C. Borner has stated that the unpaired piercer is attached directly to the base of the left maxilla.
Ia, frons; b, clypeus (the pointed labrum beneath it); II, mandible; III, first maxilla; (a, base; b, sheath; c, piercer), III', inner view of sheath; IV, second maxillae forming rostrum (b, mentum; c, ligula).
The poison-bag lies on the side of the head between the eye and the mandibular joint and is held in position by strong ligaments which are attached to this joint and to the maxilla so that the act of opening the jaws and concomitant erection of the fangs automatically squeezes the poison out of the glands.
In the skull the zygomatic arch is slender and the jugal bone small and not extending far forwards, being supported by the long zygomatic process of the maxilla, while the infra-orbital foramen is mostly large, and there are no post-orbital processes.
The most obvious feature the Apteryges afford is the presence of a back toe, while the extremely aborted condition of the wings, the position of the nostrils - almost at the tip of the maxilla - and the absence of an after-shaft in the feathers, are characters nearly as manifest, and others not less determinative, though more recondite, will be found on examination.