Observations on the nervous system of the flight apparatus
in the locust Schistocerca gregaria
By D. M. GUTHRIE
(From the Department of Zoology, University of Leicester)
With 3 plates (figs. 3, 5, and 10)
Summary
The hair sense-organs of the head are part of a sensory system affecting the activity
of motor neurones to the flight muscles. They possess curved hollow hair shafts
inserted in a complex socket. A large neurone is present beneath the socket and is
partly surrounded by a large formative cell, the trichogen-tormogen cell. The distal
process passes up into the expanded base of the hair shaft. Fine connexions between
the outer region of the formative cell and the inner part round the neurone process,
possibly limiting angular sensitivity, can be seen in some specimens, although the
form and fine structure of the hair shaft is almost certainly important in this respect.
The axons from the 5 areas of hair organs are collected together into a dorsal
tegumentary nerve, those from area three forming a short subocellar nerve. Electronmicrographs of this nerve show that there are a number of large fibres (1 to 5 /JL), and
many more smaller fibres (i-o to o-i fj.) with no sheaths. There were estimated to be
5,500 fibres in each dorsal tegumentary nerve.
Within the central nervous system, the dorsal tegumentary fibres may follow one
of 4 routes, as follows. They may (i) pass forward into the protocerebrum, (ii) end
in zones of terminals in the deutocerebral region, (iii) a few thick fibres pass down
into the suboesophageal ganglion and then cross over to the opposite side giving off
collaterals before descending to the pterothoracic ganglia, (iv) most of the fine
descending fibres probably end at the suboesophageal level, a proportion of them
crossing over here.
The motor neurones to the longitudinal indirect muscles M81 and M82 consist of
4 anterior and 1 posterior cell respectively, and possess large and striking cell bodies,
whose collaterals could be seen in the dorsal zones of motor terminals. The probable
internuncial links between the sensory and motor arcs are outlined.
Introduction
I N locusts, the flight mechanism involves an important nervous pathway
between the hair sensillae of the head and the motor neurones of the flight
muscles. Weis-Fogh (1956), Haskell (1958), and Wilson (1961) showed that
stimulation of the hair sensillae by air currents could initiate and help to
maintain the activity of the motor centres of the thoracic ganglia. In addition
the hairs can detect angular differences in the direction of air flow, and may
be concerned in orientation. Proprioceptors in the thorax also play an important part in the control of flight (Wilson 1961, Gettrup 1962).
The main purpose of the work described in this paper was to investigate
the anatomy of the pathways between the head hairs and the wing muscles.
[Quart. J. micr. Sci., Vol. 105, pt. 2, pp. 183-201, 1964.]
184
Guthrie—Nervous system of flight apparatus of locust
The enumeration of the nerves and muscles is based on Campbell (1961),
but in appropriate instances Ewer's numbers (1957) are given in addition, as
they have been employed by Weis-Fogh and Wilson.
Methods
Light microscope histology
Vital staining with methylene blue gave good results with peripheral nerve,
the stain being prepared in the reduced form by Unna's method (Pantin,
FIG. 1. Lateral view of the right side of the head ca^
5 hair areas of Weis-Fogh.
1946) and fixed in ammonium molybdate. Most of the sectioned material was
brought on to slides after fixation in Duboscq-Brasil, and the standard paraffin
wax procedures. Sections were stained in ripened Hansen's trioxyhaematin,
or by a modification of Holmes's silver technique using lutidine as a mordant
according to Blest (1961), although pyridine gave some good results. The
Palmgren method was not so successful with locust material as it was with
Hemiptera (Guthrie, 1961).
Thin sections of the hair sensillae in the 0-5 to 3-0 /x range were prepared
using the techniques described by Steedman (i960), but embedding in hard
paraffin wax was found to be as satisfactory as his agar-ester wax method for
this material. Sections were stained in Heidenhain's iron haematoxylin.
Guthrie—Nervous system of flight apparatus of locust
185
Electron tniscroscope histology
Material was fixed for about 1 h at 2° C in veronal buffered 1 % OsO4>
dehydrated in chilled ethyl alcohol, and embedded in methyl methacrylate.
Many of the blocks containing small pieces of nerve were re-orientated in
carnauba wax. Sections were cut on a Servall Porter-Blum microtome, and
some were stained for a few seconds with 1 % phosphotungstic acid before
examination with a Siemens Elmiskop electron microscope.
Electrophysiology
A number of recordings were made using silver wire electrodes connected
to a double-sided preamplifier, signals being displayed on a Cossor 1049
oscilloscope.
