Meiosis is described in males of a lepidopteran species, Phragmatobia fuliginosa (Arctiidae), using transmission electron microscopy of ultrathin sections. Emphasis is on the behavior of spindle-associated membranes. Chromosome segregation is regular in both generations of spermatocytes. The spindles are unusual, however, in comparison to those of higher eukaryotes. Spindles in prometaphase to early telophase are surrounded by stacks of membranous cisternae. In late telophase, when reformation of the nuclear envelope around the daughter nuclei is completed, the membranous layers around the spindle fragment. In addition, the intraspindle membrane system is remarkable. The nuclear lumen is devoid of membranes in prophase I and the spindle area gradually fills with irregular membrane elements throughout prometaphase I. In metaphase I, some intraspindle membranes form tubules scattered throughout the spindle area. In contrast, in metaphase II, the intraspindle membranes mainly consist of relatively regular long tubules aligned parallel to the spindle axis. This arrangement offers itself for a closer analysis aimed at the question whether the intraspindle membranes form a continuum. This information is relevant in the context of intraspindle membrane function. The presence of a continuum surrounded by intraspindle membranes would lend credence to the hypothesis that intraspindle membranes-in analogy to the sarcoplasmic reticulum of muscles-play a part in the control of spindle function via sequestration and release of Ca2+ ions. The analysis of a series of cross sections through a metaphase II spindle at high resolution revealed numerous interconnections between individual tubules of the intraspindle membrane system. We assume, therefore, that the intraspindle membranes surround a continuous space. Although less cogent, the observations also suggest that the intraspindle membranes form links with the perispindle membrane layers. (C) 1995 Academic Press, Inc.