Lytta vesicatoria (Linnaeus, 1758)

Spanish Fly

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POLYPHAGA Emery, 1886

TENEBRIONOIDEA Latreille, 1802

MELOIDAE Gyllenhal, 1810

MELOINAE Gyllenhal, 1810

LYTTINI Solier, 1851

LYTTA Fabricius, 1775

This widespread southern and central European species is generally common across warmer areas around the Mediterranean but more local and scarce further north to southern Scandinavia and the UK; it extends east through Asia Minor and Russia to Siberia and has been recorded from the United States but has not become established. It is not native to the UK but there have been occasional and scattered records from Kent, Dorset, Norfolk and the Isle of Wight, among other sites, going back over the centuries but how successful it has been in breeding here is not known. Adults occur from early May until August or later depending on latitude, they feed on leaves of a range of trees and shrubs, mostly on ash, lilac and privet but also willows, honey suckle, rose and various fruit trees among others and in years when the beetles occur in abundance they can be destructive to commercially grown stock. Mating occurs after a period of maturation feeding early in the season and females oviposit from May or June; numerous batches of 40-50 eggs are laid in the soil near the nesting sites of various solitary bees, among those recorded as hosts for Lytta are species of Anthophora Latreille, Colletes Latreille, Nalictus Latreille, Megachile Latreille and Osmia Panzer. The triungulin larvae emerge en-masse and are immediately active; they climb nearby stems and assemble on flowers to wait for visiting bees. At this stage mortality is high as they will attach to any visiting insect but suitable hosts will invariably carry them away to their nests, often in numbers and bees have been seen covered in them. Triungulins feed upon debris in the nest and soon moult into the scarabaeiod form of larva which will consume the bee larvae and their food supplies, these grow rapidly and pass through 5 or 6 instars before entering into a non-feeding and quiescent ‘coarctate’ stage which will overwinter inside a chamber in the nest. A further scarabaeiod form appears in the spring and this will remain in the chamber and pupate to produce an adult soon afterwards which will leave the nest and fly to nearby trees and shrubs to feed.

The common name refers to a toxic chemical called cantharidin which is produced by male beetles and passed to females during copulation, eggs are coated with the poison during oviposition and this protects them from predators which find the taste extremely repulsive, and larvae consume the chorion which gives them the same protection. Adults emit a strong acidic smell which is obvious from several metres away even to humans and it is obvious from observation that this gives them general protection from predators etc. Cantharidin is extremely toxic (a phosphatase 2A inhibitor); in contact with the skin it quickly produces painful swellings and blisters, ingested it causes a wide range of metabolic dysfunction and is claimed to have aphrodisiac properties but while it does affect the urinary and genital systems these are not of an aphrodisiac nature, and in any case as little as 10mg can be fatal. Historically it has been popular as a poison and there have been a few notorious examples of its use, each beetle contains up to 0.7mg cantharidin and so it was, presumably, easy to obtain in the required quantity. Harvesting the toxin is straightforward; when disturbed the beetles produce the poison in a milky fluid from the mouth and the leg joints but historically the beetles have been dried in numbers and then crushed to a fine iridescent powder which smells and tastes awful but is easily administered. At the time of writing several products are available on line called Spanish fly and all have the usual extravagant claims.

The large size and brilliant colour will identify this species with certainty among the UK fauna. 10-22mm. Dorsal surface entirely brilliant metallic green, sometimes with a bluish, coppery or red reflection, ventral surface usually shiny coppery and smooth. Head finely and diffusely punctured, broadly transverse from above, with dilated and widely rounded temples and a deep longitudinal median furrow. Antennae long and slender; scape elongate, curved and broadened towards the apex, second segment small and abruptly dilated apically, remainder elongate. Pronotum transverse and about as broad as the head, widest in front of the middle and strongly narrowed to a rounded or angled anterior margin, less strongly narrowed to distinct posterior angles,  surface roughly sculptured and finely punctured. Elytra long and near-parallel-sided, with well-developed shoulders and separately rounded apical margins, surface roughly sculptured and each with 2 weak and often ill-defined raised longitudinal lines. Distal antennal segments black, appendages otherwise entirely metallic. Legs long and slender, femora and tibiae unarmed, tarsi 5-5-4 without contrasting bilobed segments, claws smooth and without a basal lobe. Males are distinct; anterior tibiae with a single broad spur, antennae longer than half the body length, basal pro-tarsomere emarginate and terminal abdominal sternite deeply incised. Females have 2 pro-tibial spurs, shorter antennae, normal pro-tarsi and an entire or only weakly sinuate apical margin to the terminal abdominal sternite.

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