Breathing organ situated anteriorly on each side of the thorax (Fig.3a.jpg). Although the rule in terrestrial species, the thoracic horn is rarely absent in aquatic forms but may break off quite easily. Unbranched or branched at least once (tribe Chironomini - 103b.jpg; 115f.jpg).
See also: The cephalothorax.

Alternative forms for thoracic horn : Th, thoracic horns.