From Ito et al. (1995) Roux's Archiv Dev. Biol. 204: 284-307
The lateral (A), horizontal (B), and perspective (C) views of the larval
CNS just after hatching. C shows the neuropile of the T1 neuromere and the
abdominal ventral nerve cord. One larval abdominal neuromere (A1-A6) just
after hatching is 11-13 µm thick, 70-80 µm wide, and 35-40 um high. Caudal
neuromeres (A7 and A8/9) are narrower and distorted. The thoracic neuromeres
are thicker, narrower, and higher than the abdominal ones.
The metameric Cartesian coordinate system is shown in A and B. "%NAP"
(Percent Neuromere Antero-Posterior) indicates positions along the longitudinal
axis; "0 %NAP" corresponds to the anterior segment border, "100
%NAP" to the posterior segmental border. The mediolateral axis is represented
by "%NML" (Percent Neuromere Medio-Lateral); 0 %NML corresponds
to the midline and 100 %NML to the lateralmost surface of the CNS. The vertical
axis is indicated as "%NVD" (Percent Neuromere Ventro-Dorsal);
0 %NVD is the ventral surface and 100 %NVD dorsal.
The segment borders are marked by the "dorsoventral channel" (DV
channel), a duct-like structure penetrating the nervous system vertically
on the midline. The inner surface of the channel is contiguous with the
outer VNC surface (Fig. 3A and H). In early embryos the channels first appear
as the space between a pair of longitudinal connectives and the neighboring
neuromeres (Fig. 6K and 14E).
With numerous intrinsic axon fibers associated with the commissures and
longitudinal tracts, the larval neuromere is much larger and more complex
than in the early embryos. The midline region between the commissure neuropile
is almost devoid of cell bodies (see also Fig. 3). Each neuromere carries
three nerves: a pair of peripheral nerves and a neurohemal organ. The neurohemal
organ connects to the dorsal VNC at the segment border. The abdominal neurohemal
organ forms a V-shaped bifurcation to send a pair of dorsal nerves (Hertweck,
1931; "transverse nerves" in Gorczyca et al., 1994) to both sides
of the body wall, where they are called segment boundary nerves (Bodmer
and Jan, 1987). The bifurcation occurs just above the VNC in the embryo.
In larvae, especially in late stages, the stalk between the VNC and the
bifurcation point becomes elongated, running above the dorsal midline (see
Fig. 1 of White and Kankel, 1978 for late larvae). The thoracic segments
lack the lateral projections from the neurohemal organ (A and B, see also
Fig. 3 of Naessel et al., 1988 for Calliphora).
The peripheral nerve consists of two separate fiber bundles until embryonic
stage 15-16 (see Fig 11B and C); they fuse during stage 16-17. The level
where the nerves leave the VNC is relatively ventral in thoracic segments
and more dorsal in the abdomen (A). The peripheral nerve has two nerve roots.
The intersegmental nerve root (ISNR) crosses the segment border and forms
two branches. The anterior branch enters the neuropile of the anterior segment
in the dorsalmost region (90 %NVD) at about 50-70 %NAP (see also Fig. 3D).
The posterior branch enters the neuropile slightly more ventrally (75 %NVD)
and posteriorly (65-80 %NML; see also Fig. 3E). Both branches are associated
with the mediolateral fiber tracts in the dorsalmost neuropile. The segmental
nerve root (SNR) forms many small branches that enter the ventralmost region
of the neuropile (50-60% NVD) at various anteroŠposterior levels and invade
the ventralmost neuropile. In midŠstage embryos, all the nerve roots run
perpendicularly to the body axis. As the nervous system contracts, most
of them are skewed to pass through the cortex obliquely. This distortion
of the nerve roots does not affect the cortex structure.