FIGURE 3 Living and extinct orders of insects, their possible relationships, and chronology. Width of lineages is a rough approximation of diversity. Some groups with a meager or nonexistent fossil record (i.e., Phthiraptera, related to Psocoptera) are not included.
(extinct, primitive arachnids). Two major hypotheses on the origins of the hexapods (including insects) are that they are most closely related to either the myriapods (centipedes and/or millipedes) (all comprising the Tracheata, or Atelocerata) or the Crustacea. Crustacea may actually have the oldest fossil record of all animals (formerly held by the trilobites) because some Precambrian fossils have recently been reinterpreted as crustaceans. If hexapods are closely related to crustaceans, it is most likely to be a group within Crustacea, and the earliest evidence of this stem group will probably be found in the Silurian.
Various kinds of fossil insects, modes of fossilization, and degree of preservation. Different scales. (A-C) Iron hydroxide concretion of a heteropteran in Cretaceous limestone from Brazil, showing preservation of thoracic muscles [B, light micrograph; C, scanning electron micrograph (SEM)]. (D) Silvery carbon film of a belostomatid on fine-grained, Triassic shale (Virginia). (E) Nymph of fHerdina (in this article, f signifies an extinct group) in ironstone concretion from the Carboniferous of Mazon Creek, Illinois. (F) Trichopteran case of sand pebbles in volcanic shale from Florissant, Colorado (Late Eocene/Early Oligocene). (G) Part of the head of tabanid fly from Florissant, showing the eye facets. (H) Portion of the wing of f Typus (“Protodonata”) , from the Permian of Elmo, Kansas, with the wing fluting preserved