(A) TEM images of mitochondria in photoreceptors of control (y w FRT19A) and fh mutant clones. The insets show a single mitochondrion (red box). Scale bar: 500 nm. (B) ETC complex activities in control (y w FRT19A), fh mutant, and genomic rescued animal (fh; gfh). Complex activities are normalized to citrate synthase (CS), which act as a control of mitochondrial mass. n = 3. (C) ADP/ATP ratio in control (y w FRT19A), fh mutant, and rescued animal (fh; gfh). n = 3. (D) ROS levels are measured by DHE in the larval ventral nerve cord. Neuronal down-regulation of ND42 (nSyb>ND42 RNAi), a subunit of CI, acts as a positive control. Quantification of DHE fluorescence is on the right. n = 8. Scale bar: 50 µm. (E) Day 3 ERG of control (y w FRT19A; Rh1-GAL4) and fh mutants in which ROS scavengers are overexpressed in photoreceptors. Data are presented as mean ± SEM. **p<0.01, ***p<0.001, Student’s t-test. ND, no significant difference. L/D, flies are raised in 12 hr light and dark cycle.