Temporal range: Lower Cretaceous–recent 140–0 Ma
|Distribution of Gryllotalpa, Scapteriscus, Neocurtilla|
The mole cricket is a member of the family Gryllotalpidae, in the order Orthoptera (grasshoppers, locusts and crickets). Mole crickets are cylindrical-bodied insects about 3–5 centimetres (1.2–2.0 in) long, with small eyes and shovel-like forelimbs highly developed for burrowing. They are present in many parts of the world and where they have been introduced into new regions, may become agricultural pests.
Mole crickets have three life stages, eggs, nymphs and adults. All stages occur underground, but adults have wings and disperse in the breeding season. They vary in their diet; some species are vegetarian, mainly feeding on roots, others are omnivores, including worms and grubs in their diet, while a few are largely predacious. Male mole crickets have an exceptionally loud song; they always sing underground, in a specially constructed burrow which has the shape of an exponential horn. The song is an almost pure tone, modulated into chirps. It is used to attract females, either for mating, or for indicating favourable habitats for them to lay their eggs.
In Zambia, mole crickets are thought to bring good fortune, while in Latin America they are said to predict rain. In Florida, where Scapteriscus mole crickets are non-native, they are considered pests, and various biological controls have been used. Gryllotalpa species are used as food in West Java, Vietnam, and the Philippines (where it is known as Camaro).
Mole crickets vary in size and appearance, but most of them are of moderate size for an insect, typically between 3.2 and 3.5 cm (1.3 and 1.4 in) long. They are adapted for underground life and are cylindrical in shape and covered with fine, dense hairs. The head, forelimbs, and prothorax are heavily sclerotinised but the abdomen is rather soft. The head bears two threadlike antennae and a pair of beady eyes. The two pairs of wings are folded flat over the abdomen; in most species, the fore wings are short and rounded and the hind wings are membranous and reach or exceed the tip of the abdomen; however, in some species the hind wings are reduced in size and the insect is unable to fly. The fore legs are flattened for digging but the hind legs are shaped somewhat like the legs of a true cricket; however, these limbs are more adapted for pushing soil, rather than leaping, which they do rarely and poorly. The nymphs resemble the adults apart from the absence of wings and genitalia; the wingpads become larger after each successive moult.
Taxonomy and phylogeny
The Gryllotalpidae are a monophyletic group in the order Orthoptera (grasshoppers, locusts and crickets). Cladistic analysis of mole cricket morphology in 2015 identifies six tribes, of which four were then new: Indioscaptorini (Scapteriscinae), Triamescaptorini, Gryllotalpellini and Neocurtillini (Gryllotalpinae), and two existing tribes, Scapteriscini and Gryllotalpini, are revised. The group name is derived straightforwardly from Latin 'gryllus', cricket, and 'talpa', mole.
|Marchandiinae||Lower Cretaceous of France|
Mole cricket fossils are rare. A stem group fossil, Cratotetraspinus, is known from the Lower Cretaceous of Brazil. Two specimens of Marchandia magnifica in amber have been found in the Lower Cretaceous of Charente-Maritime in France. They are somewhat more abundant in the Tertiary amber of the Baltic and Dominican regions; impressions are found in Europe and the American Green River Formation.
Mole crickets are not closely related to the "pygmy mole crickets", the Tridactyloidea, which are in the grasshopper suborder Caelifera rather than the cricket suborder Ensifera. The two groups, and indeed their resemblance in form to the mammalian mole family Talpidae with their powerful front limbs, form an example of convergent evolution, both developing adaptations for burrowing.
Most species of mole cricket can fly powerfully, if not with agility, but males do so infrequently. The females typically take wing soon after sunset, and are attracted to areas where males are calling, which they do for about an hour after sunset. This may be in order to mate, or they may be influenced by the suitability of the habitat for egg-laying, as demonstrated by the number of males present and calling in the vicinity.
