- The Aptera are divided into two divergent sub-orders, theThysanura or bristle-tails, and the Collembola or springtails.
In most Collembola the spring appears to belong to the fifth abdominal somite, but Willem, by study of the muscles, has shown that it really belongs to the fourth.
Lubbock (Lord Avebury), "Monograph of the Collembola and Thysanura," Ray Society, 1873.
Oudemans, Bijdrage tot de Kennis den Thysanura en Collembola (Amsterdam, 1888); B: Grassi, who published between 1885 and 1889 a series of memoirs entitled "I progenitori dei Miriapodi e degli Insetti," in the Atti Accad.
In addition to this work of Willem, valuable anatomical papers on Collembola have been published by H.