The sense organs
The wind-sensitive structures consist of a number of trichoid sense organs
disposed in 5 groups on the anterior and dorsal regions of the head as described
by Weis-Fogh (1956), whose enumeration has been followed here (fig. 1).
These areas are made conspicuous by their including a large proportion of the
hair organs with shafts over 70 \x. in length (70 to 250 /x).
When a current of air is passed over the head at increasing speeds, at certain
critical speeds some of the hairs can be seen to enter a state of rapid vibration,
while neighbouring ones may remain motionless. Within a hair area there
are small differences in the orientation of the shafts, but greater differences
exist in this respect between the separate hair areas. When a current of air
is directed on to the hair beds, regular trains of impulses can be picked up
from the central nervous system as far posteriorly as the anterior longitudinal
connectives of the mesothoracic ganglion (fig. 3, A).
A description of the hair organs based on thin sections viewed with the
light microscope is given here, but it is hoped to present further details based
on electron microscope work in a later communication.
The structure of a wind-sensitive hair is shown in fig. 2, and it can be seen
that it has a general resemblance to those figured by Sihler (1924) in Gryllus,
in the structure of the socket, large size of the nerve-cell body, and the
apparent absence of a distinct tormogen cell. The hair shaft is curved and
hollow, the central canal extending to near the tip.
The trichogen-tormogen cell is a large element lying directly below the
hair socket. It contains a large vacuolated cavity through which a process,
described here as the column, passes up to the base of the hair. The column
supports the distal process of the nerve-cell, and also appears to have connexions with the side walls of the trichogen-tormogen cell. These connexions,
which were not observed in all sense organs, consist of tubular structures
lying across the trichogen-tormogen cell cavity, and at an angle to the column
(figs. 2; 3, F).
The sense cell is a large bipolar neurone lying to one side of the trichogentormogen cell (fig. 2). It is surrounded by several sheath cells (fig. 2), the
186
Guthrie—Nervous system of flight apparatus of locust
dense zones in
central channel
sheath
cells
FIG. 2. A hair sense organ as seen in sections made in the plane of the hair shaft. The lower
part of the shaft is shown, and above it a small segment of the middle region. On the right
are shown sections cut in a plane transverse to that of the hair shaft: compare these with
figure 3.
number being dependent on the size of the nerve-cell; thus the larger neurones
may possess at least 3 tiers of associated sheath cells with 2 or 3 cells in each
tier. Whether these are all derived from the single neurilemma cell of a
tetrad as in Rhodnins (Wigglesworth, 1953) is not easy to determine, although
developmental stages apparently consisting of 4 and 5 cells were visible in
FIG. 3
D. M. GUTHRIE
Guthrie—Nervous system of flight apparatus of locust
187
the freshly moulted epidermis. The neurone cell body contains a large
nucleus and distinct nucleolus, the cytoplasm appearing coarsely granular and
containing numerous tubules and vesicles. The proximal process is a dense
axon passing out through the basement membrane of the epidermis. The
axons of the larger neurones are surrounded by a collar of sheath cells at this
point. Smaller axons can be seen to converge on the larger (fig. 2, axons).
The distal process of the neurone is a hollow tube containing fine strands of
material aggregated at most levels into 4 or 5 ill-defined columns arranged
against the inner surface of the tube. At its lower end the tube originates
from finer tubules in the neurone cell body. At its upper extremity the tube
emerges from the column, and is often coiled in this region. A fine central
strand within the tube can be observed here in some specimens. Near the
base of the hair shaft the tube is expanded into a bulb with 4 dense zones in
it, the 2 nearest the main cavity of the trichogen-tormogen cell being the most
distinct. These may be the scolopale rods of other authors. Beyond this
point the tube narrows abruptly as it enters the basal channel of the hair
shaft. The densely staining material inside the tube is at this point clearly
aggregated into 4 cylinders that pass up some way into the tapering end of
the tube. The basal channel of the hair shaft is a keyhole-shaped slot in the
button-like expansion of the shaft, and the neurone process appears to lie on
one side of this, cytoplasmic material from the trichogen-tormogen cell
passing up the other side. The neurone process appears to end here at the
upper extremity of the basal channel, but it is difficult to see whether any
material from the neurone passes into the narrow main channel of the hair shaft.
The form of the hair is that of a tapering cylinder with a central symmetrical
canal between one-third and one-quarter of the outside diameter of the hair.
