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Fig. 4. Transmission electron microscopy of the two ommatidial types, round and
trapezoidal, of the yellow tip. (A–E) Transverse sections of an
ommatidium with an approximately circular-cylindrical, round rhabdom.
(F–J) Transverse sections of an ommatidium having a trapezoidal rhabdom
at the middle level of the retina. Bar, 1 µm. (K) Summary diagram of the
two ommatidial types. The total length of the ommatidium, including the
dioptrical apparatus, is about 420 µm. In the round type, the
photoreceptors R1–R4 contribute their rhabdomeres over about the distal
half of the ommatidium, and the R5–R8 do the same over most of the
proximal half. The R9 photoreceptor adds some microvilli at the base of the
rhabdom. Clusters of pigment granules exist in the cell bodies of the
photoreceptors R5–R8 in most of the distal half of the round type
ommatidium, over a distance of about 100–300 µm from the corneal
surface, but no pigment clusters exist in the proximal half of the ommatidium.
In the trapezoidal-type, the rhabdomeres of R1–R4 form the distal two
thirds of the rhabdom, those of R5–R8 form most of the proximal
one-third, and the R9 photoreceptor again adds some microvilli at the base of
the rhabdom. Clusters of pigment granules exist in the cell bodies of the
photoreceptors R5–R8 in virtually only the proximal one-third of the
trapezoidal-type ommatidium. The levels of the transverse sections A–J
are indicated by arrowheads at the left side of the diagram. Four tracheal
extensions (T) invade the retina. BM, basement membrane; D, dorsal, V,
ventral; P, posterior, A, anterior.
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