Comparative studies on the compound eyes of larvae and adults of an aeschnid dragonfly, Anax nigrofasciatus nigrofasciatus. (I) Dorsal part


In the dragonfly Anax nigrofasciatus nigrofasciatus (Aeshnidae, Anisoptera), the X-tissue is the tissue which occupies the posterior corner of the larval eye and which forms the large facet region in the dorsal part of the adult compound eye after emergence. In the early period of the last larval instar, an ommatidial retinula of the X-tissue shows the same arrangement of retinular cells and crystalline cone processes as that observed in the dorsal part of the adult compound eye, although it has no rhabdom. The rhabdom starts to be formed in the middle stage of the last larval instar and the electrical responses by light can be recorded from that stage, but the chromophore retinal of visual pigment cannot be detected from the larval X-tissues. The rhabdom observed remains rudimentary till emergence. During the period of emergence, the corneal facets in the X-tissue expand to nearly twice their original size as a result of rapid enlargement of the ommatidia. On the day of emergence, the threshold light intensity of the electrical respose is about 2 log units lower than that of the X-tissue. The chromophore retinal of visual pigments then is detectable from the dorsal part of the compound eye. After maturation of the dragonfly, the sensitivity of the dorsal ommatidia further increases, corresponding with an increase of the amount of chromophore retinal. The dorsal region of the adult compound eye does not have the tiered structure of the rhabdom as seen in the ventral region. It has its maximum spectral sensitivity in the UV region.

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