The transverse profile of the outer surface of the hair is smoothly circular in
the region of the socket but further distally the outer surface is fluted, there
being 8 to 12 concave facets (figs. 2; 3, B). In this region the outer zone of
the shaft appears to be denser than the inner parts, and each facet forms one
surface of a wedge-shaped segment of the shaft (figs. 2; 3, B, C). At its base
FIG. 3 (plate), A, discharge in anterior connectives of the mesothoracic ganglion dying
away as the airflow over the head is shut off. Several fibres are active.
B to I are sections cut transversely across the main axis of the sense organ. These photographs should be compared with fig. 2.
the middle region of the hair shaft. Note the facets.
the hair shaft just above the socket. Material can be seen in the central channel,
the base (button) and basal channel of the hair shaft, showing the eccentric end of the
se cell, and less dense central material,
the free portion of the distal neurone process,
the region of the tubular cytoplasmic connexions.
the neurone process surrounded by the column of the trichogen-tormogen cell,
the lower region of the neurone process.
, through the region of the neurone cell body.
J, this section is through the main axis of the sense organ. The hair shaft has fallen out,
and the end of the neurone process can be seen.
b, button; n, neurone; s, socket; tc, tubular connexions; tn, tube of distal neurone process;
tt, trichogen-tormogen cell.
2421.2
O
188
Guthrie—Nervous system of flight apparatus of locust
the hair is expanded into the button-shaped structure already noted. This
button is supported below by a thick circular ridge, and at the sides by thinner
diaphragms. The hair shaft emerges from the socket through a cuticular cone
broadly continuous with the rest of the cuticle. There is also a thin plate
with a central aperture for the neurone process below the button marking the
upper boundary of the trichogen-tormogen cell (fig. 2).
The identification of the material within the upper parts of the hair shaft
proved difficult. In some sections there appeared to be definite peripheral
and central aggregations of material; in others the central channel appeared
empty. This may have been partly due to variable success in fixation but
oblique sections showed serially repeated zones of density of the form indicated in fig. 2. This would account for the variable appearance of transverse sections. The greater part of the canal is almost certainly lined with
a thin layer of cytoplasm derived from the trichogen-tormogen cell.
One or two points need to be made here concerning the symmetry of the
sense organ, as this may be relevant to any discussion of the angular sensitivity
of the organ. Most parts of the organ are radially symmetrical, but the hair
shaft is curved along one axis; this is also the axis of the distal neurone process
and may be called the primary axis. The greatest width of the slot-like basal
channel appears usually to lie at right angles to this primary axis, so that the
neurone process which occupies an eccentric position within the hair base
follows a slightly curved path round the column. The 2 tubular struts lying
across the trichogen-tormogen cell cavity are set off at equal angles of about
45° on either side of the primary axis.
The axons of the neurones from hair areas 1 to 5 pass into the central
nervous system in the dorsal tegumentary nerve of Albrecht (1953).
The topography of the dorsal tegumentary nerve
Most of the cuticular sense organs on each side of the upper part of the
head capsule, above the level of the median ocellus, combine their fibres to
form the dorsal tegumentary nerve (fig. 4). A few proprioceptive hairs
adjacent to the antennal socket contribute fibres to the antennal nerve.
The distribution of the nerve as it appears in sections through the whole
head and in dissection is shown in fig. 4. The main nerve-trunk originates
from the supra-oesophageal ganglion in the region of the deutocerebrum, and
passes dorsally across the upper surface of the protocerebrum. In this region
the nerve is about 50 p in diameter and incompletely surrounded by fatty
tissue. Near the upper margin of the protocerebrum, the main trunk divides
into an anterior branch (fig. 4, A) and a posterior branch (fig. 4, B), and these
in turn subdivide as shown in fig. 4. For the purposes of this description the
main branches are numbered, but there is undoubtedly a good deal of variation in the topography of this nerve.
The correspondence between hair areas and nerve branches was as follows.
Areas 1 and 2 are largely innervated by branch A2, although a few fibres from
Guthrie—Nervous system of flight apparatus of locust
189
the ocellar region pass into Ai. Area 3 is innervated by the sub-ocellar part
of branch Ar. This nerve does not seem to have been described by other
authors. A small nerve A3 can be seen in some specimens apparently running
63
lateral ocellus
ocerebrum
ocerebrum
FIG. 4. The distribution of the branches (AI to B3) of the dorsal tegumentary
nerve of the left side of the head.
to a central non-cuticular region of the head, but its termination was not
identified. The posterior nerve-trunk B constitutes the main outlet for hair
areas 4 and 5, and for more posterior sensilla of small size which contribute
to nerve B3. The layout of the dorsal tegumentary nerve appears to be
influenced by the density of cuticular sense organs, but in addition the axons
190
Guthrie—Nervous system of flight apparatus of locust
of large neurones may affect the orientation of small axons, and therefore have
a relatively great effect on the pattern of nerve branches. It can often be
observed that the small axons converge on the larger ones (fig. 2).
