Meloe Proscarabaeus

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While many beetles look the same, did you know that they differ in their abilities? Some are more of a pest, and some are actually more useful than you would think. So, which category does this beetle fall into? Let’s take a look.
Meloe Proscarabaeus

The beetle Meloe proscarabaeus is a member of the Blister Beetle (Meloidae) family. It belongs to the genus Meloe, which is commonly known as the Oil Beetle. Oil Beetles can be distinguished from other beetles by the apparent lack of functional wings and a shortened elytra (front wings). They are flightless in nature and, when disturbed, release hemolymph from their joints.

The hemolymph is a blood-like fluid that contains Cantharidin, in the case of all blister beetles. Oil beetles also secrete an oily substance, which offers protection from predators due to its bad smell and taste. A well-known vesicant, Cantharidin is known to cause severe blisters and inflammation in other animals, including larger mammals and humans. The terpenoid chemical acts as their natural defense system.

Etymology and Scientific Classification

The Meloe proscarabaeus is one of the most well-known oil beetles, and can be placed under the famed Scarab category of beetles. According to ancient Egyptian rituals and beliefs, scarabs signified immortality. Impression seals and amulets depicting scarab beetles have been found alongside the mummies of ancient Egyptian pharaohs. The “immortality” context can be partly attributed to the complex life cycle of the beetles.

Noted Swedish botanist Carl Linnaeus made the scientific binomial nomenclature of the black oil beetles in 1758. The Meloe proscarabaeus falls in the order of Coleoptera under the class of Insects. Coleoptera, or the order of the beetles, is the largest among all orders. It constitutes about 400,000 species, approximately 40% of all insects and 25% of known members of the Animalia Kingdom. They are associated with the Family Meloidae (Blister beetles) and the Genus Meloe (Oil Beetle).

Availability

The Meloe proscarabaeus can be found living in the side of fields and meadows. It is most noticeably found living on the northern side of the continent, including the forests of Nottinghamshire in England. It is also found in Ireland and Scotland. Its numbers have been declining steadily due to the lack of habitat in the present day, with the felling of forests and lack of open green spaces.

Physical Appearance

Like all the members of the Genus Meloe, the Meloe proscarabaeus lacks hind wings and has extremely shortened elytra (modified hardened forewings). These features make them a flightless beetle, which lives on the ground most of the time. They are generally black in color, with sparkling blue or purple limbs. Their size may vary from specimen to specimen, but generally adults may grow up to 20 millimeters in length. There are generally two variants: the black oil beetle and the violet oil beetle.

Life Cycle

The females lay their eggs (up to 1000) near the ground nests of solitary bees. When they hatch, the larvae (known as Triangulins) become extremely active. They climb up trees into flowers, waiting to attach themselves to the female host bee. The bees then take them back to their hives, where the larvae feed on the host bee’s eggs and pollen. It is inside the beehive where the larvae grow into full adults- passing through the stage of pupation. Once fully grown, they emerge from the nest of their host.

Directly after coming out of the nest, the adult black oil beetles seek out a partner for mating. After the rituals are complete, the females burrow deep into the soil to lay her eggs. The depth may vary according to the length of the beetle, but it’s-generally 1 cm more than the length of the females. The depth of the tunnel may go as far as 6cm.

Several females may line up and lay their eggs within a distance of each other. The burrowing is generally done in warmer climates, during the beginning of Summer. The eggs hatch within two weeks after being laid, with the initial larvae color turning from yellow to orange within the span of a few hours.

Cantharidin

The secretion of Cantharidin is one of the signature traits of the entire genus Meloidae (Blister beetles). The organic compound is used as both a defense mechanism and as an aphrodisiac. It helps in the courting and mating rituals of most of the blister beetles.

In general instances, the secretion of Cantharid is exclusive in the male blister beetle. Often initially procured from an external source by ingestion, the secretion of the terpenoid chemical begins at the Cephalic cavity present in the lower section of the head of the male blister beetles. The highly vesicant chemical (which causes blisters or inflammation) is then transferred to the females during the mating sessions. Male beetles present the majority of the secreted Cantharidin to the females. It is a “nuptial gift,” along with spermatozoa for fertilization. In some cases, the females cover their laid eggs with Cantharidin for the protection of the larvae against predators.

In terms of organic chemistry, the IUPAC nomenclature for Cantharidine is 2,6-Dimethyl-4,10-dioxatricyclo-[5.2.1.0 2.6]decane-3,5-dione. It belongs to the terpenoid category of organic compounds. Cantharidin has been historically used as a sexual stimulant. Recent research, however, suggests that the feelings of warmth and arousal occur due to inflammation and severe blistering rather than any aphrodisiac characteristics.

Poisoning by Cantharidin

Cantharidin is an extremely potent naturally occurring toxin. Though the beetles are not strong enough to cause any significant damage to humans by biting, Cantharidin can cause a lot of damage in contact with the human skin. Known as a powerful vesicant, it causes severe blisters and irritation. It is even more harmful when ingested. Complications such as hematuria (blood in urine), hematemesis (vomiting of blood), mouth-burns, difficulty in swallowing, as well as nausea, may arise. Above the lethal dose of 10 mg, it may prove fatal.

Eating habits

Meloe proscarabaeus, like all other blister beetles, have a peculiar feeding habit. The young larvae are omnivorous and feed mainly of the host bee’s eggs and pollen, as well as decaying pieces of wood from trees and shrubs. The adults, on the contrary, are almost exclusively herbivorous in nature. They feed on flowers, leaves, and other edible parts of the tree. They are considered an agricultural pest, feeding on clover, lettuce, and peas. Their food preferences include the yellow spring flower (also known as the Yellow Star of Bethlehem), among many other plants.

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