Haltere in the context of Campaniform sensilla

Genetic assimilation is a process described by Conrad H. Waddington by which a phenotype originally produced in response to an environmental condition, such as exposure to a teratogen, later becomes genetically encoded via artificial selection or natural selection. Despite superficial appearances, this does not require the (Lamarckian) inheritance of acquired characters, although epigenetic inheritance could potentially influence the result. Waddington stated that genetic assimilation overcomes the barrier to selection imposed by what he called canalization of developmental pathways; he supposed that the organism's genetics evolved to ensure that development proceeded in a certain way regardless of normal environmental variations.

The classic example of genetic assimilation was a pair of experiments in 1942 and 1953 by Waddington. He exposed Drosophila fruit fly embryos to ether, producing an extreme change in their phenotype: they developed a double thorax, resembling the effect of the bithorax gene. This is called a homeotic change. Flies which developed halteres (the modified hindwings of true flies, used for balance) with wing-like characteristics were chosen for breeding for 20 generations, by which point the phenotype could be seen without other treatment.

Bittacidae is a family of scorpionflies commonly called hangingflies or hanging scorpionflies.

The genus Bittacus, comprising approximately 75% of all species within the family, occurs worldwide. Other genera are mostly confined to South America or Australia. Members of this family may be confused with crane flies, in the order Diptera, but can be distinguished by their two pairs of wings and lack of halteres.

The mesothorax is the middle of the three segments of the thorax of hexapods, and bears the second pair of legs. Its principal sclerites (exoskeletal plates) are the mesonotum (dorsal), the mesosternum (ventral), and the mesopleuron (lateral) on each side. The mesothorax is the segment that bears the forewings in all winged insects, though sometimes these may be reduced or modified, as in beetles (Coleoptera) or Dermaptera, in which they are sclerotized to form the elytra ("wing covers"), and the Strepsiptera, in which they are reduced to form halteres that attach to the mesonotum. All adult insects possess legs on the mesothorax. In some groups of insects, the mesonotum is hypertrophied, such as in Diptera, Hymenoptera, and Lepidoptera), in which the anterior portion of the mesonotum (called the mesoscutum, or simply "scutum") forms most of the dorsal surface of the thorax. In these orders, there is also typically a small sclerite attached to the mesonotum that covers the wing base, called the tegula. In one group of insects, the Hemiptera, the dorsal surface of the thorax is typically formed primarily of the prothorax, but also in part by the enlarged posterior portion of the mesonotum, called the scutellum; in the Coleoptera, the scutellum may or may not be visible, usually as a small triangular plate between the elytral bases, thus similar in position to the Hemipteran scutellum. In Diptera and Hymenoptera the mesothoracic scutellum is also distinct, but much smaller than the mesoscutum.