The dorsal tegumentary nerve appeared to be wholly sensory from a study
of sections and dissections of normal specimens, but in those in which half
the head surface had been cauterized a few fibres in the nerve persist. These
may be (i) fibres from contralateral sense organs, i.e. there is overlap between
the sensory fields of the tegumentary nerves, (ii) motor fibres, (iii) fibres of
uncauterized sense organs. The latter is the most likely explanation as it is
difficult to destroy all sense organs, in particular those bordering the eyes.
The fine structure of the main trunk of the dorsal tegumentary nerve
The main nerve-trunk consists of a sheath 1 to 2 fi thick, a layer of sheath
cell nuclei of about the same thickness, and some 5,000 nerve-fibres between
o-i fj, and 5-0 /A in diameter. The distribution of fibre sizes is shown in fig. 6,
and it can be seen that the commonest fibre diameter is 0-2 fj,. This estimate
of the number of nerve-fibres in the trunk was made by comparing the
numbers and diameters of fibres in different regions of the nerve, of known
area, with the total area of the nerve occupied by fibres. For this purpose
an outer cortex 5 /x thick and mainly composed of fine fibres has to be distinguished from an inner medulla with a higher proportion of large fibres;
this can be seen in fig. 5, A, B, and c.
Large fibres were distinctive, possessing definite sheaths, and being over
1 -o /A in diameter, while small fibres had no sheaths and were usually less than
i-o JJ, in diameter (fig. 5, c). The actual estimates obtained for the 2 groups
of fibres were 28 large fibres and 2,800 small fibres in the cortex, and 150
large fibres and 2,500 small fibres in the medulla. It must be noted, however,
that measurements of the diameter of fibres based on transverse sections
are liable to error due to irregularities in thickness as demonstrated by
Whitear (i960); where reasonably large counts are involved these errors should
cancel out.
It is difficult to equate the number of fibres evidently present in the nerve
with the number of cuticular sense organs visible externally. Not more than
200 large hairs, and 500 small hairs and pits can be seen on each side of the
head in the area innervated by the dorsal tegumentary nerve; about 90 of
these are the large hairs of hair beds 1 to 5. It seems probable that there are
many small sensilla that cannot be clearly observed externally, and also
numerous sub-cuticular sense organs contributing to the total of 5,300 fine
fibres.
The detailed histology of the dorsal tegumentary nerve as shown by electron
micrographs can be compared with the histology of cockroach leg nerve as
described by Hess (1958).
The fine fibres as shown in fig. 5, B and c are simple cylinders of axoplasm,
containing a few mitochondria and other inclusions, bounded by a faintly
FIG. 5
D. M. G U T H R I E
Guthrie—Nervous system, of flight apparatus of locust
191
double membrane. The large fibres are more complex. The axoplasm contains numerous mitochondria of varied appearance (fig. 5, c), large vesicles
similar to the agranular vesicles of Van Breemen, Anderson, and Reger
(1958), and small vesicles. The finely granular appearance of much of the
axoplasm may be due to the presence of neurofibrils as suggested by Hess
(1958), or to discontinuities in the embedding medium.
The sheaths surrounding the large fibres are thinner and less regular than
those surrounding the fibres in the central nervous system, or in the leg
nerve of Periplaneta as figured by Hess. There is usually only a single
globulated fold of sheath cell round each axon. The similarities between
these sheath cells and vertebrate Schwann cells were pointed out by Hess,
but considerable differences remain in point of regularity and complexity.
If functional analogies can be made with vertebrates, the large sheathed fibres
may be expected to exhibit higher conduction velocities and greater electrical
separateness than fine fibres. Katz and Schmitt demonstrated electrical interaction in unmyelinated fibres (1940). The lack of distinct nodes at which ion
movements can occur may be compensated for by the irregular distribution
of the globules.
