Both kinds of photoreceptors nauplius and compound eye, were examined in the fishlouse Argulus coregoni. A 1 st stage metanauplius larval compound eye resembles that of the adult fishlouse compound eye (Meyer-Rochow et al. 2001), but its diameter measures only approximately one tenth of that of the mature eye of an adult. However, in relation to total body length the adult eyes were actually smaller. All ommatidia in the metanauplius compound eye had a well developed dioptric apparatus as well as a rhabdom already at the time of hatching. A seemingly functioning nauplius eye was found in all postembryonic developmental stages of A. coregoni. The size of the nauplius eye increased less rapidly than body length did. The same held true for the compound eye. In the nauplius eye of 1st stage larvae the two lateral ocelli were elongated, but in 8th stage larvae all three ocelli were circular like those in the adult eye. The nauplius eye continued to grow throughout the postembryonic development, but the structure of the eye remained mostly unchanged. There was no reason to doubt that both types of photoreceptor, nauplius and compound eyes, were functional even in the metanauplius larvae. Thus, it seems entirely possible that even the smallest larvae could use vision to detect and attack a possible host.
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