Ovarian development and parity determination in Culiseta melanura (Diptera: Culicidae)

A laboratory colony of Culiseta melanura (Coquillett) was used to follow ovarian development from emergence to oviposition and to validate the accuracy of using follicular dilatations to age grade females. We observed no change in the size of the primary follicles in unfed females from emergence to 3 d of age. Sugar feeding stimulated follicular growth and produced the following 3 types of ovarioles: (1) large primary follicles that eventually developed into functional ovarioles; (2) lesser numbers of small primary follicles that developed small amounts of yolk after blood feeding but degenerated in the latter stages of development; and (3) small primary follicles that did not develop yolk after blood feeding, exhibited accelerated growth, and became rogue ovarioles with multiple false dilatations. The yolk of the small primary follicles that degenerated after blood feeding was resorbed during stage IV of egg development, and the degenerating follicles resembled gonotrophic dilatations in the latter stages of the cycle. This process produced gravid females with some secondary follicles that appeared to possess a gonotrophic dilatation. Other authors have termed these artifacts false or agonotrophic dilatations. Degenerating ovarioles bearing these artifacts were used to determine physiological age in blood fed and gravid Cs. melanura. Nulliparous females in the latter stages of gonotrophic development have single false dilatations on degenerating ovarioles, 1-parous females have 2 false dilatations on degenerating ovarioles. In unfed, nulliparous females, false dilatations can be distinguished from true dilatations because they are attached to secondary follicles that are much smaller than the primary functional follicles that fill most of the ovary. In blood fed and gravid females, follicles that support false dilatations always lack yolk. Rogue ovarioles are unreliable indicators of physiological age and should not be used for diagnostic purposes.