Serotonin Mediates Behavioral Gregarization Underlying Swarm Formation in Desert Locusts

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Science  30 Jan 2009:
Vol. 323, Issue 5914, pp. 627-630
DOI: 10.1126/science.1165939

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  1. Fig. 1.

    (A) Final larval instar solitarious and gregarious locusts. Scale bar, 1 cm. (B) Trajectories (over 500 s) of a solitarious (upper) and gregarious (lower) locust in the behavioral arena. A group of 20 long-term gregarious-phase locusts was placed behind a clear partition on the left. (C) Solitarious locusts undergo rapid behavioral gregarization with appropriate stimulation; median Pgreg of locusts treated for 0 to 4 hours by either forced crowding with gregarious locusts (circles), stroking a hind femur (squares), electrically stimulating the principal hind-leg nerve (diamonds), or exposure to the sight and smell of other locusts (triangles). See SOM text for analysis. (D) The CNS of a locust consists of the brain, which receives olfactory and visual information, and a chain of segmental ganglia linked by paired connectives. The three thoracic ganglia receive direct mechanosensory and proprioceptive inputs from the legs.

  2. Fig. 2.

    The amount of serotonin in the thoracic CNS is correlated with the degree of behavioral gregarization. Relationships between the amount of serotonin in the thoracic CNS (loge scale) and the degree of behavioral gregarization (Pgreg) after solitarious locusts have been (A) crowded for 1 hour (triangles), 2 hours (squares), or unstimulated controls (circles); (B) stroked on the left hind femur for 2 hours; (C) given patterned electrical stimulation to metathoracic nerve 5, simulating the effect of mechanosensory stimulation for 2 hours; or (D) presented with the sight and smell of ∼1000 locusts for 2 hours.

  3. Fig. 3.

    Serotonin is necessary to induce behavioral gregarization. Behavior of locusts injected with substances that block either the action or the synthesis of serotonin and then exposed to sensory stimuli that normally induce gregarization (left column) is shown. Saline-injected controls are shown in the right column. (A) Locusts injected with serotonin-receptor antagonists ketanserin and methiothepin (1 mM) and given either 1 hour of femoral mechanosensory stimulation or 1 hour of olfactory and visual stimulation from other locusts. (B) Locusts injected with 0.1 mM AMTP, an inhibitor of serotonin synthesis coupled with 2 hours of mechanosensory stimulation. Arrows indicate median Pgreg values.

  4. Fig. 4.

    Serotonin is sufficient to induce gregarious behavior. Behavior of locusts after they have been treated with (A) serotonin topically applied to the thoracic ganglia (left column) and paired saline controls (right column); (B) a mixture of serotonin agonists (1 mM α-methylserotonin and 1 mM 5-carboxamidotryptamine) and paired saline-injected controls; and (C) the serotonin precursor 5-HTP, either with or without 30 min of crowding with other locusts.

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