Changes in midgut ultrastructure and surface tegument were examined in two laboratory-reared populations of unfed, adult, lone star ticks. Many solitary, electron-dense, iron-containing siderosomes were present in the midgut epithelium at 0-mo. Siderosomes became progressively smaller in the aging epithelium. The midgut epithelium was devoid of large, lysed siderosomes by 6 mo. Glycogen was plentiful at 6 mo. Nineand 12-mo ticks had numerous myelinosiderosomes, structures that contain myelin figures and electron-dense, iron-containing particles produced by the lysosomal degradation of hemoglobin. The cervical groove ratio (anterior margin of the scutellum: cervical groove length) decreased with age in the unfed female from 4.057 to 2.357 at 0 mo and 12 mo, respectively. The cervical groove in the unfed male did not change with age. We propose that changes in the midgut epithelium and cervical groove ratio may be used as indicators of physiological age of laboratory-reared, unfed, adult, lone star ticks. The unfed, adult, lone star tick, Amblyomma americanum (L.), has been observed to live up to 3 yr in the field without taking a blood meal. Ticks differ from other arthropods in that the digestion is mostly intracellular (Balashov, 1972). Such digestion is a unique phenomenon allowing the tick to store and utilize nutrients within the midgut cells. The slow depletion of food stores over time is important to the ticks' continued survival and likely a key reason for their ability to survive long periods without feeding. Balashov (1972) has suggested that histological changes in the midgut epithelium (hemoglobin, hematin, and glycogen) might be reliable indicators of tick age. Until recently, little work has been done on the midgut ultrastructure of ixodid ticks. Raikhel (1978) has described much of the ultrastructure of the ixodid tick midgut, but did not elucidate age-related changes in the unfed adult. The present investigation sought to determine changes in the midgut ultrastructure and surface tegument of laboratory-reared lone star ticks to gain insights into unique characteristics of intracellular digestion and to determine morphological features that could be used to estimate the chronological age of ixodid ticks. Received 30 July 1982; revised 24 January 1983; accepted 27 May 1983. * Department of Physiological Sciences, Oklahoma State University, Stillwater, Oklahoma 74078. MATERIALS AND METHODS