The formation, structure, and fate of paracrystalline endoplasmic reticulum (pcER) is described in male germ cells of the red firebug, Pyrrhocoris apterus (Pyrrhocoridae, Hemiptera, Insecta), using transmission electron microscopy of ultrathin sections. The pcER forms in spermatogonia close to cytoplasmic bridges interconnecting the cells. Compact aggregates of ER elements are visible there. The membrane masses appear very irregular and consist of anastomosing elements of agranular ER with an electron-dense lumen. Toward the end of spermatogonial divisions, the first signs of pcER formation are detectable in the form of circular and semicircular membrane profiles. In spermatocytes the pcER is fully developed. Most obvious in ultrathin sections through pcER blocks are circular, undulating, and almost straight membrane arrays. In advanced spermatids, pcER is missing. In order to determine whether pcER is widespread in Hemiptera, oocytes, spermatogonia, and spermatocytes of the cotton stainer, Dysdercus intermedius (Pyrrhocoridae), were also studied. Whereas oocytes and spermatogonia possessed pcER, it was absent from spermatocytes and spermatids. There were no hints to the function of the pcER, but the occurrence in germ cells of representatives of the Pyrrhocoridae renders them convenient systems for further experimental approaches. (C) 1995 Academic Press, Inc.