First published online October 10, 2003
Mechanosensory-induced behavioural gregarization in the desert locust Schistocerca gregaria
Stephen M. Rogers1,2,*,
Thomas Matheson1,
Emma Despland2,
Timothy Dodgson2,
Malcolm Burrows1 and
Stephen J. Simpson2
1 Department of Zoology, University of Cambridge, Downing St, Cambridge CB2
3EJ, UK
2 Department of Zoology, University of Oxford, South Parks Rd, Oxford OX1
3PS, UK
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Fig. 1. Mechanosensory stimulation directed to just part of the hind femur elicited
rapid behavioural gregarization. (A-F) Histograms showing the frequencies of
locusts falling into different categories of behavioural phase state after 4 h
of mechanosensory stimulation directed towards the regions indicated in black
on a hind femur. The white areas were painted over to prevent the activation
of mechanoreceptive hairs. (G,H) In two control treatments, a hind femur was
entirely painted over and either stimulated as in the other treatments or left
unstimulated. N=12 locusts per treatment.
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Fig. 2. Median Psol values of the data shown in
Fig. 1, obtained after locusts
were mechanically stimulated on different regions of the femur as indicated on
the x-axis. Values close to 0 indicate that the locusts behaved in a
gregarious manner in the assay; values near to 1 indicate that they remained
solitarious. Asterisks indicate degree of significance of Dunnet's two-tailed
post-hoc tests against the unstimulated controls (white bar):
*, P<0.05; **, P<0.01;
***P<0.001.
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Fig. 3. Mechanical stimulation of a hind femur of locusts unable to move their legs
produced only a small change in behavioural phase state. Distribution of
Psol values obtained after 4 h of mechanical stimulation
in restrained locusts (N=77). 45% of restrained locusts remained
highly solitarious (Psol>0.95) after mechanical
stimulation, but more animals showed some increased gregariousness compared
with unstimulated controls. The asterisk indicates the degree of significance
(t-test) of the rank normalised data against the controls in the
previous experiment. *, P<0.05.
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Fig. 4. Numbers of tactile hairs and basiconic sensilla on the hind femur of
final-instar nymphs differed between phases. (A) Numbers of tactile hairs on
the outward faces (viewed laterally) of the front, middle and hind legs.
Solitarious locusts (black bars) had significantly more tactile hairs on the
hind femur than did gregarious locusts (white bars). (B) These additional
tactile hairs occurred on the dorsal distal and most ventral proximal regions
of the anterior face of the hind femur. (C) On the distal anterior hind femur
(black region of inset), solitarious locusts had significantly fewer basiconic
sensilla but significantly more tactile hairs than did gregarious locusts.
Significance (t-test) is indicated by asterisks: *,
P<0.05; **, P<0.01; ***,
P<0.001; N=10 locusts per group throughout.
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Fig. 5. Numbers of tactile hairs on the front and hind legs of adult locusts
differed between phases. (A) Numbers of tactile hairs on the front and hind
legs. Solitarious locusts had fewer sensilla on the front leg and on the hind
tarsus but had more sensilla on the hind femur than did gregarious locusts.
(B) The distribution of tactile hairs on the hind femur was similar to that of
final-instar nymphs (see Fig.
4B). (C) Maps showing the positions of the hairs from nine
individuals overlaid onto averaged femur outlines; most hairs were confined to
the longitudinal ridges on the femur. Significance (t-test) is
indicated by asterisks: *, P<0.05; **,
P<0.01; ***, P<0.001; N=9 locusts
per group throughout.
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Fig. 6. Patterned electrical stimulation of metathoracic nerve 5 can elicit
behavioural gregarization. (A-I) Histograms showing the distributions of
Psol values obtained after immobilised locusts were
electrically stimulated for 4 h by electrodes attached to the indicated
branches of metathoracic nerve 5 or the extensor tibiae muscle. (J) Ventral
view of the thorax and proximal hind leg dissected to show the metathoracic
ganglion, nerve 5 and its principal branches and the extensor tibiae muscle.
Values in bold are median Psol values obtained after
electrical stimulation at each location. Nerve 5A runs between two muscles in
the coxa, hence its apparent sudden termination. Significance (Dunnet's
post-hoc tests of ANOVA) is indicated by asterisks: *,
P<0.05; **, P<0.01; ***,
P<0.001. Scale bar, 2 mm.
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Fig. 7. Frequency histograms showing the distribution of Psol
values of free-moving locusts in which metathoracic nerve 5 had been cut
before the hind femur was mechanically stimulated for 4 h. Either (A) the
whole of nerve 5, (B) nerve 5A or (C) nerve 5B was cut, or (D) the whole of
nerve 5 was cut but the insect was stimulated on the contralateral (intact)
femur. The median Psol values are indicated next to each
graph; significance is indicated by an asterisk (Dunnet's post-hoc
tests of ANOVA; P<0.05, N=9 locusts for all groups).
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Fig. 8. Frequency histograms showing the distribution of Psol
values obtained in the behavioural assay after locusts were stimulated at 1
min intervals with either (A) 5 cm3 air, (B) 0.5 cm3
acetic acid odour mixed with 4.5 cm3 air, (C) 5 cm3
acetic acid odour drawn from the headspace of a bottle of glacial acetic acid,
(D) stroking with a small paintbrush or (E) simultaneously stroking with a
paintbrush whilst 5 cm3 acetic acid odour was applied. Median
Psol values for each distribution are indicated next to
each graph; significance is indicated with asterisks (Dunnet's
post-hoc of ANOVA analyses): *, P<0.05;
**, P<0.01, N=8 locusts for all groups.
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Fig. 9. Summary of the innervation of the exteroceptors, proprioceptors and muscles
of the hind leg and their possible roles in eliciting behavioural phase
change. We have shown that the nerves or sense organs in white boxes need to
be stimulated in order to elicit phase change. Nerves or sense organs in the
lightest grey boxes are implicated in signalling appropriate mechanosensory
gregarizing stimuli. Nerve 5A and its innervated structures, in mid-grey
boxes, may contribute to signalling gregarizing stimuli, but it is not
necessary to stimulate this nerve for phase change to occur. We have either
demonstrated or can infer that those nerves, muscles and sense organs in the
dark grey boxes have no role in mechanosensory-elicited phase change. FCO,
femoral chordotonal organ.
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© The Company of Biologists Ltd 2003
