This report analyses the role of neuronal activity in shaping the axon
al arborizations of sensory neurons from individually identified filif
orm hairs on the prosternum of locusts (Locusta migratoria), and their
connections with a pair of identified interneurons (A4l1). Afferents
from lateral filiform hairs terminate in the ipsilateral neuropil and
connect only with the ipsilateral interneuron in all instars. Afferent
s from ventral filiform hairs possess ipsi- and contralateral branches
and make monosynaptic connections with both interneurons in first ins
tars. In later instars, the ipsilateral branch, and its synaptic conne
ction to the ipsilateral interneuron, is gradually reduced until it is
lost in the adult, whereas the contralateral branch, and its synaptic
connection with the contralateral interneuron, is strengthened. There
fore, after an initial overgrowth of fibers and synapses, segregation
of fibers occurs involving the loss of synaptic connections. This loss
of branches and synapses was prevented by immobilizing a subpopulatio
n of ventral and lateral filiform hairs, or each group independently,
so that their normal activity was blocked. In such treated animals aff
erents from ventral filiform hairs retain their ipsi- and contralatera
l branches until adulthood. We therefore conclude that afferent activi
ty plays an important role in shaping the final structure and connecti
vity of afferents, as neither the peripheral position of the receptors
nor the hormonal environment was changed by these manipulations.