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Oil Beetles -- Pleasures and Threats


(The following article appeared in the May 1993 issue of the Newsletter)

Beetles belong to the insect order Coleoptera and represent the most prolific of all life forms in the present evolutionary state of the earth’s history; nearly one quarter of all extant described life forms on the planet are beetles. Estimates of the total number of beetle species are variously put between one and ten million! Out of so many species, relatively few are harmful to mankind and many are actually beneficial. Just a few species are poisonous, including a group known as the Oil beetles. The long association of these insects with mankind as a source of love potions, medicines and poisons is a fascinating story and is dealt with in this article. No less fascinating is the strange reproductive biology of these beetles and their parasitic habits, which will be the subject of the next article in this series.

Warning colors of Oil or Blister beetles

Not many desert insects could be considered gaudy; even most of the butterflies might be described as subdued in coloration. There are, of course, exceptions, but where these occur, there are often good reasons for showiness.

Two large butterflies, often common in Al Ain, are very brightly marked. Why? The bodies of the Asian Citrus Swallowtail (Papilio demoleus) and the Plain Tiger (Danaeus chryshippus) contain poisons which their caterpillars extracted and accumulated from the respective larval foodplants – citrus fruit trees and Sodom’s Apple. The bright colors of these two butterflies serve to remind potential predators (birds, lizards and the like) that they are unpalatable.

To biologists these bright color schemes are often called "warning colors". Such warning coloration is by no means confined to butterflies – it is common amongst the bees and wasps and a host of other insects, including some groups of beetles.

In fact, springtime in the desert brings forth an amazing and almost endless succession of brightly colored beetles, all from the same family and all advertising the fact that they are poisonous species. These beetles belong to the family Meloidae and they are commonly known as Oil Beetles or Blister Beetles for reasons that we will come to. Many of the Oil Beetles are very brightly colored indeed and pleasing to the eye, some being bright scarlet with black markings, others bright yellow with a pattern of black spots.

One common species has a smart color scheme of bright chestnut brown with darker markings, but a few species are more or less cryptic in their coloration, being metallic gray or else some somber shade of brown or black. In contrast to the majority of beetles, whose bodies are very heavily armored, Oil Beetles, especially those with warning coloration, are soft-bodied.

Dozens of species occur in our region from late February until early June in large numbers, so that even the non-naturalist easily notices their bright coloration. One can be walking on the gravel plains of Oman or in the paddock zone of the Al Ain Zoo in April and spy from a distance what appears to be a patch of unidentified bright red flowers. However, on close approach, the flowers turn out to be brilliant red Oil Beetles, hundreds of them cling in groups to the available vegetation.

Unlike most desert beetles, these make no attempt to escape. If you try to disturb them, they may fly away awkwardly to the next plant. If you pick one up, you will soon notice that your hand becomes stained with an orange-yellow fluid released by the beetle. This is the time to drop the beetle and to look around for a bar of soap and a copious supply of water with which to wash away the offending fluid, before it has a chance to get to work on your skin!

This, if you like, is the second and final warning given by the oil beetle of its unpalatability. All parts of the beetle are poisonous, including the haemolymph ("blood"), and by molesting the beetle, it is stimulated to release drops of oily haemolymph from the leg joints.

Reflex bleeding like this is a fairly common chemical defense mechanism in beetles and occurs for example in most Ladybirds (Chrysomelidae), including the aptly named Bloody-nosed Beetles (Timarcha), as well as in the Meloidae. The haemolymph secreted by these other beetles is, in general, not especially poisonous and instead has a bitter taste causing would-be predators to spit out the insect. Meloid haemolymph, however, is deadly poisonous and contains large amounts of cantharidin, a vesicant agent causing severe skin blistering (hence, Blister Beetle), internal poisoning, and death.

Other poisonous beetles

Only very few other beetles are quite as poisonous as the meloids. Small blue and red Rove Beetles of the widely distributed genus Paederus contain a powerful analogue of cantharidin, known as paederin. These beetles often fly into houses and may land and crawl upon the skin. If an attempt is made to brush off the insect, it may release the poison, causing painful blisters that take weeks to heal.

