Mole Crickets (Orthoptera: Gryllotalpidae)

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Mole crickets (Gryllotalpidae) are cosmopolitan (except the poles), 3.2 – 3.5 cm (average, they can be larger than 5 cm!) relatives of crickets (suborder Ensifera, superfamily Grylloidea), named after the mole, since they are both animals that are highly-dependent on burrowing, and they kind of resemble each other (see drawing of Gryllotalpa hexadactyle to the left, from Hebard (1934)). Other common names include churr worms, eve churrs, and changas. Gryllotalpid burrows form networks below the surface of river- and lakeside soil (Ratcliffe & Fagerstrom, 1980), with occasional spacious 1 cm-large egg chambers. This surface network is temporary; they also have more deep permanent burrows.

Their burrowing nature is obvious not only from their behaviour (they can’t jump), but also from their anatomy: their front legs are broadened. If you see an orthopteran that can’t jump and with broad front legs, it’s always a gryllotalpid. Specifically, it’s the tibia that’s specially modified, as seen above (Frost, 1959); in many other burrowing insects, e.g. scarabs or cicada nymphs, the tibia is also the modified segment. The “claws” are immobile and called dactyls.

Another feature of note on the tibia is the tympanum on the medial (inner) side, which serves as the hearing organ; they share this trait with the crickets, whereby it’s not sure if it evolved independently in those groups or whether it’s a synapomorphy (or even an ancestral trait for all ensiferans!). Also important for burrowing is a blade on the trochanter.

Other distinctive characteristics derived from their burrowing lifestyle include the reduced eyes (who needs eyes in the darkness of the underground?; they also only have 2 ocelli) and their short, sturdy antennae (long ones are awkward to maintain underground). Additionally, their ovipositors are reduced. They have retained their wings though, and are considered strong fliers. Some exceptions are brachypterous (lost their wings); this can even vary within the same species.

Also related to burrowing is their highly-sclerotised pronotum, with its characteristic shape forming a dorsal shield that even covers the sides of the insect; this can easily be imagined as making travelling through soil easier.

As a sidenote, they should not be confused with the pygmy mole crickets (Tridactyloidea), which are not actually cricket-relatives, but grasshopper-relatives (suborder Caelifera). They are only superficially similar, having convergently evolved a similar habitus due to a similar burrowing lifestyle.

As can be seen from the compilation above (Jost & Shaw, 2005; citations for originals available on request), a sister-group relationship between the Gryllotalpidae and Gryllidae (true crickets) has never been doubted, based on very close matches in wing venation, male sexual organs, dactyls and the morphology of the abdomen.

There are 90-100 species of gryllotalpid, although that number may be completely wrong – the literature is very inaccurate on species numbers. Taxonomically there are two tribes or subfamilies, depending on which classification you follow (Gryllotalpini/Scapteriscini as tribes; Gryllotalpinae/Scapteriscinae as subfamilies). The difference lies in the number of dactyls: gryllotalpines have 4, scapteriscines have 2-3.

The most common genus is Gryllotalpa, with 65 species around the world; the second-largest is Scapteriscus with 21 species in South America (one has sicne invaded N. America). The other four genera are Neocurtilla (Americas, 6 spp.), Indioscaptor (Asia, 4 spp.), Gryllotalpella (S. America, 2 spp.) and the monospecific genus Triamescaptor (New Zealand). Refer to the OSF for all this info.

There are few characters that can be used for universal species identification: the mating song is species-specific, but unknown for many species. Same goes for burrow structure. Otherwise, one has to rely on regional keys, but they are not reliably applicable worldwide. This means that there is also no reliable phylogenetic analysis for the relationships between the gryllotalpids yet – there are some based on communication type and behaviour, and of course the ubiquitous molecular ones, but nothing rigorous has been conducted so far, and I would classify the family as in need of revision.

