A new book on local odonates has helped clear the air on a group of insects that have mastered the sky and munch their way through mosquitoes and myriad other pests. As nymphs, they wreck mayhem on fish and tadpoles in small water bodies, but the relative stenotopicity of their larval stages has meant that species favouring still, open pools have prospered while jewels that sparkle in deep shade have lost much of their habitat. A few have vanished entirely, as what remained of the island's wild riffles gave way to streams of rock and roll.
One particularly helpful section of the book deals with the identification of a group of dragonflies that confound casual observers who can do little better than declare the insects to be either 'red' or 'blue'. Looking at the latter shade, the differences become apparent as one homes in on a suite of features that include eye and body colour, size, habitat preference and posture. The robust build of this deep blue libellulid in a forest clearing pointed to Orthetrum, but the genus has two very similar species with devilish details that seldom show up in the field due to the insects' differing hangouts.
A sidelong glance reveals the above to be a slender blue skimmer (Orthetrum luzonicum) for the markedly more slender abdomen with a hint of a 'waist' after the girth of the first three segments. The abdomen of the common blue skimmer (Orthetrum glaucum), in contrast, is both more evenly and stoutly built, and the mature male has greenish blue eyes and a dark blue thorax, whereas adult males of O. luzonicum (the picture shows a young stud) bear bright blue eyes and a thorax of powder blue. The latter also tend to occur close to ponds and marshes, while the former wanders farther afield to colonise the edges of forests, where individuals can be usually seen perching on low twigs close to the ground.
The azure coat of some dragonflies arises from the production of a powdery secretion known as pruinescence, which appears as a waxy grey tint in some species and a powdery blue tone in others. Anisopterans from diverse genera may don near-identical blooms but differ in nearly all other aspects. One blue dragon that can be easily distinguished is the male blue dasher (Brachydiplax chalybea), which is visibly smaller than the skimmers, with a stouter body and thorax bearing brownish markings. The eyes are also distinct and a yellowish tint at the base of the hindwings is diagnostic.
A common dragonfly around urban ponds, drains and reservoirs, the blue dasher is often spotted in the so-called obelisk pose, which is thought to help them keep their cool in the afternoon heat. Phoretic mites also seem to target this species, which can be found throughout tropical Asia in waterbodies created or disturbed by man. A closely related species with completely hyaline wings dwells only in swamp forests, while the mangrove dwarf (Raphisma bispina), which also resembles the dasher but lacks the wing markings, is restricted to the Rhizophora zone.
The obelisk is also a favourite pose of dropwings (Trithemis spp.), which often bask on the tips of exposed twigs with characteristically 'depressed' wings. This posture, along with the dainty size and dark eyes, suggest that this individual perched above a swampy ditch is a male indigo dropwing (Trithemis festiva), despite the absence of yellow-brown specks on the abdomen. Not as common and widespread locally as its crimson cousin, this dragonfly is reliant on swifter streams that continue to lose ground to plans that hold no water and seek to populate the future with a vision free of floods.
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