Everyone loves shieldbugs – why else would we have given them such a heroic and heraldic name? These distinctive bugs are relatively large, stout and brightly coloured – and robust enough that they can be picked up with impunity. They walk across the hand with a friendly clockwork gait and take to the wing from the end of a finger with an assured model-aeroplane whirr. They are highly photogenic, and there are just enough of them (about 70 British species) to pique the interest of non-specialists.
‘Bug’ (also bugg or bugge) is an old word. Though it is often used nowadays to refer to almost any small, mean creepy-crawly, for the strict entomologist it means a member of the insect order Hemiptera. Characterised by long piercing and sucking mouthparts and an incomplete metamorphosis (see page 61), this large group includes about 100,000 species worldwide, with cicadas, spittlebugs, leaf-hoppers, aphids, scale insects, water boatmen, back-swimmers, bedbugs, capsids and shieldbugs among their number.
Traditionally, ‘shieldbug’ meant a member of the family Pentatomidae, named for their five antennal segments (most other bugs have four) – but this rather unfairly excludes many four-segmented species (family Coreidae) that are still very shield-shaped. Shieldbugs are, in fact, a slightly arbitrary grouping of several related bug families lumped together.
Some books claim shieldbugs are actually named for the scutellum – the large triangular plate in the centre of the body. Part of the upper surface of the second segment of the thorax, scutellum literally means ‘small shield’ and is the Latin diminutive of scutum – the heavy, slightly curved shield carried by Roman legionnaires. As well as providing a tough, protective body segment, the scutellum forms an anchor for the stiffened front wings of the bug, which nestle against it at rest and are held in place by a series of submicroscopic curved bristles, rather like Velcro. The delicate membranous rear wings are folded, origami style, under these wing cases. In some groups, the scutellum is broad and curved, a bit like a giant thumbnail, and nearly completely covers the rear body of the insect.
Aromas and appetites
In North America, by contrast, shieldbugs are usually known as stink bugs. True, they do have a certain characteristic scent, described as a mild cocktail of diesel and marzipan by connoisseurs. The bugs emit this smell via a liquid excretion from the metathoracic glands – two small, slit-like holes on their underside, just above the rear legs. Though probably originally derived from species-specific sexual pheromone scents, by which the bugs locate each other to mate successfully, this bitter liquor is now a powerful anti-predator deterrent.
Only the adults have these thoracic glands. In the nymphs, a similar repellent purpose is achieved by paired glands down the back of the abdomen. These are the dark spots against the bright body, which make some shieldbug nymphs closely resemble ladybirds. It’s no coincidence, since the strong pink, red, white or yellowish nymphal colours (wholly different to the camouflaged browns and greens of the adults) offer the same warning to would-be predators as do the equally foul-tasting ladybirds – “do not, under any circumstances, eat me”.
Shieldbug don’t bite, yet this belies the fact that several are fairly ferocious predators. The spiked shieldbug, Picromerus bidens, a common woodland species, specialises in attacking moth and sawfly caterpillars on trees. Its skewer mouthparts are thicker than those borne by conventional plant-feeding, sap-sucking species. In damp meadows and woodland edges, Britain’s only metallic-blue shieldbug, Zicrona caerulea, particularly seems to target the grey, grub-like larvae of the similarly metallic-blue Altica flea-beetles.
Few other British shieldbugs are carnivorous like this, but evolutionary studies suggest that the primitive shieldbug ancestors were predominantly insect-hunting and that the herbivorous tendencies evolved later.
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