Like with many orthopterans, some gryllotalpids can be a nuisance to agriculture, as they can be voracious root feeders. They count as severe rice pests in Africa, and in southeastern USA, they cause over 77 million US$ in damage annually as they feed on crops (Frank, 1998), e.g. on sugarcane, although they have also been reported as wreaking havoc on golf lawns, which is a good thing because those things waste too much space anyway for a boring rich cocksucker game. They have also become adapted for urban environments – they can live in lawns and parks.

The most popular way of controlling them is to infect them with a biocontrol agent, the steinernemtatid nematode Steinermia scapterisci – this is done by luring them with an audio lure to a chamber full of the infectious nematode stage; from there, the nematode will spread among the population. A less sophisticated method, for the independent farmers reading this, is to use poisoned food: they are attracted to wheat bran, poultry mash or cracked corn.

That said, some are carnivorous, feeding on soil insects; they thus count as facultative predators (Coll & Guershon, 2002).

Audio is a big part of a mole cricket’s life: next to the openings of the burrows, there is a chamber, kept continually moist, that acts as a loudspeaker, amplifying their mating and communication calls. The opening of the burrow is also modified and can have any of the patterns shown above (Hill et al., 2006). When working properly, the calls can be up to 70 db loud (Daws et al., 1996). Each call is species-specific, and can be heard from sunset on during flight season, most of the time in spring, sometimes in autumn. Since it’s mostly females that fly at this time, this also counts as a mating season. Very notably, gryllotalpids are among the very few insects where females of some species also produce sounds (Baumgartner, 1910).

The sound itself is produced by the stridulatory file, a vein near the wingbase of each forewing; the files of the wings are rubbed together at ~65 Hz to make sound (Bennet-Clark, 1970); it is marked with an asterisk and crudely painted red in the above picture (Desutter-Grandcolas, 2003). This is not as highly-developed in the gryllotalpids as it is in crickets.

I mentioned the trap used to lure mole crickets is acoustic. This is the most effective way to collect mole crickets, should you be so inclined. Simply blast a male mating call, place a bucket or something beneath the sound emitter and reap the rewards. They’re, as far as I know, the only insects where an acoustic trap is actually really successful – so successful that you will also catch those parasites that locate them via their calls, e.g. tachinid flies (Frank et al., 1996) – those are interesting from an evolutionary point of view (how to avoid getting parasitised, when you must call to reproduce), but that’s a topic for another post; see Zuk & Kolluru et al. (1998) for a review.

In addition to the auditory part, there seems to be a vibrational aspect to the call as well, with bibrations being sent through the soil (Hill & Shadley, 1997); however, whether this is functional or merely a side-effect has, as far as I know, not been investigated.

The result of mating is a bunch of eggs, layed in the special egg chamber. In the Gryllotalpini, there is evidence in some species of presocial behaviour, i.e. taking active care of the eggs (Gwynne, 1995). The nymphs that hatch from the eggs develop slowly relative to other orthopterans, and go through between 7 and 10 instars before reaching the adult stage. Young adults have been observed cannibalising each other (pers. obs.).

I mentioned in my insect brains post that the size of an insect’s mushroom bodies and its components is correlated with its ecology. Gryllotalpids are an excellent example. Their accessory calyx is greatly enlarged; this is the area responsible for mechanical feeling and taste – the two senses most important for a burrowing animal. In fact, a similar modification of the mechanosensory areas in the mole is found for the exact same reason (Catania, 2000).

As for their fossil record, they are rare. Their stem-group is represented by Cratotetraspinus from the Early Cretaceous of Brazil (Martins-Neto, 1995). Earliest representatives of the crown group are known from 2 specimens, named Marchandia magnifica, from the Early Cretaceous ambers of France, pictured above (Perrichot et al., 2002), with more numerous Tertiary fossils from the Baltic and Dominican ambers, and as impressions from various European deposits and from the Green River Formation (USA).

References:

Baumgartner WJ. 1910. Observations on the Gryllidae: III. Notes on the classification and on some habits of certain crickets. Kansas University Science Bulletin 5, 309-319.