The outer sheath of the nerve consists of a layer of cells similar to those
figured by Ashhurst and Chapman (1961) bounded by a fibrous neural lamella
which seems to have a more regular arrangement of its fibres than occurs in
the thicker neural lamellae of the central nervous system as figured by Ashurst
(fig. 5, D). In the tegumentary nerve, the transverse fibres form a series of compartments in which the longitudinal fibres lie. Dense zones can be seen in
some of the fibres, perhaps corresponding to the dense collagen bands so
clearly demonstrated by Ashhurst and Chapman.
Now that Treherne (1961) has shown the extent of the permeability of the
outer nerve sheath to ions, the structure of the sheath may be regarded as of
less critical interest.
The descending tracts in the ventral nerve cord
Physiological records suggest that impulses pass directly down the ventral
nerve cord to the meso- and metathoracic ganglia, and that many of them
cross over to the opposite side of the nerve cord in the region of the suboesophageal ganglion.
An attempt was made to see whether the pattern of fibres described by
anatomical methods corresponded with physiological observations. Two main
methods were employed: examination of complete series of 10 p sections
FIG. S (plate). The fine structure of the dorsal tegumentary nerve as seen in transverse
section.
A, low power photograph of whole nerve to show the partial separation into a cortex (c) and
a medulla (me), and the neural lamella («/).
B, the closely packed fine fibres of the cortex.
C, large sheathed and small unsheathed fibres in the medulla; m, mitochondrion; s, sheath.
D, the neural lamella of the nerve showing longitudinal (If) and transverse (tf) fibres.
192
Guthrie—Nervous system of flight apparatus of locust
extending from the brain to the metathoracic ganglion, and examination of
selected regions of the nerve cord after hot wire cautery of the hair areas
on one side of the head. The locusts were left for at least 3 weeks for degeneration to occur. Neither of these methods was entirely satisfactory for
the following reasons. Only the largest fibres in the tract can be seen at all
clearly in the central nervous system, as although they may be 1 to 3 /x thick
0
0 - 4 0 * V 2
\-6
20
2-4
Diameters of fibres in p
2:8
3-2
3-6
FIG. 6. A histogram illustrating the size distribution of 258 fibres in the dorsal tegumentary
nerve measured from electron micrographs.
in the tegumentary nerve they become attenuated on entry into the CNS
and at this point appear seldom to be much more than 1 /JL across. The
descending tracts become diluted by many other sensory fibres of similar
size and appearance, (this is especially so at the more posterior levels of the
nervous system. Furthermore the distance through which the fibres must be
traced makes this operation arduous; 2 cm of nervous system provide 2,000
sections at 10 JX each. The following brief description is thus conditioned by
these factors. The course of the fibres is shown in fig. 7.
The fibres composing the dorsal tegumentary nerve enter the central nervous system in the region of the deutocerebrum as noted by Satija (1958).
A number of fibres pass forward into the protocerebral region, a few others
end in a centrally placed glomerulus at the deutocerebral level, but the
greater proportion of larger fibres pass posteriorly through the longitudinal
connectives into the sub-oesophageal ganglion. Within the posterior region
Guthrie—Nervous system of flight apparatus of locust
193
dorsal tegumentary nerve
protocerebrum
deutocerebrum
tritocerebrum
sub-oesophageal
ganglion
prothoracic
ganglion
mesothoracic
ganglion
>ral (internuncial)
glomerulus
ventral {sensory) glomerulus
metathoracic
ganglion
FIG. 7. A diagram of the main pathways followed by nerve-fibres of
the dorsal tegumentary nerve in the central nervous system. Stippled
areas represent zones of terminals.
of this ganglion, many of the larger fibres cross over to the opposite side, and
descend further posteriorly on this side. At the point at which the fibres
pass diagonally across the sub-oesophageal ganglion, there is a well defined
zone of terminals apparently contributed to by these fibres. This glomerulus
is in communication with the large synaptic zones, to the-sides of the ganglion.
194
Guthrie—Nervous system of flight apparatus of locust
The fibres then descend into the prothoracic ganglion, where again there are
connexions with the lateral 'Punktsubstanz'. The fibre tract is at this point
very indistinct, but continues to occupy a position near the centre of the connectives. As can be seen from electron micrographs of the connectives in
this region, small fibres are scattered in a rather haphazard manner between
the larger fibres (fig. 10, F), and do not form well defined tracts. In the mesothoracic ganglion some reduction of glomeruli and tracts is visible in the
median ventral region of the ganglion (fig. 10, E) when the hair areas have
been cauterized. The glomeruli receive sensory fibres from the wing through
iB of Campbell (1961); this is nerve iA of Ewer (1957), as will be described
in more detail presently. Similar areas of the metathoracic ganglion appear to
receive dorsal tegumentary nerve-fibres, but the evidence for this is slight.
