Follicle developmental stages in blood-seeking horseflies (Diptera: Tabanidae) in Alberta, Canada

The developmental stages of the terminal follicles in 1855 females of 11 species of Hybomitra collected in Manitoba traps and from round horses in Alberta were recorded. It is believed all the flies were seeking blood-meals. Between 76% and 97% of the nullipars in any one species had yolk in the terminal oocytes. The stage of follicle development in a parous fly varied with the fly’s physiological age and the physiological character (anautogenous or autogenous) of the species to which it belonged. Within every physiological age-class significantly more flies with "yellow bodies" than those in the sac-stage had yolk in the terminal oocytes. Significantly more 1-parous flies of anautogenous species than 1-parous females of autogenous species had yolk in the terminal oocytes. Flies that were 2-parous were relatively rare. It is suggested that some parous flies need more than one blood-meal to develop a batch of eggs.