The structure of the unpaired ventral nerves in the blowfly larva

The anatomy and histology of the unpaired ventral nerves are described from lightand electron-microscopical studies. Seven unpaired nerves arise from the median dorsal surface of the ventral ganglionic nerve-mass, and terminate in abdominal segments 2 to 8 respectively. Each unpaired nerve bifurcates, sending a lateral nerve to both sides of its respective segment. Subsequently the lateral nerve gives rise to 2 nerve-branches. The first branch runs to a multipolar neurone that is associated with the tracheal system, and the second branch passes beneath the ventral longitudinal muscles and terminates on a ventral transverse muscle. Structurally, the unpaired nerve consists of axons ensheathed by Schwann-cells, the cytoplasmic processes of 2 giant Schwann-cells forming the entire neurilemma sheath of the nerve and its branches. A basement membrane, or connective-tissue sheath, about 2 n thick surrounds the Schwann-cells. Four nerve-fibres are found in the unpaired nerve; 2 of these bifurcate, sending a fibre into both lateral nerves, but the other 2 fibres pass without dividing, into the left and right lateral nerves respectively, making a total of 3 fibres in both lateral nerves. One fibre from the lateral nerve enters the nervebranch leading to the multipolar neurone associated with the tracheal system, and eventually joins its cell-body. This fibre is the axon of the neurone. The remaining 2 fibres in the lateral nerve pass into the nerve-branch that runs to the ventral transverse muscle. A comparison is made between the unpaired nerves of the blowfly larva and those in other insects.