The spectra of C8H4 and C10H5 were observed in a butadiyne/argon discharge under the same conditions as those whereby C7H2 and C9H3 have been found to dominate the spectroscopy. It is shown that the species C8H4 and C10H5 most likely originate from methyl substitution of a hydrogen in C7H2 and C9H3 such as not to significantly disturb the chromophore. Assuming comparable oscillator strengths for the substituted species, it is found that the ratio of abundances of the unsubstituted and substituted species is around 27:1. This is discussed in relation to the chemistry of plasmas and the interstellar medium.