Saturday, 8 February 2020

So good they named it once

Oecanthus pellucens (Scopoli, 1763) - the Italian tree cricket - was recently found breeding in Dungeness, Kent, UK (Sutton, Beckmann & Nelson, 2017).

There are a number of recordings of this species in the BioAcoustica repository: recordings of Oecanthus pelluscens on BioAcoustica.

Like many tree crickets it has a certain translucent quality, and it is this that Giovanni Antonio Scopoli alludes to when giving it the specific name pellucens. Torre-Bueno's A Glossary of Entomology gives the definition:
pellucid, pellucidity, pellucid, transparent, whether clear or coloured.
(This definition is somewhat incorrect, something transparent is clear, it can be coloured or colourless, but I digress)

 That Scopoli refers to this property is clear in the original description: "Caput album, subpellucens". Subpellucens is best translated as opaque.


All very well, but so far not particularly interesting. The (minor for most) interest comes from an alternative definition of pellucid, here borrowed form the Oxford English Dictionary.
Of music or other sound: clear and pure in tone.
Crickets in general are known for their pure bell like songs, and that of Oecanthus pelluscens is no exception. Shown below are three chirps from the recording above (time on x-axis, frequency on y-axis). This shows a strong resonance frequency, with three clear, regularly spaced, harmonics: a clear and relatively pure tone indeed.


Oecanthus pelluscens, a cricket so clear and pure it could be named just once.

Friday, 24 January 2020

Orthoptera Culture Group

Google Group for discussing the lab/zoo Culture of Orthoptera (grasshoppers/crickets/bush-crickets).

Thursday, 9 January 2020

Churr-worms, Eve-churrs & Mole crickets

Churr-worms, eve-churrs and mole crickets are (of course) all the same creature:
"Few country people today are acquainted with the Mole Cricket, but formerly, when presumably it was commoner, it went by such names as "Churr-worm" and "Eve-churr". These concern its strange jarring "note" which is heard chiefly during the evening in the spring." 
- G. E. Hyde. (1961) Teach Yourself Entomology. The English Universities Press Ltd, London.

But what is a churr? The Oxford English Dictionary (OED) describes it as "expressive of a somewhat deeper and hoarser sound than chirr", and chirr as "to make the trilled sound characteristic of grasshoppers, etc."

The Mole Cricket (in this case Gryllotalpa gryllotalpa - there are many others outside the UK) produces its churr in the evenings and nights, so Eve-churr seems appropriate. This vernacular name is shared, however, with the nightjar (Caprimulgus europaeus). As the name seems obsolete for both species the chances for confusion are currently minimal. Nightchurr is also an obsolete (regional) name for both these species (OED).

The OED also distinguishes a chirr from a chirp, the chirr being "often nearly equivalent to chirp , but properly expressing a more continuous and monotonous sound." As a side-note many authors in bioacoustics have followed Broughton (1976) in using the precisely defined neologism echeme in place of chirp.

There are no doubt other regional names for the Mole Cricket in the UK, John George Wood in his 1866 book Homes without hands: being a description of the habitations of animals, classed according to their principle of construction gives the delightful Croaker.

The status of the Mole Cricket in the UK has not improved and it is confined to the New Forest. Until recently the only confirmed recent record was several sound recordings, one of which is below.

On stridulation, and dictionaries...

Over the Christmas period I pulled out a few entomological and general science dictionaries to see how much bioacoustic terminology was present. The definitions of stridulation I found (with comments) are provided below, and there are some striking errors (at least for those who like their insects in the correct Order), as well as some minor issues.

The accuracy of the definitions seems to improve with the specificity of the work. The less likely you are to need the dictionary the more likely you are to have one with a correct definition?

A Glossary of Entomology (Torre-Bueno, 1962 third printing)

"stridulating organs, parts of the insect structure which are used in making sounds; in general, one part is a file-like area and the opposing one a scraper or rasp (Imms)."
"stridulation, in insects, the sound produced by rubbing one surface or one structure upon or against another, both being suitably roughened; the act of stridulating or making creaking sounds."
This definition seems concise and is hard to argue with.  

Oxford Dictionary of Science (4th Edition, 1999)

"stridulation The production of sounds by insects rubbing one part of the body against another. The parts of the body involved vary from species to species. Stridulation is typical of the Orthoptera (grasshoppers, crickets, cicadas), in which the purpose of the sounds is usually to bring the sexes together, although they are also used in territorial behaviour, warning, etc."
A major taxonomic issue here including the cicadas within the Orthoptera. Cicadas also produce sound by tymbalisation rather than stridulation. No note of the specialisation of the parts involved in sound production, but forgivable given the broad scope of the dictionary. Bonus point for mentioning the multiple purposes of sound production.

Henderson's Dictionary of Biological Terms (12th Edition, 2000)

"stridulating organs n. special structures on various parts of the body of certain insects such as grasshoppers, crickets and cicadas, which produce the characteristic "song" of these insects." 
"stridulation n. the characteristic sound made by grasshoppers, crickets and cicadas."
Again cicadas are a problem, although this time they are at least not placed within the Orthoptera. The specialisation of the structures is mentioned, although no mention of their movement against each other.

A Dictionary of Entomology (Gordh & Headrick, 2001)

"STRIDULATING ORAGNS Hardened parts of the insect body that are used in making sounds. Typically one part is a file-like surface and the opposing one a scraper or rasp (Imms)."
"STRIDULATION (Latin, stridulus = creaking, squeaky; English, -tion = result of an action. Pl., Stridulations.) 1. Friction of rigid parts on modified surfaces. Insects: The sound produced by rubbing a series of hard projections (spines, Acanthae) against a file-like surface, Stridulatory methods of acoustical communication are widespread in the Insect and probably the most generally used form of sound communication. Anatomy of a system consists of a Pars Stridens which forms a rasp (file) composed of tubercles and a Plectrum which forms a scraper (Strigil). 2. The act of stridulating or making creaking sounds."
This is about as comprehensive as one could expect in a dictionary. Not all stridulation in insects is by spines or tubercles, sometimes ridges on the integument are used, but this point seems relatively minor.

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