twi^RY50/twi^v50 animals raised at the non-permissive temperature from the second larval instar stage till eclosion show abnormal indirect flight muscle development, in particular only three dorsal longitudinal muscle fibres are seen. Development of the direct flight muscles is not severely affected, although individual muscles are slightly thinner than in wild-type flies.

Raising twi^v50/twi^RY50 animals at the restrictive temperature either during the larval stage until the middle of the third instar or during the early mid-pupal stage has moderate effects on the number of dorsal longitudinal indirect flight muscles (DLMs) in the adult; the number of DLMs is slightly reduced compared to wild-type. Raising twi^v50/twi^RY50 animals at the restrictive temperature for a single period at the late larval stage results in a more severe reduction in the number of DLMs. twi^v50/twi^RY50 animals raised at the restrictive temperature of 30^oC from pupariation onwards usually have three persistant larval oblique muscles per hemithorax early during pupariation, but by 18 hours after puparium formation (APF) at 30^oC, there are only 3-4 fibres per hemithorax, in contrast to the 6 seen in wild-type flies.

63% of twi^RY50/twi^v50 larvae raised at the permissive temperature (18^oC) until hatching and then shifted to the restrictive temperature (30^oC) develop into adults, compared with 82% of control larvae. Several of these twi^RY50/twi^v50 adults have defects in wing positioning. twi^RY50/twi^v50 adults raised at the permissive temperature have five dorsal longitudinal indirect flight muscle (DLM) fibres in each half thorax, compared to six in wild-type flies. twi^RY50/twi^v50 adults raised at the restrictive temperature have three DLM fibres in each half thorax. Patterning defects are also seen in other thoracic and abdominal muscles, including the tergal depressor of trochanter muscles and the lateral transverse muscles of the abdomen.

At 18^oC, twi^v50/twi^RY50 animals survive to adulthood, whereas at 29^oC the twi^v50/twi^RY50 combination is lethal during embryogenesis. When embryos are shifted to 29^oC after gastrulation, visceral mesoderm and heart develop normally whereas differentiation of somatic muscles is deranged.

twi^v50/twi^RY50 transheterozygotes are temperature sensitive: they survive at 18^oC and die at 28^oC. The temperature sensitive period is 2--4 hours after egg laying. Embryos are partially dorsalized, gastrulation is normal but fail to differentiate a mesoderm.