Thorax with markings similar to those in the dark form, but the pleurae generally paler.
The pleural cavity also contains pleural fluid, which allows the pleurae to slide effortlessly against each other during ventilation.
The back thorax (or opistothorax) consists of about 10 progressively smaller segments with insignificant pleurae.
Each segment consists of the central axial ring and the outer pleurae, which protected the limbs and gills.
The pleurae are two thin membranes, one cell layer thick, which surrounds the lungs.
During inspiration of air, the diaphragm contracts and the lungs expand drawn by pleurae through surface tension and negative pressure.
Neighbouring pleurae are displaced because of the large pleurae of the 3rd segment.
Its major characteristics are a large forward-reaching glabella, pointed pleurae or pleurae with very short spines, and a medium pygidium with well-impressed furrows.
