Figure 236 from Chapter 7 (Mitochondria) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Fusion of mitochondria during insect spermiogenesis results in the formation of a structure called the nebenkern. Here, the labyrinthine pattern of fully formed nebenkern is apparent. Following this stage, two hemispheric masses flank either side of the flagellum to form two mitochondrial derivatives that may run parallel to the axoneme for nearly the length of the sperm tail. Image by David Phillips. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.