The mole cricket is hemimetabolous meaning it undergoes incomplete metamorphosis; the eggs hatch into nymphs which increasingly resemble the adult form as they grow and pass through a series of about ten moults. After mating, there may be a period of one or two weeks before the female starts laying eggs. She burrows into the soil to a depth of 30 cm (12 in), (72 cm (28 in) has been seen in the laboratory), and lays a clutch of 25 to 60 eggs. She then retires, sealing the entrance passage, but in one species, the northern mole cricket, the female has been observed to remain in an adjoining chamber to tend the clutch. Further clutches may follow over several months, according to species. Eggs need to be laid in moist ground and many nymphs die because of insufficient moisture in the soil. The eggs hatch in a few weeks, and as they grow, the nymphs consume a great deal of plant material and tunnel their way gradually towards the surface.
The adults of some species of mole cricket may move as far as 8 kilometres (5.0 mi) during the breeding season. Mole crickets are active most of the year, but overwinter as nymphs or adults in cooler climates, resuming activity in the spring.
Mole crickets live almost entirely below ground, digging tunnels of different kinds for the major functions of life, including feeding, escape from predators, attracting a mate (by singing), mating, and raising of young.
Their main tunnels are used for feeding and for escape: they can dig themselves underground very rapidly, and can move along existing tunnels at high speed both forwards and backwards. Their digging technique is to force the soil to either side with their powerful, shovel-like forelimbs, which are broad, flattened, toothed and heavily sclerotised (the cuticle is hardened and darkened).
Males attract mates by constructing specially-shaped tunnels in which they sing. Mating takes place in the male's burrow; the male may widen a tunnel to make room for the female to mount, though in some species mating is tail-to-tail. Females lay their eggs either in their normal burrows or in specially-dug brood chambers which are sealed when complete.
Male mole crickets sing by stridulating, always underground. The song is based on an almost pure tone at 3.5 kiloHertz, loud enough to make the ground vibrate 20 cm all round the burrow. In Gryllotalpa gryllotalpa the burrow is somewhat roughly sculpted; in Gryllotalpa vineae, the burrow is smooth and carefully shaped, with no irregularities larger than 1 millimetre. In both species the burrow takes the form of a double exponential horn with twin openings at the soil surface; at the other end there is a constriction, then a resonating bulb, and then an escape tunnel. A burrow is used for at least a week. The male positions himself head down with his head in the bulb, his tail near the fork in the tunnel.
The mole cricket stridulates like other crickets by scraping the rear edge of the left forewing, which forms a plectrum, against the lower surface of the right forewing, which has a ratchet-like series of asymmetric teeth: the more acute edges face backwards, as do those of the plectrum. The plectrum can move forwards with little resistance; but moving it backwards makes it catch each tooth, setting up a vibration in both wings. The sound-producing stroke is the raising (levation) of the wings. The resulting song resembles the result of modulating a pure tone with a 66 Hertz wave to form regular chirps. In G. vineae, the wing levator muscle, which weighs 50 milligrams, can deliver 3.5 milliWatts of mechanical power; G. gryllotalpa can deliver about 1 milliWatt. G. vineae produces an exceptionally loud song from half an hour after sunset, continuing for an hour; it can be heard up to 600 metres away. At a distance of 1 metre from the burrow, the sound has a mean power over the stridulation cycle of up to 88 decibels; the loudest recorded peak power was about 92 decibels; at the mouths of the burrow, the sound reaches around 115 decibels. G. gryllotalpa can deliver a peak sound pressure of 72 decibels and a mean of about 66 decibels. The throat of the horn appears to be tuned (offering low inductive reactance), making the burrow radiate sound efficiently; the efficiency increases when the burrow is wet and absorbs less sound. The mole cricket is the only insect that constructs a sound-producing apparatus. Given the known sensitivity of a cricket's hearing (60 decibels), a night-flying G. vineae female should be able to detect the male's song at a range of 30 metres; this compares to about 5 metres for a typical Gryllus cricket that does not construct a burrow.
The loudness of the song is correlated with the size of the male and the quality of the habitat, both indicators of male attractiveness. The loudest males may attract 20 females in one evening, when a quieter male may attract none. This behaviour enables acoustic trapping: females can be trapped in large numbers by broadcasting a male's song very loudly.
Mole crickets vary in their diets; some like the tawny mole cricket are herbivores, others are omnivores, feeding on larvae, worms, roots, and grasses, and others like the southern mole cricket are mainly predacious, feeding on insects and other small arthropods. As well as consuming roots underground, mole crickets leave their burrows at night to forage for leaves and stems which they drag underground before consumption.
