Heider bryonic membranes are vestigial or entirely wanting.
In some Psocidae the wings are in a vestigial state, and the fully winged species rarely if ever fly.
In the Achorutidae the head is forwardly directed, the tergum of the prothorax conspicuous, and the spring small or vestigial.
The proboscis-pores are highly variable, and frequently only one is present, that on the left side; sometimes the pore-canals of the proboscis unite to open by a common median orifice, and sometimes their communication with the probosciscoelom appears to be occluded, and finally the pore-canals may be quite vestigial.
Although an integral portion of the gut, it has ceased to assist in alimentation, its epithelium undergoes vacuolar differentiation and hypertrophy, and its lumen becomes more or less vestigial.
