Flightless, with a penetrance of 85-100%; cannot jump more than 2 cm. Wings held up vertically (penetrance 63-90%). In some flies, large amount of disorganized fibrillar muscle material present in the thorax; in others, most of the fibrillar muscles are absent. Both dorsal longitudinal muscles (DLMs) and dorsoventral muscles (DVMs) are reduced; in the electron microscope, myofibrillar structure appears disorganized and Z-band structure disturbed, although the indirect flight muscle fibers that do form are superficially normal. The abnormal behavioral and muscular phenotypes are thought to be due to a primary defect in embryonic cells that map (in mosaic experiments) to the dorsal region of the blastoderm. Viability of mutant alleles is about 95%. Recessive allele, stronger than vtw². Penetrance of flightless phenotype 100%. Penetrance of vertical wing phenotype 90%.

Severe defects in dorsoventral muscles.

vtw¹ flies are 95% viable, 100% flightless and cannot jump more than 2cm. 90% hold their wings vertically.

vtw¹ flies lack dorsal longitudinal fibres (DLMs) 1-4 and dorsoventral fibres (DVMs) 7-9. In about 8% of flies fibres 5 and 6 are present and in 30-40% one to four of DVMs 10-13 are present. At no time are normal indirect flight muscles seen during development.

vtw¹ flies show minor disturbances of the cuticular structure just anterior to the distal portion of the second wing vein.

The thorax of vtw¹ adult flies contain spherical bodies containing 20-40 smaller spheres that are normally only found in early pupae.