Bennet-Clark HC. 1970. The mechanism and efficiency of sound production in mole crickets. Journal of Experimental Biology 52, 619-652.

Catania K. 2000. Cortical organization in insectivora: the parallel evolution of the sensory periphery and the brain. Brain Behavior and Evolution 55, 311–321.

Coll M & Guershon M. 2002. Omnivory in terrestrial arthropods: mixing plant and prey diets. Annual Reviews of Entomology 47, 267–297.

Daws AG, Bennet-Clark HC & Fletcher NH. 1996. The mechanism of tuning of the mole cricket burrow. Bioacoustics 7, 81-117.

Desutter-Grandcolas, L. 2003. Phylogeny and the evolution of acoustic communication in extant Ensifera (Insecta, Orthoptera). Zoologica Scripta 32, 525-561.

Frank JH. 1998. How risky is biological control? Comment. Ecology 79, 1829–1834.

Frank JH, Walker TJ & Parkman JP. 1996. The introduction, establishment, and spread of Ormia depleta in Florida. Biological Control 6, 368–77.

Frost SW. 1959. Insect Life and Insect natural History. 2nd Edition.

Gwynne DT. 1995. Phylogeny of the Ensifera (Orthoptera): a hypothesis supporting multiple origins of acoustical signalling, complex spermatophores and maternal care in crickets, katydids, and weta. Journal of Orthoptera Research 4, 203-218.

Hebard M. 1934. The Dermaptera and Orthoptera of Illinois. Bulletin of the Illinois Natural History Survey 20.

Hill PSM & Shadley JR. 1997. Substrate Vibration as a Component of a Calling Song. Naturwissenschaften 84, 460-463.

Hill PSM, Wells H & Shadley JR. 2006. Singing from a constructed burrow: why vary the shape of the burrow mouth? Journal of Orthoptera Research 15, 23-29.

Jost MC & Shaw KL. 2006. Phylogeny of Ensifera (Hexapoda: Orthoptera) using three ribosomal loci, with implications for the evolution of acoustic communication. Molecular Phylogenetics and Evolution 38, 510-530.

Martins-Neto RG. 1995. Complementos ao estudo sobre os Ensifera (Insecta, Orthopteroida) da Formacao Santana, Cretaceo Inferior do Nordeste do Brasil. Revista Brasileira de Entomologia 39, 321-345.

Panov A. 1966. The correlation of ontogenetical development of the central nervous system of Gryllus domesticus L. and Gryllotalpa gryllotalpa L. (Orthoptera, Grylloidea). Revue d’Entomologie de URSS 45, 326–340.

Perrichot V, Néraudeau D, Azar D, Menier J-J & Nel A. 2002. A new genus and species of fossil mole cricket in the Lower Cretaceous amber of Charente-Maritime, SW France (Insecta: Orthoptera: Gryllotalpidae). Cretaceous Research 23, 307–314.

Ratcliffe BC & Fagerstrom JA. 1980. Invertebrate lebensspuren of Holocene floodplains: their morphology, origin and paleoecological significance. Journal of Paleontology 54, 614-630.

Zuk M & Kolluru GR. 1998. Exploitation of sexual signals by predators and parasitoids. Quarterly Review of Biology 73, 415–438.

Research Blogging necessities :)

PERRICHOT, V., NERAUDEAU, D., AZAR, D., MENIER, J., & NEL, A. (2002). A new genus and species of fossil mole cricket in the Lower Cretaceous amber of Charente-Maritime, SW France (Insecta: Orthoptera: Gryllotalpidae) Cretaceous Research, 23 (3), 307-314 DOI: 10.1006/cres.2002.1011

Hill, P., Wells, H., & Shadley, J. (2006). Singing from a constructed burrow: why vary the shape of the burrow mouth? Journal of Orthoptera Research, 15 (1), 23-29 DOI: 10.1665/1082-6467(2006)15[23:SFACBW]2.0.CO;2