As noted previously, there is evidence that the larger fibres in this tract
taper at both the tegumentary and the central ends. Thus the diameter of the
largest fibres observed was as follows: at the base of the sense cell, 4 /u, (with
sheath, 6 JU.); in the anterior branch of the tegumentary nerve (A), 4 to 5 /x; in
the main trunk of the tegumentary nerve the largest fibre diameters are usually
found to be about 5 /x, but in a few specimens some kind of swelling seems to
have taken place and fibres 7 L/A thick were found; at the point where the nerve
enters the CNS, 3 /x; in the descending tracts, 1 p to 2-5 /x (the fibres appear
thinner in the connectives than in the ganglia).
One final point about the descending fibres. It can be shown by cutting
away one side of the head and dividing the interganglionic connectives at
various levels that large impulses (several millivolts recorded externally),
cross in the sub-oesophageal ganglion to the opposite side of the cord, but
there is also an enhanced activity of small impulses on both sides during
stimulation perhaps due to elements in synaptic contact with the descending
fibres at the sub-oesophageal level.
The alary nerves and the flight motor neurones
An attempt was made to elucidate the anatomical pathways followed by
impulses in the descending tracts having an excitatory effect on flight motor
neurones. As the pterothorax of an orthopteran, such as the locust, contains
many more separate units than occur in insects of more advanced orders such
as Hemiptera (Guthrie, 1961) or Diptera (Power, 1948), only a part of this
neuromuscular system was examined. This mainly comprised the large
longitudinal indirect muscle M81, the small oblique muscle M82, and the
nerves to these muscles, although some attention was given to other sensory
and motor structures associated with these ones. The nerve-fibres of this
part of the flight apparatus form a highly characteristic pattern of nerves
whose central roots are formed by the posterior and anterior nerves of the
pro- and mesothoracic ganglia respectively. The main advantage of studying
these nerves is that most of the fibres are associated with flight structures.
There is some variation in the topography of this part of the nervous
system, which will be referred to as Prn-Mi (prothoracic recurrent nerve and
Guthrie—Nervous system of flight apparatus of locust
195
prothoracic
recurrent nerve
1st mesothoracic
nerve
FIG. 8. Variations in the branching of nerves to flight structures arising from the prothoracic
recurrent and first mesothoracic nerves. A, normal form; B, variant. The nerves designated
a contain a pair of thick fibres probably to M6i. The identity of nerve iA was uncertain.
first mesothoracic nerve of Campbell). Fig. 8, A shows the commonest form
of Prn-Mi, in which the large sensory trunk iC (Ewer iA) is broadly joined
to the motor roots of the pro- and mesothoracic ganglia. In fig. 8, B can be
seen an extreme form in which the motor roots have remained largely separate
from nerve 1C. The distribution of the major branches agrees broadly with
196
Guthrie—Nervous system of flight apparatus of locust
Campbell's description of Locusta, but nerve iDib (fig. 9) appears to be
combined with 1D2 to form a trunk equivalent to iBb of Ewer. Furthermore,
the branch to the oblique muscle M82 (almost an oblique segment of M81),
FIG. 9. Diagram of the nerves arising from the prothoracic recurrent nerve and the first
mesothoracic nerve of one side to show the distribution of the 6 thickest fibres and their
cell bodies.
separates as a distinct nerve-trunk containing a single large axon, before the
rest of the nerve reaches M8i. A similar variability of the head nerves was
described by Donges (1954) in the weevil Cionus, and it would seem that there
is a tendency towards motor and sensory fibre segregation, usually masked
by other factors during growth.
Guthrie—Nervous system of flight apparatus of locust
197
In successful silver and iron preparations, fibres can be traced in various
parts of this system, and the appearance of transverse sections through some
of the nerve branches is shown in figs. 9 and 10 A to D. The fibres can be most
conveniently divided into 4 categories of diameter, and the nerve branches
classified as follows:
Prothoracic recurrent
First mesothoracic
(a) sensory
(b) motor
iDib+iD2 (iBb)
iDiai (iBai, to M81)
iDia2 (iBa2, to M8z)
iB (to M59)
B
3t05f
4
2
about 7
D
less than i fj.
few
—
—
2
—
3
about 50
6
more than 200
about 20
1
—
—
10
—
—
—
—
A
6 to 8
4
1
1
JU.
C
1 to 2 fj.
