The double dactyl was invented in 1951 by Anthony Hecht and Paul Pascal. In 1966 it was introduced to the public, first in an Esquire article, then in Jiggery Pokery: A Compendium of Double Dactyls edited that year by Hecht and John Hollander (and including contributions from, among others, founder Pascal, Donald Hall, Richard Howard, and James Merrill).
The form has a pleasing, sing-song rhythm; its stringent rules provide a challenge to the poet:
• The poem has eight lines, divided into two equal stanzas;
• all lines except those at the ends of the stanzas are double-dactylic, having two dactylic feet (STRONG weak weak STRONG weak weak);
• lines at the ends of the stanzas are shorter (STRONG weak weak STRONG);
• the stanzas rhyme;
• the first line is a piece of nonsense ("higgledy- piggledy" is often used, and double dactyls are sometimes called higgledy-piggledies);
• the second line is the double-dactylic name of the poem's subject, usually a famous person;
• another line of the poem, most commonly the sixth, must be a single double-dactylic word.

Although it suggests most commonly the sixth, the first example on the page is one of Hecht’s which places the hex on the seventh line, so confirming the variability.

http://www.ddaze.com/04LVResource/zDactyl.htm