Variations of larval ommatidia in four species of Bittacidae (Insecta: Mecoptera)

The larvae of Bittacidae normally bear a pair of compound eyes on the lateral sides of the head. The compound eyes each are reported to consist of seven ommatidia. However, whether the eyes have variations in number among species and individuals remain unclear to date. Here, ommatidial variations of compound eyes were investigated in four species of Bittacidae larvae using light microscopy. The results show that the number of ommatidia remains constant for a larval eye across instars of the same larva, but numerical variations of ommatidia were found for different individuals and different species of Bittacidae. The compound eyes of larvae each consist of six to ten ommatidia in the most widely distributed Bittacus cirratus Tjeder, but are composed of seven to nine ommatidia in B. planus Cheng and B. lii Zhou, and seven to eight ommatidia in Terrobittacus longisetus Tan & Hua. Seven ommatidia are the most common number, likely representing the ground plan of larval compound eyes in Bittacidae. The distribution patterns of ommatidia can be categorized into five types. The significance of larval ommatidial variation of Bittacidae is briefly discussed with behavior of larvae and evolution of visual organs of holometabolous larvae.