The mouth lies usually in the centre of the anterior vzs and sub-terminal sucker or between two adoral suckers, but in Gasterostomum and its allies it is mid-ventral.
In the Argasidae the anterior portion of the dorsal surface of the body is extended forwards above the capitulum, so that this structure is concealed from above; the integument is fairly uniformly granular or coriaceous above and below; the palpi are simple and unmodified; there is no sucker beneath the claws in the adult, and there is only a slight structural difference between the sexes.
There seems no doubt that the suctorial mouth-organs of the Diptera have arisen quite independently from those of the Lepidoptera, for in the former order the sucker is formed from the second maxillae, in the latter from the first.
The sucker carries a series of three or four leather cups, which are pressed against the inner surface of the working barrel by the weight of the column of oil.
Thus in the recently discovered arctic genus Prosorhynchus the muscular and glandular extremity is protrusible, but in the allied Gasterostomum this organ is represented by a sucker with fimbriated or tentacular margins.
The oral sucker may alone be present (Monostomidae), more usually a second is developed on the under surface, but may be mid-ventral (Distomidae) or terminal.
The alimentary canal, which may be represented by a vestigial structure, is accordingly not functional, and the larva does not become pelagic. A pyriform organ is present in most Gymnolaemata as well as the sucker by which fixation is effected.
Visceral sac very much reduced; without shell or mantle; anus posterior; foot provided with sucker in male only.
If successful, the larva throws off its cilia and develops a dorsal papilla, a median ventral sucker and an additional pair of lateral suckers.
Should it, however, encounter another Diporpa, the mid-ventral sucker of either is applied to the dorsal papilla of the other, and complete fusion takes place across the junction.