IUCN threat status:

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Family Gryllotalpidae (mole crickets) in North America north of Mexico

No other crickets have mole-like forelegs. Antennae shorter than body; foretibia with two or four strong blade-like projections; first two segments of foretarsus developed into blades; body cylindrical and covered with short dense pubescence; hindfemur not reaching tip of abdomen. Length 19-50 mm.

Mole crickets include the largest and the most destructive crickets in North America. Four of our seven species are immigrants.

Burrowing

Mole crickets spend nearly all their lives underground. They occupy extensive tunnel systems within which they can move, forward or backward, with great speed--as can be demonstrated by allowing one to tunnel in damp sand in a container with a transparent bottom (such as a glass bowl or clear plastic dish). When dug up, they do not leap away like other burrow-inhabiting crickets but dig their way back underground with powerful strokes of the forelegs. The dirt is simply forced aside. Other burrowing crickets have much slower digging techniques. Short-tailed crickets (Anurogryllus), for example, carry dirt away one mouthful at a time. Mole crickets often tunnel immediately beneath the surface and leave trails of pushed-up soil that resemble, in miniature, the surface tunnels of mammalian moles. The burrowing techniques of mole crickets restrict them to sandy, friable, or nearly saturated soils.

Flight

Although mole crickets appear heavy and clumsy, many are capable fliers. In some species, all individuals have the long hindwings necessary for flight. In others, some or all individuals have short hindwings and cannot fly. Capturing mole crickets as they end their flights at simulations of their conspecific calls or at bright lights is far easier than extracting them from soil.

Food

Mole crickets eat both plant and animal matter, with some species being mostly herbivorous and others mostly carnivorous. Our three species of two-clawed mole crickets (Scapteriscus) damage lawns, golf courses, pastures, and crops, mostly by feeding on roots and leaves but also by cutting roots and uprooting seedlings as they tunnel.

Oviposition and social behavior

Female mole crickets lack an external ovipositor and lay eggs in their burrows or in special chambers that they then seal off. The females of northern and European mole crickets stay with their eggs and young nymphs. It is not known when the relationship is terminated nor to what extent the mothers provide protection and food. Mole crickets have anal glands from which they can expel a sticky liquid that may thwart insect predators such as ants.

Mating occurs within the burrow and the male's courtship song can be heard issuing from the ground during the day as well as at night. Observations in glass-sided burrows reveal that the male sometimes enlarges the burrow to give the female room to mount. In at least one species, the northern mole cricket, matings are initiated tail-to-tail with the male lying on his back! A female may mate several times with the same male at intervals of 10 or 15 minutes, but once she no longer responds to the male's courtship, the pair fights and one leaves the burrow system. Perhaps in the wild the male leaves or is driven away, with the female taking over the tunnel system for egg laying

Life cycles

Mole crickets generally require a year or more to develop and they overwinter in all stages except the egg and small nymph. The only populations that have more than one generation per year are southern and short-winged mole crickets in south Florida. Two-year life cycles have been documented for the northern mole cricket in the Carolinas. Longer life cycles seem likely for more northern populations and for larger mole crickets, but these have not been studied.

Acoustic behavior

Since soil transmits sound poorly, crickets that remain earthbound are unlikely to use sound for long range communication. Indeed, the calling of mole crickets seems to be directed largely or entirely to those that are flying. In the two European and three U.S. species that have been most thoroughly studied, males construct special burrows for calling. These have horn-shaped openings to the outside and are of appropriate dimensions and design to augment the sound and project it skyward. Calling to the sky would make little sense except that in these species females fly prior to mating and can locate mating partners by flying to the source of calls. Such a link between flying and calling is evident in the three species of two-clawed mole crickets (Scapteriscus). The two that fly project their calls upward and flying conspecifics land at or near the calls. The one that cannot fly does not call, though it retains a courtship song.

Sound production by female mole crickets is somewhat a mystery. On rare occasions, when in contact with other mole crickets, females have been heard making sounds as they rub their wings together. S.M. Ulagaraj tape recorded and analyzed such sounds of a female tawny mole cricket. D.A. Nickle and T.C. Carlysle (1975) found toothed crossveins that may function in stridulation on the upper surface of the left forewing of females of tawny and southern mole crickets. They also reported male-like files (beneath the right forewing) in females of European and prairie mole crickets and summarized what was known of female sound production. Females seem to produce sounds during aggression or defense.

Two aspects of hearing in mole crickets are puzzling. Firstly, the young nymphs of two-clawed mole crickets have perfectly-formed, completely exposed tympana. In other crickets, the tympana are not evident in the early instars and are fully developed only in the adult. Dave Yager, in a pilot study, established that some southern mole cricket nymphs had hearing similar to, but generally less sensitive than, the adult. The function and exact nature of precocious hearing in mole crickets are unexplored. Secondly, all songs of mole crickets have dominant frequencies of less than 5 kHz, yet Nobuo Suga determined that the northern mole cricket (or a close Amazonian relative) hears best at 20-30 kHz. The specimens he studied had flown to lights. Perhaps mole crickets that fly have their hearing evolutionarily tuned to detect the ultrasonic pulses of insectivorous bats.

Remarks

The so-called "pygmy mole crickets" (Tridactylidae), once assumed to be closely related to mole crickets are now placed in a different suborder of Orthoptera (Caelifera). The sharing of mole-like features by these two insect groups is a result of evolutionary convergence rather than their having a mole-like common ancestor. Pygmy mole crickets are less than 10 mm and their forelegs have no tibial tympanum or tarsal blades.

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Creative Commons Attribution 3.0 (CC BY 3.0)

© Thomas J. Walker

Source: Singing Insects of North America

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