ALBERT

Notes: Summary 1. The cerci of locusts are paired, unsegmented, cone-shaped structures arising from a depression on either side of the tip of the abdomen (Fig. 2). 2. The cerci bear hair sensilla of two basic types (Fig. 3): (a) filiform hairs, which emerge from a cuticular pit, are flexibly mounted, and are sensitive to wind and to mechanical displacement; and (b) bristle (or trichoid) hairs, which originate directly from the surface of the cercus, and are less flexibly mounted. The axons of neurones innervating these hairs group together into successively larger bundles before joining the cercal nerve which runs to the terminal ganglion (Fig. 4). 3. In the terminal ganglion the majority of cercal afferents (Fig. 5), and all identified filiform afferents (Fig. 6) end in contact with a cercal glomerulus formed by the densely interwoven arborizations of giant and non-giant interneurones (Figs. 5, 7,9). 4. A transverse section of the ventral nerve cord anterior to the terminal ganglion reveals four axons, one medial and three grouped together dorsolaterally, with distinctly larger profiles than all the others (Figs. 8, 9). The somata associated with these axons are located contralaterally in the terminal ganglion. All four interneurones have at least some projections into both cercal glomeruli. The soma of giant interneurone 1 (GIN 1), which is associated with the large medial axon, lies laterally in neuromere 9, the somata of GINs 2–4 are located ventrally in neuromere 9, ventrally in neuromere 10, and postero-dorsally in neuromere 11, respectively (Figs. 7,9). 5. All four GINs run the length of the ventral nerve cord (Figs. 10, 11, 12) and end in the brain (Fig. 11). The axon of GIN 1 is found in the ventral intermediate tract (VIT), those of GINs 2, 3, 4 are initially in the lateral dorsal tract (LDT) but they cross to the dorsal intermediate tract (DIT) at the level of the most anterior free abdominal ganglion. All have medial branches in the abdominal ganglia (Figs. 10, 12). GIN 1 has a lateral branch (sometimes absent) only in the metathoracic ganglion, while the other GINs have lateral branches in all of the thoracic ganglia (Figs. 10, 11, 12). 6. The cercal receptor/giant interneurone system ofLocusta is compared to those of other orthopteroid insects, with special attention to the origin and evolution of giant interneurones.