Typical thoracic setae are composed of longitudinally arranged cylinders ending as teeth at the oblique distal surface. Most thoracic ‘blades' are rounded in cross section and show bilateral asymmetry. Some have teeth in oblique rows (“cross striations”) on the side adjoining the tube wall. Sickle setae found posteriorly on the thorax have distal portions like abdominal setae. These are flat, each composed mainly of a palisade of spatulate units. On one side cylindrical elements may also be present, in patterns which are characteristic of some taxa. Uncini are made of rows (sometimes single) of shorter cylindrical elements with hooked ends pointing anteriorly. Anterior thoracic hooks are usually blunt, but pointed in the genus Janua. The collar setae assist withdrawal into the tube, the abdominal setae help with emergence, whilst other thoracic setae act as distance pieces, pushing the body to anchor the thoracic uncini against the opposite wall. The other uncini are brought into use by the prehensile abdomen. The flaccid ends of the sickle setae are readily lost in some taxa and their development or loss may reflect unimportant variations in strength of the adjacent abdominal field. Fin and blade setae are not very different from simple setae and the natural grouping of the genera into subfamilies shows that changes from one form of collar seta to another have occurred independently several times.

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The strurturc or the spiracle in the adult female of Ixodrs ricinus (L.) is assessed. Gaseous exchange between the tracheal system and the external atmosphere is shown to occur only via the aeropyles in the surface of the sieve plate and is regulated by the atrial valve. The elaborate structure is shown to be an effective passive mechanism to retard the transpiration of water vapour from the tracheal system. The prdicels reduce airflow within the labyrinth to a minimum, while the size and arrangement of the iieropyles allow thr formation of a cloud of water vapour above the surface of the spiracle. It is from this cloud which diffusion of water vapour takes place, with no significant rhanges in water vapour pressure occurring within the tracheal system or the spiracle itsel[ 'I'he mechanics of the system are analysed.

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