The Sialidae or alder-flies (q.v.) differ from other Neuroptera in the jaws of the larva - which is aquatic, breathing by paired, jointed abdominal gills - resembling those of the imago, and being adapted for the mastication of solid food.
Before commencing the mastication it is generally necessary to warm the apparatus by means of steam; but as the operation proceeds the heat produced requires to be moderated by streams of cold water flowing through channels provided for the purpose.
The mastication causes a copious flow of saliva of a brick-red colour, which dyes the mouth, lips and gums. The habit blackens the teeth, but it is asserted by those addicted to it that it strengthens the gums, sweetens the breath and stimulates the digestive organs.
Owen's description showed this view to be groundless, and he attributed the extraordinary development of the toucan's beak to the need of compensating, by the additional power of mastication thus given, for the absence of any of the grinding structures that are so characteristic of the intestinal tract of vegetable-eating birds - its digestive organs possessing a general simplicity of formation.
Mouth situated more forwards than in Delobranchia, no share in mastication being taken by the basal segments of the 5th and 6th pairs of prosomatic appendages.
The mouth is usually more or less protractile and always toothless; the lower pharyngeal bones, which are large and falciform, subparallel to the branchial arches, are provided with teeth, often large and highly specialized, in one, two or three series (pharyngeal teeth), usually working against a horny plate attached to a vertical process of the basioccipital bone produced under the anterior vertebrae, mastication being performed in the gullet.
When the summits of these are worn by mastication their surfaces present circles of dentine surrounded by a border of enamel, and as attrition proceeds different patterns are produced by the union of the bases of the cusps, a trefoil form being characteristic of some species.
The object of taking no liquid with meals is that it ensures mastication being more complete, because persons cannot wash the unmasticated food down by drinking, and it prevents the gastric juice from being greatly diluted, and so allows it to digest more rapidly.
In the appendages near the mouth one or both of the protopodal segments may bear inwardly-turned processes, assisting in mastication and known as gnathobases.