The mushroom bodies of the honeybee are important neuropils for learning and memory. Therefore, knowledge about their input and output connections is essential to understanding how these neuropils function. A newly described input tract to the mushroom body is presented here, which is called the subesophageal-calycal tract (SCT) and connects the subesophageal ganglion with the calyces of the mushroom bodies. The neuronal somata of the SCT neurons lie in one cluster between the lobula of the optic lobe and a neuropil area that is formed from the fusion of the tritocerebrum and the subesophageal ganglion. Within the subesophageal ganglion, the dendritic fibers of SCT neurons overlap with terminals of sensory neurons from the proboscis. Therefore, we conclude that the SCT neurons might process gustatory and mechanosensory information from the proboscis. Individual SCT neurons receive unilateral input within the subesophageal ganglion and may connect to either the ipsilateral or the contralateral mushroom body. On their way to the mushroom bodies, the SCT neuron axons meet the roots of the antennocerebralis tracts (ACTs) and from this point follow the same path as the median ACT neurons for a short distance. Within the calyces, the SCT neurons innervate two separate areas, a small area within the dorsal collar just below the lip and a part of the basal ring. Double-labeling experiments show that the projections of the SCT neurons do not overlap with the projections of the olfactory projection neurons and visual projection neurons from the dorsal medulla. The possible function of the SCT neurons and the relation of the SCT to known input tracts of the mushroom bodies in other insects are discussed.

In honeybees (Apis mellifera), the biogenic amine octopamine has been shown to play a role in associative and non-associative learning and in the division of labour in the hive. Immunohistochemical studies indicate that the ventral unpaired median (VUM) neurones in the suboesophageal ganglion (SOG) are putatively octopaminergic and therefore might be involved in the octopaminergic modulation of behaviour. In contrast to our knowledge about the behavioural effects of octopamine, only one neurone (VUMmx1) has been related to a behavioural effect (the reward function during olfactory learning). In this study, we have investigated suboesophageal VUM neurones with fluorescent dye-tracing techniques and intracellular recordings combined with intracellular staining. Ten different VUM neurones have been found including six VUM neurones innervating neuropile regions of the brain and the SOG exclusively (central VUM neurones) and four VUM neurones with axons in peripheral nerves (peripheral VUM neurones). The central VUM neurones innervate the antennal lobes, the protocerebral lobes (including the lateral horn) and the mushroom body calyces. Of these, a novel mandibular VUM neurone, VUMmd1, exhibits the same branching pattern in the brain as VUMmx1 and responds to sucrose and odours in a similar way. The peripheral VUM neurones innervate the antennal and the mandibular nerves. In addition, we describe one labial unpaired median neurone with a dorsal cell body, DUMlb1. The possible homology between the honeybee VUM neurones and the unpaired median neurones in other insects is discussed.

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We report some of the neural and muscular circuitry that allows honeybees to control head movements. We studied neck motor neurons with cell bodies in the suboesophageal ganglion, axons in the first cervical nerve (IK1) and terminals in neck muscles 44 and 51 (muscle classification: Snodgrass in Smithsonian Misc Coll 103:1-120, 1942). We show that muscle 44 actually comprises five separate bundles of muscle fibres (subunits), while muscle 51 is split into two subunits. Eight motor neurons innervate muscles 44 and 51. Two motor neurons have cell bodies in the ventral-median cell body group (one innervates a subunit in muscle 44, the other a subunit in muscle 51). One motor neuron has a ventrally located contralateral cell body (innervating a subunit in muscle 44) and five have laterally located ipsilateral cell bodies. Of the five lateral cells, one innervates a subunit in muscle 51, three selectively innervate subunits in muscle 44 and one co-innervates a subunit in muscle 44 with the contralateral cell. Extracellular recordings revealed three types of visually driven, direction-selective cell-types in each IK1 tuned for leftward, rightward and downward motion over the eyes. The spatiotemporal tuning of the units is similar to that of other visual interneurons in the bee brain.