Gymnosoma rotundatum (Linnaeus, 1758)

Scientific name: Gymnosoma rotundatum (Linnaeus, 1758)
Common name:
French name: Gymnosome arrondi
Order: Diptera
Family: Tachinidae
Wingspan : 6-7 mm
Biotope: Woodland edges, flower meadows, parks and gardens.
Geographic area: Europe.
Observation period : June to September.

Gymnosoma rotundatum has an orange and rounded abdomen marked with three round patches.
The female's thorax is black, the male's one is a golden colour.
The larvae grow as parasites of shield bugs of the Pentatomidae family.
Gymnosoma rotundatum over winters as a larva inside the host insect, exiting after to pupate in the ground.
There are several species of the Gymnosoma gender and they are very difficult to tell apart.
The shape of the black patches on the abdomen, though it is known that they are usually small, well separated and rounded on Gymnosoma rotundatum and triangle-shaped and sometimes touching on Gymnosoma nudifrons, shall not be used to tell species apart. These characteristics vary very much and experts consider that these are not reliable criteria.
Here are some points to try to go further than Gymnosoma sp.:
Gymnosoma nitens is a very smaller size (3 to 4 mm). The sides of the thorax are black.
Gymnosoma clavatum and Gymnosoma dolycoris show a central whitish line from the apex of the scutellum to the head of the thorax
Gymnosoma nudifrons has black stripes on the inner side of the eyes which run downwards to the antennae.


Gymnosoma rotundatum - Yvelines, France - July 7th 2009
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Gymnosoma rotundatum
This small Gymnosoma rotundatum, observed on the side of a forest track, was used to coming back again and again to a small corner of vegetation.
I have then been able to try to shoot different pictures and I have seen how difficult it is to have a precise focus point with macro pictures and handheld camera.
You could not display this picture a much larger size as the focus point is rather on the leaf than on the insect



Gymnosoma rotundatum - Yvelines, France - July 7th 2009
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Gymnosoma rotundatum
I have identified this insect as a male Gymnosoma rotundatum by eliminating Gymnosoma clavatum and Gymnosoma dolycoris by the lack of a white line on the thorax (previous picture) and by eliminating Gymnosoma nudifrons by the short length of the black lines on the inner side of the eyes.



Gymnosoma rotundatum - Yvelines, France - May 14th 2011
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Gymnosoma rotundatum
I have observed this mating on the banks of a forest pond.



Gymnosoma rotundatum - Yvelines, France - May 14th 2011
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Gymnosoma rotundatum
It has been rather easy to approach but, having no tripod with me, I had to use the flash to avoid moving blur, then getting too hard colours and reflections on shiny areas.



Gymnosoma rotundatum - Yvelines, France - May 14th 2011
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Gymnosoma rotundatum
It is clear on this picture that the shape of the black spots must no be used to tell species apart. The male, which is slightly smaller, shows well separated rounded spots. The female shows triangle-shaped spots which are closer from each other.

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