Structural organization of the ommatidium in the ventral compound eye of the dragonflySympetrum

Summary

In the dragonflySympetrum, the circumferential sequence of retinular cells *R5*R4, R3*R2, R1*R8, R7, R6 (in which R5 & 8, R2 & 3, R1 & 4 comprise three receptor pairs, R7 and R6 an unmatched pair with long visual fibres, and asterisks denote the positions of cone cell processes) is homologized to the general pattern of odonate retinulae. This sequence runs in an anticlockwise direction for ommatidia of the right ventral retina viewed from outside inwards, that in the left retina runs clockwise. The proximo-distal sequence of contributions of these cells to the retinula (presence of nucleus, contribution to the tiered rhabdom, Fig. 1) has R1 & 4 in the basal third (Fig. 10) beneath R5 & 8, and R2 & 3 (Fig. 6); R7 has a large distal rhabdomere beneath which R6 contributes a few microvilli for most of the rhabdom's length. There is no twist to the rhabdom, and neighbouring ommatidia have consistent orientations. R1 is dorsal and R2 & 3 anterior. Rhabdom diameters are shown in Table 1; individual rhabdomere volumes are as follows: R7, 320 μm³; R5 & 8, 650 μm³ each; R2 & 3, 430 μm³ each; R1 & 4, 230 μm³ each.