Predators, parasites and pathogens
Besides birds, lizards and insectivorous mammals, the natural enemies of mole crickets include ants which feed on the eggs, beetles which eat the larvae, nematodes and mites. The nematode Steinernema scapterisci kills Scapteriscus mole crickets by introducing bacteria into their bodies, causing an overwhelming infection. Some parasitic wasps attack developing mole crickets; the parasitoid Larra polita has been used in Hawaii to control oriental mole crickets that were damaging sugarcane crops. The Brazilian red-eyed fly is a specialised parasitoid of mole crickets in the genus Scapteriscus. The fly's eggs hatch inside her abdomen; she is attracted by the call of the male mole cricket and deposits a larva on any individual with which she comes in contact. Another specialist predator is the larva of the ground beetle Pheropsophus aequinoctialis. The adult beetle lays eggs beside the burrows of Scapteriscus mole crickets, and the beetle larvae find their way to the egg chamber and eat the eggs. Fungal diseases can devastate mole cricket populations during winters with sudden rises of temperature and thaws. The fungus Beauveria bassiana can overwhelm adult mole crickets.
Mole crickets evade predators by living below ground, and vigorously burrowing if disturbed at the surface. As a last-ditch defence, they eject a foul-smelling brown liquid from their anal glands when captured; they can also bite.
Mole crickets are relatively common, but because they are nocturnal and spend nearly all their lives underground in extensive tunnel systems, they are rarely seen. They inhabit agricultural fields and grassy areas. They are present in every continent with the exception of Antarctica; by 2014, one hundred and seven species had been described and more species are likely to be discovered, especially in Asia. Neoscapteriscus didactylus is a widespread pest species, originating in South America it has spread to the West Indies and New South Wales in Australia. Scapteriscus also originated from South America and is now present in North America. Gryllotalpa africana is a major pest in South Africa, and other Gryllotalpa species are widely distributed in Europe, Asia and Australia. They are not native to mainland Britain, the former population of G. gryllotalpa being apparently extinct, but are starting to appear in the Channel Islands.
In human culture
In Zambia, Gryllotalpa africana is held to bring good fortune to anyone who sees it. In Latin America, Scapteriscus and Neocurtilla mole crickets are said to predict rain when they dig into the ground.
The main damage done by mole crickets is as a result of their burrowing activities. As they tunnel through the top few centimetres of soil they push the ground up in little ridges, increasing evaporation of surface moisture, disturbing germinating seeds and damaging the delicate young roots of seedlings. They are also injurious to turf as they feed on the roots of the grasses, leaving the plants prone to drying-out and damage by use.
In their native lands, mole crickets have natural enemies that keep them under control. This is not the case when they have been accidentally introduced to other parts of the world. In Florida they are considered pests and are described as "a serious problem". A University of Florida Entomology report suggests that South American Scapteriscus mole crickets may have entered the United States at Brunswick, Georgia in ship's ballast from the West Indies around 1899. One possible remedy is biological pest control using the parasitoidal wasps Larra bicolor. Another remedy that has been successfully applied is use of the parasitic nematode Steinernema scapterisci. When this is applied in strips across grassland, it spreads throughout the pasture within a few months and not only controls the mole crickets, but remains infective in the soil for future years. The Brazilian red-eyed fly (Ormia depleta) has been released in Florida to help control mole crickets; it has established itself in the south of the state but is unable to survive over winter in the north.
There are several genera of mole cricket, separated into tribes:
- tribus Gryllotalpini
- tribus unknown (fossil taxa)
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|Wikimedia Commons has media related to Gryllotalpidae.|
|Wikispecies has information related to: Gryllotalpidae|
- Mole Cricket at Australian Museum
- Houston, Terry (2011) Information sheet: Mole Crickets at Western Australian Museum
- Prendergast, Amy (2012) "Solving the Mystery of the Hidden Callers of the Night" at Australian Wildlife Secrets
- Mole Cricket Knowledge Base at University of Florida / Institute of Food and Agricultural Sciences
- On the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures website