Solving the Mystery of the Hidden Callers of the Night

by on Jun.17, 2012, under Fauna, Invertebrates

Text and images by Amy Prendergast

If you are a resident of suburban Perth, Western Australia, I’m sure many of you will hear following sunset a chirping chorus commence, yet are unable to espy the owner of these nighttime vocalizations. On many a night during my twilight dog-walks, I’d been surrounded by the sounds of what appears to be a cross between a cricket chirp and a frog’s croak, yet no matter how hard or long I looked, staring intently at the location where the sounds seemed to be sourced from, I never saw the mysterious creatures responsible for these nighttime noises. Even when I appeared to be directly over the site from which the calls were broadcast, I still could not locate the creature producing these reverberations!

Then, one night not long ago in my very backyard, I heard the memorable sounds start up. Honing in on the spot where they appeared to be coming from, I noticed a slight depression in the sandy soils characterizing Perth. Before the enigmatic creature could cease its song and leave its location undetectable, I plunged my hands into the sand and scooped up a handful. I felt a squirming in my hand – success! Being nighttime, the light was scant so I raced inside and deposited my handful of sand into a glass jar and with anticipation finally laid eyes upon the identity of the creature (insect, amphibian?) that had thus far foiled my attempts to detect it visually. The sight that met my eyes was certainly not anticlimactic! The creature thus unveiled was a large, robust-bodied insect, about the length of my index finger (5cm), glossy brown, with small veined wings extending to the mid-thorax that completely cover the tiny hind-wings, slightly curled, annulated antennae, and a large, heavily sclerotinized shield (protothorax) extending back from the head over the thorax. The most remarkable feature however was the forelimbs: these were disproportionately large, sclerotinized, “clawed” and shovel-like.

 

I then set to the task of taxonomically identifying this specimen and discovered this creature was a mole cricket. Mole crickets are insects closely related to “true” crickets (family Gryllidae); however, mole crickets are recognized as being taxonomically unique and thus are assigned their own distinct family – Gryllotalpidae. Along with “true” crickets, as well as locusts and grasshoppers, mole crickets are classified in the Order Orthoptera. Unlike “true” crickets mole crickets have poor leaping abilities; but what they lack in the jumping arena is made up with regards to their digging abilities. Like true crickets however the males sing to attract mates, producing their sounds by stridulation: rubbing a row of “teeth” on one forewing against a ridge-like vein or scraper on the other forewing. Calls of mole crickets however are much deeper than those of true crickets.

The reason I previously had never spied a mole cricket was the fact that these nocturnal insects are fossorial, subterranean creatures, spending most of their time not far beneath the surface of the soil inhabiting extensive underground tunnel systems that they dig. The morphology of their amazing modified forelimbs I was so impressed with is an adaptation to their burrowing lifestyle, with the powerful robust forelimbs bearing stout spines enabling them to be effective burrowers.

Following nightfall, males commence calling from just below the surface from the entrance of special vertical burrows they construct. The burrow acts as an amplifying, resonating chamber, broadcasting their calls out into the night in the hope of enticing a female. The amplification of the male’s chirping courtship call is due to the structure of these burrows: using their amazing forelimbs, they dexterously sculpt their burrows into a double exponential horn shape, which causes the burrow to act as a megaphone. Males characteristically commence singing at dusk but will cease after a few hours. Once mated, the female lays her fertilized eggs in an underground egg chambers she has dug and will guard her clutch until they hatch.

The mole cricket family has quite an extensive distribution, with various species of the five recognized genera occurring in Asia, Africa, Europe, North and South America, and Australia. All Australian species are placed in the genus Gryllotalpa. This genus, comprising 22 species, is also represented in Africa, Europe and Asia. However, whilst native to eastern Australia, mole crickets are an introduced species to WA. In fact, mole crickets only recently made their way here, as prior to the 1990s records of mole crickets were nonexistent. The species recorded in WA are Gryllotalpa pluvialis and Gryllotalpa species (australis-group). Gryllotalpa pluvialis males produce strident, rapid, chirping songs, contrasting with the slower, quieter trills of the male Gryllotalpa sp. (australis group) mole crickets.

Whilst being remarkable creatures, I don’t advise picking one up: when disturbed or threatened, in defence a mole cricket will discharge a foul-smelling brown liquid from its anal glands (I was fortunate not to receive this nasty surprise when I snared my mole cricket). One of WA’s introduced species, Gryllotalpa pluvialis, also secretes a clear, viscous substance from its rear-end which likely functions in entangling insect or spider predators.

No longer is the identity of the creatures that produce the chirping cricket x frog-like cacophony an enigma. Whilst they still evade my observation during my twilight wanders, I now know the callers are the mysterious mole crickets whose calls fill the air yet whose remarkably modified bodies remain hidden beneath the sandy soil surface.

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