10
5
S
5
It can be seen that M81 receives 4 large fibres; 2 of these remain near the
surface of the muscle, while the other 2 pass to muscle-fibres situated in the
deeper part of the muscle. Branches of these fibres could be traced to their
endplates.
The origin of the 6 large fibres identified peripherally was investigated,
especially of the 4 fibres to muscle 81. From the work of von Nuesch (1954)
on Antheraea, data on the effect of recurrent nerve section on locust flight
described by Wilson (1961), and from the point of view of a rational explanation of the fusion of the 2 central motor roots it would seem likely that either
1 or 2 of the M81 fibres originated from the mesothoracic ganglion. But this
seems not to be so. The results of degeneration experiments, and attempts
to trace these fibres throughout their length indicated that the 2 large fibres in
the mesothoracic motor root innervate M82 through iDia2 (iBa2), and M59
through iB, so that all the M8r fibres pass into the prothoracic ganglion.
A portion of the ganglionic course of the 6 large fibres was worked out, and
this is illustrated in figs. 9 and 10, D. The 4 anterior fibres branch extensively
in the dorso-lateral zone of terminals, although it may be noted that the mode
of branching of 2 of the cells is almost identical, and differs from that of the
other 2. This similar pair have cell bodies adjacent to one another (a neurones),
while the second pair (/S neurones) are less closely associated. The cell bodies
of the flight motor neurones are of large size and striking appearance as can
be seen from fig. 10, D, with many foldings of the cell membrane.
Many fibres are visible in the lateral 'Punktsubstanz' that could be responsible for transmitting electrical activity from the ventral zones of fine sensory
terminals, including those of the dorsal tegumentary tract, to the dorsal
motor neuropile, but defined pathways were not observed. The internal
separation of motor, sensory, and internuncial zones within the ganglia falls
into the normal insect pattern (Zawarzin, 1924; Guthrie, 1961).
Peripherally, most of the sensory fibres join the sensory root of the mesothoracic nerve 1, and this appears to be the fate of nerve 1D2 (iBb). This
198
Guthrie—Nervous system of flight apparatus of locust
nerve contains a large fibre (5 /x), besides some 20 fibres of lesser diameter.
Recordings from i D i b show a steady discharge of large potentials, even in the
wings closed position, so it seems likely that the large fibre is part of the
stretch receptor described by Gettrup (1962).
The large sensory tract of the first mesothoracic nerve passes down to the
ventral sensory neuropile within the ganglion, as in other insects (Guthrie,
1961), and most fibres terminate here on the ipselateral side. Some fibres,
notably some of the thickest, pass into longitudinal tracts and may pass to the
vicinity of the M81 motor neurones. Other fibres pass across the ganglion to
the sensory neuropile of the contralateral side.
The fine structure of the central nervous system
Some work was done on the central nervous system of Schistocerca with
the electron microscope; much of this ground has now been covered by the
work of Ashhurst and Chapman (1961, 1962), therefore only a few points
relevant to this study will be made.
A much greater quantity of sheath material is present in the inter-ganglionic
connectives than in the dorsal tegumentary nerve; only a few of the smallest
fibres being in direct axonal contact (fig. 10, F). Some of the large fibres in
the connective are separated by 4 or 5 globulated sheath layers from their
neighbours. Judging by the electron micrographs of Hess (1958), sheath
material is more abundant in locust connectives than in even a large peripheral
nerve in the cockroach; however, the irregularity of both membrane folds and
droplets is similar. Pipa and others (1959) have published a figure of cockroach sheaths seen in longitudinal section which would suggest that the
globules, which Hess showed to be of a fatty nature, are part of a branching
and anastomosing network.
The descending sensory fibres have synaptic relationships with other
neurones at different levels as already described, and therefore some material
was prepared from the lateral synaptic zone of the sub-oesophageal ganglion.
This region consists of large and small axon extensions, with little evident
sheath material, and many large mitochondria (fig. 10, G). A few of the
FIG. 10 (plate). A to c show transverse sections through nerves of the Prn-Mi complex
demonstrating the large motor fibres. Compare with fig. 9.
A, the prothoracic recurrent nerve.
B, nerve iDia near muscle 82.
C, the motor root of the first mesothoracic nerve.
D, a longitudinal section through the prothoracic ganglion. In this part of the section the
a neurones (n), and the 4 large fibres and one rather smaller fibre of the recurrent nerve (rn),
can be seen.
E, a transverse section through the mesothoracic ganglion of an insect in which one side of
the head had been cauterized. The arrow indicates the position of the degenerated tract.
