NORMAL GLOMERULAR ORGANIZATION OF THE ANTENNAL LOBES IS NOT NECESSARYFOR ODOR-MODULATED FLIGHT IN FEMALE MOTHS

A prominent hypothesis for the function of the glomerular structures i n the primary olfactory neuropil of many groups of vertebrate and inve rtebrate animals is that they enable the processing and coding of info rmation about the chemical compounds that compose complex odors. Previ ous studies have indicated that various degrees of glomerulus formatio n in the antennal lobes of the brain of the moth Manduca sexta can be effected by reducing the number of olfactory sensory axons that grow f rom the antenna into the antennal lobe during metamorphosis. To test t he hypothesis that the presence of glomerular structure is necessary t o process and identify odors, we substantially reduced, by surgery, th e number of antennal segments in developing moths and upon metamorphos is we observed and quantified behavioral responses known to be elicite d by odors. Intact and lesioned adult female moths were challenged to fly upwind to the source of an attractive host-plant odor in a wind tu nnel. Some of the moths that had developed with reduced olfactory inpu t flew upwind to the odor source. The flight behavior of these individ uals was similar to the odor-mediated flight typically observed in mot hs that had developed normally. Histological analysis of the moths' an tennal lobes revealed that the lobes of more than half of the responde nts that had been lesioned during development lacked normal glomerular organization. The neuropil of these abnormally developed antennal lob es was mostly aglomerular, but with a few isolated, clearly abnormal g lomerulus-like structures. This suggests either that even a few abnorm al glomeruli are sufficient to mediate this specific behavior or that ''canonical'' glomerular organization per se is not necessary for this odor-mediated behavior.