Mole cricket


Mole crickets are members of the insect family Gryllotalpidae, in the order Orthoptera. Mole crickets are cylindrical-bodied, fossorial insects about long as adults, with small eyes and shovel-like fore limbs highly developed for burrowing. They are present in many parts of the world and where they have arrived in new regions, they may become agricultural pests.
Mole crickets have three life stages: eggs, nymphs, and adults. Most of their lives in these stages are spent underground, but adults have wings and disperse in the breeding season. They vary in their diet: some species are herbivores, mainly feeding on roots; others are omnivores, including worms and grubs in their diet; and a few are largely predatory. Male mole crickets have an exceptionally loud song; they sing from a burrow that opens out into the air in 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 Neoscapteriscus mole crickets are not native, they are considered pests, and various biological controls have been used. Gryllotalpa species have been used as food in West Java, Vietnam, Thailand, Laos, and the Philippines.

Description

Mole crickets vary in size and appearance, but most of them are of moderate size for an insect, typically between long as adults. They are adapted for underground life and are cylindrical in shape and covered with fine, dense hairs. The head, fore limbs, and prothorax are heavily sclerotised, 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 wing pads become larger after each successive moult.

Taxonomy and phylogeny

The Gryllotalpidae are a monophyletic group in the order Orthoptera. Cladistic analysis of mole cricket morphology in 2015 identified six tribes, of which four were then new: Indioscaptorini, Triamescaptorini, Gryllotalpellini and Neocurtillini, and two existing tribes, Scapteriscini and Gryllotalpini, are revised.
The group name is derived straightforwardly from Latin gryllus, cricket, and talpa, mole.
Within the extant subfamilies, genera include:

Gryllotalpinae

  • tribe Gryllotalpellini Cadena-Castañeda, 2015
  • * Gryllotalpella
  • tribe Gryllotalpini Leach, 1815
  • * Gryllotalpa
  • tribe Neocurtillini Cadena-Castañeda, 2015
  • * Leptocurtilla
  • * Neocurtilla
  • tribe Triamescaptorini Cadena-Castañeda, 2015
  • * Triamescaptor
  • tribe incertae sedis
  • * †genus Pterotriamescaptor
  • * †genus Burmagryllotalpa Burmese amber, Myanmar, Cenomanian

    Scapteriscinae

  • tribe Indioscaptorini Cadena-Castañeda, 2015
  • * Indioscaptor
  • tribe Scapteriscini Zeuner, 1939
  • * Scapteriscus
  • * ''Neoscapteriscus''

    †subfamily Marchandiinae

  • †Archaeogryllotalpoides Martins-Neto 1991 Crato Formation, Brazil, Aptian
  • †Cratotetraspinus Martins-Neto 1997 Crato Formation, Brazil, Aptian
  • †Marchandia Perrichot et al. 2002 Charentese amber, France, Cenomanian
  • †Palaeoscapteriscops Martins-Neto 1991 Crato Formation, Brazil, Aptian

    ''Incertae sedis''

  • †Tresdigitus rectanguli Xu, Fang & Wang, 2020 Burmese amber, Myanmar, Cenomanian
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.
File:Pigmy Mole Cricket .jpg|thumb|Pygmy mole crickets are members of the suborder Caelifera.

Convergent evolution

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.

Behavior

Adults of 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 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.

Life cycle

Mole crickets undergo incomplete metamorphosis; when nymphs hatch from eggs, they increasingly resemble the adult form as they grow and pass through a series of up to 10 moults. After mating, a period of 1–2 weeks may occur before the female starts laying eggs. She burrows into the soil to a depth of,, and lays a clutch of 25 to 60 eggs. Neoscapteriscus females then retire, sealing the entrance passage, but in Gryllotalpa and Neocurtilla species, 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 must 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 either underground or on the surface.
The adults of some species of mole crickets may move as far as 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.

Burrowing

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, mating, and raising of young.
Their main tunnels are used for feeding and for escape; they can dig themselves under ground 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 fore limbs, which are broad, flattened, toothed, and heavily sclerotised.
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 in the case of the genus Neoscapteriscus or not sealed in the case of genera Gryllotalpa and Neocurtilla.

Song

Male mole crickets sing by stridulating, always under ground. In Gryllotalpa gryllotalpa, the song is based on an almost pure tone at 3.5 kHz, loud enough to make the ground vibrate 20 cm all round the burrow; in fact, the song is unique in each species. In G. gryllotalpa, the burrow is somewhat roughly sculpted; in G. vineae, the burrow is smooth and carefully shaped, with no irregularities larger than 1 mm. In both species, the burrow has two openings at the soil surface; at the other end 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, and his tail is near the fork in the tunnel.
Mole crickets stridulate like other crickets by scraping the rear edge of the left fore wing, which forms a plectrum, against the lower surface of the right fore wing, 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 forward 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 of the wings. The resulting song resembles the result of modulating a pure tone with a 66-Hz wave to form regular chirps. In G. vineae, the wing levator muscle, which weighs 50 mg, 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 m away. At a distance of 1 m from the burrow, the sound has a mean power over the stridulation cycle 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, making the burrow radiate sound efficiently; the efficiency increases when the burrow is wet and absorbs less sound. Mole crickets are the only insects that construct a sound-producing apparatus. Given the known sensitivity of a cricket's hearing, a night-flying G. vineae female should be able to detect the male's song at a range of 30 m; this compares to about 5 m 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, while 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.