F, a transverse section of a longitudinal connective to show the random arrangement of
small fibres. Many layers of sheath material (s) can be seen round the larger fibres, which contain numerous mitochondria (m).
G, a transverse section through the synaptic zone of the prothoracic ganglion to show a large
synaptic ending (Is) containing vesicles (v). Small fibres (sf) can be seen encroaching on the
large ending.
FIG. IO
D. M. GUTHRIE
Guthrie—Nervous system of flight apparatus of locust
199
terminals contained aggregations of vesicles, similar in their dense boundary
layers to the synaptic vesicles figured by De Robertis (1958) in vertebrate
material, and suggested by him to contain acetylcholine. These insectan
vesicles are 10 to 20 m/x in diameter, and thus somewhat smaller than the
vertebrate ones which are 18 to 60 m/A across.
Discussion
A number of points of information and theory, not easily included under
other headings, are briefly mentioned here.
It was suggested in the section on the sense cells that the form and physical
properties of the hair shaft and its socket are in all probability more important
in determining the directional sensitivity, than the asymmetries of the neurone
and associated cells. Some preliminary work has been done with a model of
the hair and its socket 1,000 times life size made of wood and rubber. Hair
shafts of various shapes were tested in air currents, and it seemed clear that
the curvature of the hair is important in limiting the oscillation response to
air currents from certain directions only. Further results will be reported in
a later communication. Incidentally it is interesting to note that manufacturers
of split bamboo fishing rods use a similar cross-section to that of the hair
shaft for the production of structures with a high resistance to lateral forces,
a necessary requirement for rapid lateral oscillation.
The repetitive nature of the distortion of the neurone process during
oscillation of the hair shaft may help to slow down adaptation, and keep a
steady flow of impulses passing to the pterothoracic ganglia, although Haskell
(1958) found a regular discharge occurring when a hair was subjected to
static bending.
The dorsal tegumentary nerve contained fibres of 2 main types: thick
fibres with sheaths, and thin unsheathed fibres. From studies on the ventral
nerve cord, it seems likely that the large fibres pass some way down the nerve
cord without synapsing, while the small elements synapse with internuncial cells in the anterior ganglia, many of them in the sub-oesophageal
ganglion. If this is so, it would seem that the larger elements, perhaps with
greater electrical separateness and complexity of sense organ structure, are
more likely to be concerned with orientation function than are the smaller
elements.
It was surprising that the smallest fibres in the dorsal tegumentary nerve
(o-i /A) were so large. Gasser (1958) described pig olfactory fibres of less
than 0-08 /J,. Nevertheless the most abundant fibres in the nerve were those
of small diameter (fig. 6), the distribution conforming to an asymmetrical
normal one, rather than a Poissonian distribution (the mean = 0-442 being
less than s2 = 1-604).
As far as the correspondence between fibre diameter and function is concerned, the great majority of the fibres in the central nervous system over
200
Guthrie—Nervous system of flight apparatus of locust
3 /A in diameter appear to be motor or internuncial fibres, but a number of
large sensory axons 5 /x or more have been identified.
The pattern of neurones concerned with the anterior flight pathways is
shown diagrammatically in fig. 11. As the sensory neuropile is to some degree
separate from the dorsal zone of terminals in which motor cells to M81
head hair organs
A
supraoesophageal
internuncials
receptors of
wing and
thoracic wall
large stretch
receptor
FIG. 11. A diagram based on anatomical findings, but in some
degree hypothetical, of the neurone pathways associated with
muscles 81 and 82. The dotted line represents the midline.
branch, it might be suggested that the effects of activity in various sensory
pathways are summed through internuncial fibres in the lateral 'Punktsubstanz' region of the ganglion, which have synaptic contacts with the motor
cells. This would fit in with the belief stated by Wilson (1961) in an unlocalized flight centre in the ganglia.
I should like to thank Dr. P. T. Haskell and other members of the AntiLocust Research Centre for their help and interest in this problem. Dr. C. T.
Lewis and Mr. H. I. Mathews, O.B.E., of Imperial College, London, made
possible the work with the electron microscope and gave much practical
help and advice. Dr. Mary Whitear made invaluable suggestions about the
Guthrie—Nervous system of flight apparatus of locust
201
presentation of the material, especially of that part dealing with electron
microscopy.
This work is part of a programme of research on insect neurones aided by
grants from the Department of Scientific and Industrial Research and the
Medical Research Council, and I must thank Mr. L. Panko for assistance in
technical procedures under the M.R.C. grant.
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