Both FoxP^del homozygotes and heterozygotes walk much lower distances than wild-type controls, with homozygotes showing a more severe phenotype. Homozygotes have significantly lower social distances (i.e average distance to the closest neighbor) than wild-type controls and do not show the expected jump reflex to a light pulse stimulus. Although the brain of FoxP^del homozygotes does not show gross morphological abnormalities, the tips of mushroom body α-lobes are significantly smaller, as compared to wild-type controls.

FoxP^del/FoxP^5-SZ-3955 transheterozygotes show a significant decrease in light-off jump habituation, as they show a considerably higher jump response during the entire course of the assay, as compared to controls.

Third instar larval neuromuscular junctions of FoxP^del homozygotes shows postsynaptic defects, but an apparently normal presynaptic side. The postsynaptic side is significantly enlarged, exhibits an enlarged subsynaptic reticulum that is less dense but is occupied by a significantly larger area of non-tubular structures, and exhibits mitochondrial defects (i.e. defective structure and cristae; mitochondria are frequently fused and surrounded by layers of membranes, forming structures that could be autophagosomes), as compared to controls. The presynaptic side shows no changes in size, no defects in the numbers of boutons or active zones, no defects in the presynaptic microtubule scaffold (i.e. futsch immunostaining), and no defects in mitochondria morphology, as compared to controls.

Third instar larval class IV da neurons of FoxP^del homozygotes have significantly smaller dendritic field areas and decreased average dendritic length, despite of insignificant changes in total dendritic length and in the number of dendritic endings, as compared to controls.