The Clickable Brain

Olfaction plays an important role in the life of a fly, not only in sexual discrimination. The brain structure that underly olfaction are correspondly rather large. The first order neuropils are the antennal lobes (one on each side of the brain). They consist of about 35 glomeruli each, of which 30 are located in the periphery of the lobe and 5 in its center. Several prominant glomeruli are recognizable by their location, size and shape; others are identifiable only by their positions relative to prominant glomeruli. No obvious sexual dimorphism of the glomerular architecture is observed in Drosophila (quite in contrast to many other insects). However, recent experiments provide strong evidence that certain glomeruli are sexually difunctional.

Golgi impregnations revealed:

1. Five of the glomeruli are exclusive targets for ipsilateral antennal input, whereas all others receive afferents from both antennae. Unilateral amputation of the third antennal segment led to a loss of about 1000 fibers in the antennal commissure. Hence, about 5/6 of the approximately 1200 antennal afferents per side have a process that extends into the contralateral lobe.

2. The maxillary palps carry - like the third segment of the antenna olfactory sensilla of the basiconic type. Their afferents project into both ipsi- and (via the antennal commissure) contralateral antennal lobes, yet their target glomeruli are not the same as those of the antennal basiconic sensilla.

3. Afferents in the antennal lobe may also stem from pharyngeal sensilla.

4. The most prominant types of interneurons with arborizations in the antennal lobe are:
   a) local interneurons ramifying in the entire lobe

   b) unilateral relay interneurons that extend from single glomeruli via the inner antenno-cerebral tract into the calyx and the lateral protocerebrum (LPR)

   c) unilateral interneurons that connect several glomeruli with the LPR only

   d) bilateral interneurons that link a small number of glomeruli in both antennal lobes with the calyx and LPR

   e) giant bilateral interneurons characterized by extensive ramifications in both antennal lobes and in the posterior brain and a cell body situated in the midline of the suboesophageal ganglion

   f) a unilateral interneuron with extensive arborization in one antennal lobe and in the posterior brain and a process that extends into the thorax.

Stocker R.F., Lienhard M.C., Borst A., and Fischbach K.-F. (1990). Neuronal architecture of the antennal lobe in Drosophila melanogaster. Cell Tissue Res 262, 9-34