Glial cells associated with elements of central neuropils in several insect species were studied with conventional light and electron microscopical techniques, the Golgi procedure, and a combination of the latter with electron microscopy. Different types of cells located in the layer of cells covering the neuropil were found to send complex arborizations into synaptic neuropils. These arborizations grow in clusters that seem to represent discrete compartments circumscribing groups of synaptic terminals. The thinnest glial processes are found deep in the neuropil and consist of compact membrane leaflets lacking cell organelles and with reduced amounts of cytoplasmic matrix. Some of these glial processes also invest neuropil tracheoles in a manner reminiscent of the association between astrocyte end-feet and blood capillaries in the central nervous system of mammals. Other glial cells reside completely in the neuropil, where they enwrap fiber bundles in a similar manner to oligodendrocytes in the central nervous system of mammals.