A frustule is the external, siliceous part of the diatom cell wall. In Latin frustule means a little piece.

The frustule is composed of two valves and the copulae, or girdle bands. One valve, or theca, is slightly larger than the other valve. The two valves overlap one another in a manner similar to the halves of a pill box, or a Petri dish.

See also theca, epitheca, hypotheca, valve, copula, and cingulum.

Gvstaurosirella Pinnata
Image Credit: Eduardo Morales
An SEM image showing the ultrastructure of the frustule of Staurosirella pinnata in girdle view.
Tetracyclus  Iwb
Image Credit: Ian Bishop
An SEM image of part of a frustule of Tabelllaria flocculosa. A valve, with its marginal spines, is visible at the bottom of the image. Attached to the valve are many of the copulae, or girdle bands, that make up the girdle.