In Brazil, I have seen grown men confined for days in their hammocks, as a result of just a few such lesions. In January 1985, a sudden plague of billions of these beetles occurred throughout the northeast of Brazil (an area bigger than Western Europe) bringing many agricultural activities such as cane cutting to a virtual standstill. Then, just as suddenly, the beetles disappeared. In the Indian sub-continent, related beetles have caused similar problems.

Other species of Paederus are found in Britain and Europe, but I have never heard of any cases of human suffering as a result of contact with them. Similar beetles probably occur in Arabia, but I have yet to find them in our region. There are two other groups of very poisonous beetles.

Certain Diving Beetles (Dytiscidae), when molested release a milky fluid which is reputed to be neurotoxic against fish and amphibians and is also said to numb the fingers of the human hand. South African Leaf Beetles of the genera Diamphida and Blepharida have been used by bushmen as a source of a deadly arrow poison in much the same way that dendrobatid tree toads are used for a similar purpose in South America.

Aphrodisiacs and Aqua Tofana

Blisters caused by cantharadin can be painful and take a long time to heal, but in the past Oil Beetles and their product have enjoyed a long and checkered association with the practice of medicine and have only comparatively recently been discredited. However, their use as an aphrodisiac under the names "Spanish Fly", "Pastilles a la Richelieu" or "Bonbons a la Marquis de Sade" is even better known and many deaths have resulted, even in modern times, from the oral ingestion of cantharidin in the pursuit of pleasure.

The aphrodisiac action of small doses of cantharidin is to cause a tingling sensation at the opening of the urinary tract, but there is nothing magical about this effect. It is merely a manifestation of the toxic properties of the compound which, if taken in larger quantities, would lead to inflammation of the urinary tract and kidney failure, amongst many other symptoms. Indeed the poisonous nature of Spanish Fly or cantharides was well known even in mediaeval times.

The notorious Medici family of Renaissance Tuscany regularly included extracts of oil beetles in the poisonous brew known as "aqua Tofana" with which they rid themselves of enemies both real and perceived. Even before that time, in ancient Greece, cantharides were used as a substitute for the Hemlock Cup in executions. The Spanish Fly is the metallic green meloid beetle Lytta vescitoria, which occurs throughout the warmer parts of Europe and is occasionally found in England and Ireland. It and other fat blue-black Oil Beetles of the genus Meloe, known as May Worms, were used as the classical source of cantharidin for both medical and amorous purposes; neither beetle is known from the Gulf region. In more recent times, these beetles were replaced as sources of cantharidin by beetles of the genus Mylabris, which were found to contain greater amounts of the toxin; Mylabris beetles and their close allies are particularly common in Arabia.

Oil Beetles and the Practice of Medicine

The use of oil beetles and extracts of their bodies in medicine is believed to have spread to Western Europe from Persia about a thousand years ago, but there is no doubt that their use was well known in Europe at much earlier times. It also seems likely, given the abundance of oil beetle species in Arabian and its proximity to Persia, that oil beetles have been used in Arabic medicine, but I have no source of information on this. However, the medicinal use and misuse of Oil Beetles was known to the Ancient Greeks and has continued uninterrupted in Europe through until comparatively recent times.

To cover the plethora of medical conditions for which cantharadin has been advocated is no easy task; it would almost be easier to list complaints for which it has found no therapeutic role. At one time or another cantharidin has been used either internally or externally as a diuretic, as an abortifacient, for removing corns, for healing skin eruptions and in treating epilepsy, cancer, leprosy, herpes, gonorrhea, jaundice, dropsy, colic, rickets, snake bite, bad nails, gout, renal stones, sterility, dysmenorrhoea, asthma and especially rabies, where it was an important component of "potio antilyssa". It has been used in many a patented hair-restorer of the more bogus kind.

Cantharidin appears in comparatively recent Pharmacopoeias from Eastern Europe, although most note that use in human medicine is considered obsolete or that the preparation is for veterinary use only. In the Andean region of South America, pulp from Oil Beetles of the genus Pseudomeloe is still used as a wart remover.

Patron: H.E. Sheikh Nahayan bin Mubarak Al Nahayan