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 Structure of the neuropils in the brain of the two-spotted cricket




Yukihisa Matsumoto(Tokyo Medical and Dental University)



Figure: Structure in the brain of the two-spotted cricket
Hitoshi Aonuma(Hokkaido University)



Brain structure


Two ganglia are located in the insect head; the brain (supraesophageal ganglion) and the subesophageal ganglion. The brain and the subesophageal ganglion are fused in honey bees, ants, silkworms, and fruit flies (Drosophila melanogaster), whereas in crickets, the ganglia are separated by paired ventral nerve cord. The brain can be broadly-divided into three areas: protocerebrum, deutocerebrum, and tritocerebrum. Several discriminative neuropil structures can be observed in the brain, such as mushroom bodies, central complex, optic lobes in the protocerebrum, antennal lobes in the deutocerebrum and lobus glomerulatus in the tritocerebrum.


The mushroom bodies (MBs)


Mushroom bodies (MBs) are the paired mushroom-shaped structures located in the protocerebrum. Composed of over 100,000 endogenous neurons called Kenyon cells, MBs are known to be the secondary olfactory center and also responsible for learning and memory. An MB in each brain hemisphere consists of  four areas, the calyx, pedunculus, vertical lobe (α lobe), and medial lobe (β lobe). The calyx is the input site of MB that receives sensory information of various sensory modalities (olfactory, visual, mechanical, etc.). Calyx of the MB has variations in appearance among insect species: honey bees, ants, and cockroaches have two calyces on each MB, whereas fruit flies have only one calyx. The MB calyx in crickets appears to be a single structure, but when examined closely, it can be divided into two parts, i.e., anterior and posterior parts (Malaterre et al., 2002; Mashaly et al., 2008). The pedunculus, α lobes, and β lobes are the output sites of MBs, providing synaptic connection between the Kenyon cells and efferent neurons.

In Gryllus campetris, another field cricket species closely related to G. bimaculatus, electrical stimulation of MB induces the calling song, courtship song and the aggressive song (Huber, 1962; Otto, 1971). In honey bees, fruit flies, and the American cockroaches, MBs have been suggested to be an olfactory learning and memory center (Menzel and Giurfa, 2006; Davis, 2011; Watanabe et al., 2011). In two field crickets (G. bimaculatus and G. campetris), the Kenyon cells in MB continue to divide and proliferate even in adults (Cayre et al., 1994, 1996). When proliferation Kenyon cells is inhibited by γ radiation, the odor learning ability is reported to diminish (Scotto Lomassese et al., 2003).

Neurotransmitters including octopamine, dopamine, serotonin, GABA, acetylcholine, glutamic acid, and nitric oxide (NO) are known to be present in the MBs of the two-spotted cricket (Spörhase-Eichmann et al., 1992; Stevenson and Spörhase-Eichmann, 1995; Hörner, 1999; Schürmann, 2000; Schürmann et al., 2000, 2008). In two-spotted crickets and honey bees, it is suggested that octopamine is involved in positive reinforcement and dopamine in negative reinforcement (Unoki et al., 2005, 2006; Nakatani et al., 2009; Vergoz et al., 2007). In both crickets and honey bees, it has been shown that NO is involved in long-term memory formation (Matsumoto et al., 2006, 2009; Müller, 1996, 2000).

The central complex (CC)

The central complex (CC) is a densely packed neuropil in the center of the protocerebrum, consisting of central body, protocerebral bridge and nodules. In crickets, damage to the CC inhibits walking activities and responses to calling song; therefore, the CC is suggested to be involved in the inhibition of the motor system and mutual inhibition of the left and right brain hemispheres. In the bow-winged grasshopper (Chorthippus biguttulus) belonging to the same order orthoptera, the central complex is involved in generation of courtship song pattern (Hoffmann et al., 2007; Weinrich et al., 2008). In the desert locust (Schistocerca gregaria), the CC and the lateral accessory lobes have been shown to play role in the processing of orientation behavior based on polarized light compass (Homberg, 2004). In Drosophila, the CC has been shown to be related to place learning by visual cues (Ofstad et al., 2011).

The optic lobes: (OLs)

The optic lobes (OLs) protrude from both sides of the protocerebrum, directly connecting to the compound eyes. An OL of crickets comprises the lamina, medulla, and lobula. The lobula plate observed in flies and butterflies is not present in crickets. The OLs are known to be processing centers for visual information, and it is also the location of biological clock for circadian rhythms (Tomioka and Abdelsalam, 2004).


The antennal lobes (ALs)

The antennal lobes (ALs) are the most conspicuous neuropil in the deutocerebrum and are the primary processing centers for olfactory information. The ALs are composed of spherical unit structures called glomeruli. In crickets, each AL is composed of 49 glomeruli, which is relatively a small number compared with other insects. Each glomerulus consists of smaller glomeruli-like structures (micro glomeruli) (Ignell et al., 2001; Yoritsune and Aonuma, 2012).

The dorsal lobe (DL) and ventral area of flagellar afferents (VFA)

The dorsal lobe (DL) and the ventral area of flagellar afferents (VFA) are neuropils in the deutocerebrum. DL is situated posteriorly to the antennal lobe, while the VFA is situated ventrally to the antennal lobe and is composed of five layers. In crickets, the DL and VFA are morphologically distinct structures, but in other insects investigated so far, they are fused together into a single structure called the antennal mechanosensory and motor center (AMMC). The DL and VFA are both involved in information processing of tactile and air flow stimuli to the antennae (Staudacher and Schildberger, 1999).

The tritocerebrum

The tritocerebrum is involved in processing tactile information from the labrum and information on air flow from sensory hair on the head. Lobus glomerulatus, the glomeruli-like structures formed by afferent neurons from maxillary palp, are the only apparent unit structures observed in the tritocerebrum.


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