Gryllotalpa major

Gryllotalpa major
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Orthoptera
Suborder:	Ensifera
Family:	Gryllotalpidae
Genus:	Gryllotalpa
Species:	G. major
Binomial name
Gryllotalpa major Saussure, 1874
Synonyms
Neocurtilla major (Saussure, 1874)

Gryllotalpa major is a prairie mole cricket of the family Gryllotalpidae. It is endemic to the United States and is the largest cricket in North America. Its natural habitat is temperate grassland. It is threatened by habitat loss and is currently only found in Oklahoma, Kansas, Missouri and Arkansas. Males of this species produce sounds by rubbing their forewings together. They sing from special burrows they construct in the prairie soil to attract females for mating, and they can be heard at distances up to 400 meters from the burrow. Males aggregate their acoustic burrows in a lek arena and are very sensitive to vibrations carried through the ground. Males communicate with neighboring males through vibrational signals, and the songs they project to flying females are harmonic chirps, rather than the trills produced by most mole crickets.

Physiology

The prairie mole cricket (Gryllotalpa major Saussure) is the largest of the North American crickets, measuring up to 5 cm long, weighing up to 2.6g, and typically brown to reddish-brown in color.^[1][2] Like other mole crickets (Gryllotalpidae), G. major has a bullet-shaped, heavy pronotum and large robust forelimbs used for digging complex subterranean burrows.^[1] Like other Orthoptera males of the species have modified forewings with a stridulatory apparatus which they use in conjunction with acoustic burrows to perform an acoustic sexual advertisement call to attract females during their brief mating season beginning in late spring.^[1][2][3]

Research by Nickle & Carlysle ^[4] found that females of G. major have similar stridulatory files as males, though smaller and weaker, and they are in fact capable of producing sound, though it seems the sound serves no function in other species sound production by females is linked to aggression and defence. Howard et al.^[5] did a study on the hearing sensitivity of G. major and found that they have hearing sensitivities from 2 kHz to an ultrasonic range at 25 kHz.

Distribution and habitat

G. major are native to in tall grassland prairie ecosystems and occupy a small range in the south central United States, found only in Kansas, Missouri, Arkansas, and Oklahoma.^[6] Prescribed burning is common throughout the prairie habitat of G. major and usually occurs during March and April, at the begging of the reproductive season of the cricket. Howard and Hill ^[1] looked to the effect of these burning on the distribution of G. major and found that recent fire burned land can be beneficial to the cricket and provide some advantages to mating behaviours. Howard and Hill ^[1] noted G. major has been observed calling on recently burned sites within 24 hours of the site being burned. It is thought the warmer soil as a result of the fire has metabolic advantages for the cricket allowing them to increase their chirp frequency, and overall the burned land allows their song to travel more efficiently increasing to attraction of females to the area.^[6] It was also noted that the belowground grass biomass of which the cricket feeds on remains intact after a burn, and no evidence of direct mortality of the cricket has been documented.^[6]

Another study by Howard and Hill ^[7] found that the grass height of the prairie land had a marked effect on the male burrows in a lek. They found that as grass height increased the spacing between each burrow also increased, as well as, an increase in the angle of opening to the burrow. This behaviour may have evolved as response to the dynamic disturbances of tallgrass prairie ecosystems.^[7]

Mating behaviour

The males of G. major builds an acoustic burrow in prairie soil arranged in a lek-like arena and perform an acoustic sexual advertisement call for the female of the species by singing from these calling chambers while the female flies over head making her choice.^[2] The call male males produce is a loud, long range, airborne song consisting of a pattern of long chirps with a frequency of 2 kHz and up to five harmonics, and these songs can be heard up to 400 m away.^[8] The female flies at a height of 1.5-5.0 m above the lek assessing each individual male presumably based on the attractiveness of the male call and the position of the male within the lek arena.^[3] Females can detect male signals from up to 76 m away and usually arrive within 20 minutes. once the female selects an appropriate male she drops down landing 3–5 m from the chosen males burrow and uses phonotaxis to find her way, once there she will, enter the burrow, mate with the male, and be on her way.^[3] Though it is expected that the female would choose the male closer to the arena due to the decrease predation risk amongst the males, Howard et al.^[3] found the female choice of males to be randomly spread across the lek with the more attractive males being located further from the center and father from the nearest calling neighbour. The preference from more isolated males could be a way of maximizing fitness as these males are able to maintain costly display territory with increased predation risk.^[3]

Research by Hill ^[2] into the mating behaviour of G. major found they fit the 3 main criteria of a classical lek system. Males serve no role in parental care, other than fertilizing female eggs, they die soon after mating season and do not tend to the eggs or young in any way.^[2] Males create lek-like arenas of evenly positioned burrow, and the females are free to fly over to make their choice without male influence, for example sequestering resources or forcing copulation upon the female.^[2] For these reasons G. major fits within a slightly modified definition of a lek system, modified only to adjust for an insect species and broaden the typical umbrella classification.^[2]

While singing males of G. major produce vibrations which travel through the soil influencing neighbouring male which are able to distinguish these ground vibrations from other background vibrations.^[8] Hill and Shadley ^[8] found these singing vibrations can travel up to 3 m in the soil depending on conditions, and the energy ranges from 30 to 300 Hz. They also found that males do not alter their calls in response to the airborne calls of other males, but rather some do respond to the ground vibrations of other and may abandon and relocate their burrows to a more optimal position.^[8] The call of G. major is unique amongst crickets as they use a chirp rather than a trill to attract females. This chirp was analyzed by Howard and Hill,^[1] and found that certain aspects of the chirp can be correlated to the morphology of the individual cricket. From their research they found that overall longer males produced a lower-frequency call that contained more syllables per chirp, though these males did not necessarily produce a louder call.^[1]

Within the lekking arena males of G. major are in close proximity to each other and are thought to exhibit some influence on neighbouring males. Hill ^[9] studied the effects of certain competitive parameter on the chirp rate of individual males and found that while males do sound louder on nights when there are more males calling, individual males did not increase their maximum call volume with increased competition from other males, nor did they increase their volume due to the availability of female. Hill ^[9] found that there was no correlation to volume and the distance to an individual males nearest neighbour, nor a correlation with the density of the population. It was found that males had a strong correlation between the temperature of the soil and their chirp rate due to an increase in metabolic energy, and also there was found to be a correlation between the number of harmonics produced by a male and the distance to that male’s closest neighbour (more research is needed to determine the effects of harmonics on females of the species).^[9] These results show that individual calling males are influenced more by their nearest one or two closest neighbours more than by the group as a whole.^[9]

Phylogeny

A study into the phylogenetic relationship of Gryllotalpidae cricket by Hill et al.^[10] grouped Gryllotalpa major in to a group with Gryllotalpa devia, Gryllotalpa pilosopes, and Gryllotalpa inermis due to the dorsal hindtibiae lacking subapical spurs.^[10]

Species status

In the late 1980s G. major was recommended to be placed under protected status as a threatened species, however a lack of ecological information has stalled protection efforts.^[8] The IUCN Red List lists the species as data deficient, as more data on the habitat and effects of decreased tallgrass prairie land on the cricket’s population is needed.^[11] NatureServe has G. major listed as G3 – Vulnerable due to scattered populations, and habitat loss.^[12]

References