing pain and extreme depression. The further characteristic symptoms are displayed in the genito-urinary tract. The drug circulates in the blood in the form of an albuminate and is slowly excreted by the kidneys. The effect of large doses is to cause great pain in the renal region and urgent wish to micturate. The urine is nevertheless small in amount and contains albumen and blood owing to the local inflammation produced in the kidney by the passage of the poison through that organ. The drug often has a marked aphrodisiac action, producing priapism, or in the female sex the onset of the catamenia or abortion.
Cantharides is used externally for its counter-irritant action. There are certain definite contra-indications to its use. It must not be employed in cases of renal disease, owing to the risks attendant upon absorption. It must always be employed with caution in the case of elderly persons and children; and it must not be applied to a paralysed limb (in which the power of healing is deficient), nor to parts upon which the patient lies, as otherwise a bed-sore is likely to follow its use. The drug is administered internally in certain cases of impotence and occasionally in other conditions. Its criminal employment is usually intended to heighten sexual desire, and has frequently led to death.
The toxic symptoms have already been detailed, the patient usually dying from arrest of the renal functions. The treatment is far from satisfactory, and consists in keeping up the strength and diluting the poison in the blood and in the urine by the administration of bland fluids, such as soda-water, milk and plain water, in quantities as large as possible. External warmth should also be applied to the regions specially affected by the drug.
A very large number of other insects belonging to the same family possess blistering properties, owing to their containing cantharidin. Of these the most remarkable is the Telini "fly" of India (Mylabris cichorii), the range of which extends from Italy and Greece through Egypt and central Asia as far as China. It is very rich in cantharidin, yielding fully twice as much as ordinary cantharides. Several green-coloured beetles are, on account of their colour, used as adulterants to cantharides, but they are very easily detected by examination with the eye, or, if powdered, with the microscope.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)