Wheeler (1889) on the embryos of two beetles - Hydrotails Lepisma and Machilis, an interesting transitional condition philus and Doryphora respectively.
- The segments are perceptible at a very regarded as the true endoderm in the hexapod embryo, for he states early stage of the development as a number of transverse bands (1897) that in the bristle-tail Lepisma and in dragon-flies they give arranged in a linear sequence.
Genera: (a) Pedibus sex; capite a thorace disereto: Lepisma, Podura, Termes, Pediculus, Pulex.
Campodea Machilis and Lepisma - to which belongs the "silverfish" (q.v.) - are the best known genera.
The air-tube system is developed in varying degree in different bristle-tails, the number of pairs of spiracles being three (Campodea), nine (Machilis), ten (Lepisma), or eleven (Japyx).