Functional morphology of the retina of Chrysops caecutiens L. and Haematopota pluvialis L. (Diptera : Tabanidae): Region around eye equator
ABSTRACT The ommatidia in the midregion of the eyes of females in 2 tabanid species, Chrysops caecutiens L. and Haematopota pluvialis L. (Diptera : Tabanidae) are compared with those in muscoid flies. They conform in their basic fine structure to ommatidia of other dipterans. However, whereas in this part of the retina of higher dipterans the rhabdomeres of all the retinula cells are twisted, in the tabanids only the peripheral rhabdomeres R1–6 twist. The central retinula cells R7 can be assigned to 3 morphological types, depending on the orientation of the microvilli and rhabdomeres: (i) dorsal (or ventral in the lower eye part); (ii) frontal; or (iii) caudal. Cells of these 3 types are distributed within the ommatidial array in an irregular pattern. The microvilli of the retinula cells R8 are all oriented in the same direction, approximately dorsoventral. All the central rhabdomeres in the midregion of the retina are untwisted. This could subserve high polarization sensitivity of the retinula cells R7/8, which may assist in host-finding by these bloodsucking flies.