In addition to this work of Willem, valuable anatomical papers on Collembola have been published by H.
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.
In their insunken mouth and their jaws retracted within the head-capsule, the Collembola resemble the entotrophous division of the Thysanura (see Aptera), from which they are probably descended.
It is better, on the whole, to regard the Thysanura and Collembola as sub-orders of a single order, the Aptera.
In Hemiptera only eleven and in Collembola only yolk, and that the mesenteric epithelium becomes reinforced by six abdominal segments have been detected.
Absolon has also written on the Collembola of caves.
The spring serves the Collembola which possess it as an efficient leaping-organ (see Springtail).