1.0 – 11 mm long. Head large and
developed laterally, often projecting posteriorly over prothorax. Tentorium
complete with exception of dorsal arms. Cranial plates and sutures often poorly
developed. Antennae capitate, four- or usually five-segmented with the third
segment (first flagellomere) pedunculate. Second and third flagellomeres bear
sensilla. Eyes variable, may be large, reduced or absent. Maxillary palps
filiform and well developed. Mandibular mouthparts (usually asymmetric) that
articulate horizontally with the head. Thoracic segments large, well developed,
and ellipsoid. Pronotum always separate. Meso- and metanotum variably fused or
separate. Single pair of ‘mesothoracic’ spiracles present on prothorax. Legs
large and variable, particularly with respect to the tarsal segments and their
angle of connection with the thorax. Paired pretarsal claws present except in
the family Gyropidae that is not known from Greenland.
Abdomen shape large and variable, typically broad and ‘fat’. Eight pregenital
segments, although segments I and II are sometimes fused. In adults the ninth
visible segment bears the genital apparatus. Segments range from highly
sclerotized to membranous. Five or usually six pairs of abdominal spiracles.
The post spiracular seta of at least one abdominal segment with two minute
associated setae. External genitalia usually well developed in both sexes,
often with distinct gonopophyses arising as lobes on some females. Hosts in Greenland: Birds.