Figure 1.
The jewel wasp stings a cockroach into the brain.
(A) A photograph and a diagram showing the presumable trajectory of the wasp’s stinger (red) inside the head of its cockroach host. The wasp holds the cockroach by the pronotum while bending the abdomen towards the cockroach’s head, inserting the stinger through the soft neck cuticle. The central nervous system of the cockroach is depicted in yellow. Br: Brain, SEG: subesophageal ganglion, NC: neck connectives. (B) Left: a lateral view of the cockroach head demonstrating the central nervous system (brain (Br) and SEG), the esophagus (Es) and the internal head skeleton (tentorium; Tnt). Right: light micrograph of a cross section of the head (taken from the plane shown as a dashed rectangle on the left), showing the brain, SEG, internal skeleton, trachea (t) and muscles (m). Different possible points of entry of the stinger through the soft neck cuticle are illustrated by arrowheads.
Figure 2.
The stinger possesses mechanosensory and dual mechano-chemosensory organs.
(A) Frontal view of the tip of the stinger (Scanning Electron Micrograph; SEM). DV: dorsal valve, VV1/2: first/second ventral valve. Dome-shaped sensilla (arrowheads) can be seen at the apex of the DV (two opposing triplets) and between serrations of the two VVs. (B) Cross-section of the stinger (light micrograph) showing the DV and two VVs enclosing the egg canal (EC). The tongue-and-groove structure of the rachis (Ra) and aulax (Au) enables intricate movements of the valves relative to each other. (C) Dorsal view of the stinger (SEM). The VVs in this image are extended distally to reveal their serrations (arrows) and a part of the EC. (D) Outlines of the stinger (distal part enlarged on the right) showing the distribution of different sensilla along the DV. Red arrows indicate the position of campaniform sensilla; black arrowheads indicate the position of dome-shaped sensilla. (E) External morphology of one campaniform sensillum (SEM). (F) A mechanosensory dendrite innervating a campaniform sensillum (Transmission Electron Micrograph; TEM). OvW: ovipositor wall, MT: microtubules, Sh: dendritic sheath. (G) External morphology of one dome-shaped sensillum (SEM). (H) A bundle of 4 chemosensory dendrites (CD) and 1 mechanosensory dendrite (MD) innervating a dome-shaped sensillum (TEM). OvW: ovipositor wall; Ap: apodeme. Sh: sheath. (I) Longitudinal section (TEM) through one dome-shaped sensillum demonstrating the apical pore (arrow) and sensillar sinus (SS). (J) Silver nitrate staining (light micrograph) of the stinger showing penetration of the tracer (black staining) through the pores of dome-shaped sensilla.
Figure 3.
Stinger afferents show spiking activity in response to mechanical stimulation.
(A) Recording set-up (top view). The wasp’s stinger and terminal abdominal ganglion (TAG) are bathed in saline but with the distal half of the stinger protruding in an approximately 450 angle above the saline. The tip of the stinger is stimulated with either hard agarose or soft agarose in a glass capillary (grey rectangle) which can be moved in the distal-to-proximal (D-P) or in the proximal-to-distal (P-D) direction along the longitudinal axis of the stinger, by means of a peristaltic pump. En passant sensory responses are recorded from stinger afferents with a suction electrode placed on the nerves between the stinger and the TAG. A confocal micrograph (bottom) shows Neurobiotin backfills from the tip of the stinger, highlighting sensory afferents ascending from stinger sensilla to the TAG. Scale bar = 50 µm. (B) Representative neuronal responses for a stinger isolated from one wasp and stimulated sequentially with hard (2.5%, top) and then with soft (0.5%, bottom) agarose. Left and right shaded areas in each trace represent the duration in which the agarose was actively pushed against (D-P) or pulled away from (P-D) the stinger, respectively. (C) Peristimulus time histogram of neuronal activity evoked by hard (blue) or soft (red) agarose stimulation. Data points represent the mean (±SEM) number of sensory spikes within 200 ms time bins. Data is pooled from 5 different wasps, each stimulated at least 10 times in each condition. Left and right grey vertical bars indicate the 500 ms of the stimulus during which the agarose is actively pushed against (D-P) or pulled away from (P-D) the stinger, respectively. **p<0.01 for hard compared with soft agarose during the first 500 ms of the stimulation (t-test; n = 5 wasps).
Figure 4.
The wasp uses mechanosensory inputs to identify the cockroach’s brain.
(A) Mean (±SD) stinging duration after different surgical manipulations on the cockroach’s brain prior to a wasp’s sting (see text for details). Control (n = 30); ‘Brainless’ (n = 19); Brain replaced with agarose pellets: 0.25% (n = 12), 0.5% (n = 9), 0.75% (n = 8), 1% (n = 6), 2.5% (n = 12); brain injected with TTX (n = 6); Brain homogenized (n = 5). **p<0.01, ***p<0.001 (Kruskal-Wallis One-Way ANOVA on Ranks versus the control group). (B) Number of red pixels, indicative of amount of injected venom in hard (2.5%, n = 10) and soft (0.5%, n = 9) agarose pellets following a wasp’s sting. Inserts are representative photomicrographs of one hard (left) and one soft (right) agarose pellet extracted from cockroach heads immediately after the sting (Scale bars = 0.1 mm). ***p<0.001 (t-test).