NO. 1788. NORTH AMERICAN ERGASILID^— WILSON. 315 



Subsequent observations, as well as the anatomy of the sexual 

 organs already given (p. 301), have only served to confirm Sars's 

 assumptions, but they still lack absolute proof. 



However, it is reasonably certain that after the spermatophores 

 are once fastened to the genital segment of the female, and their con- 

 tents have passed into the semen receptacles, a sufficient amount is 

 thus stored to fertilize all the eggs which that female can produce 

 during her after life (note size of semen receptacle, figs. 15 and 16). 



Breeding seasons. — Like the Caligidse, the Ergasilidse have no 

 regular breeding season; females with egg-strings in different stages 

 of development are found on the same host with those which have 

 no egg-strings at all. But there seem to be three periods when 

 females with fully ripe eggs are more common than at other times. 

 These periods are the months of April — May, July — August, and 

 October — November, in each instance including the last two weeks 

 of the first month and the first two weeks of the second. During 

 these times the examination of a few hosts is fairly sure to yield one 

 or more females whose egg-strings are about ready to hatch. The 

 most of the observations of the present author were made at Lake 

 Maxinkuckee, Indiana, during the summers of 1906 and 1908 while 

 in the employ of the U. S. Bureau of Fisheries. Females with ripe 

 eggs began to be found in the middle of July, became more common 

 by the last of the month, and then gradually diminished until by the 

 middle of August they were extremely rare. 



The intervals between these breeding seasons, eight or nine weeks, 

 may therefore be taken to represent approximately the time of 

 incubation plus the interval of rest which intervenes after each brood 

 has been hatched. 



Fertilization and extrusion of the eggs. — During the winter months 

 from December to March these parasites appear to remain dormant, 

 so far as any sexual activity is concerned. 



When captured during this period the females are without egg- 

 strings, and, as Nordmann states, the ovaries are usually empty, 

 leaving the body more transparent than usual. The same thing 

 happens sometimes between the breeding seasons in summer. But 

 with the advent of warmer weather the ovaries quickly fill and the 

 eggs are soon ready to be extruded. 



As in the Caligidae, each egg is fertilized when it passes out of the 

 oviduct into the external sack. Tlie latter is secreted only as there 

 is a demand for more space and is extended by the pressure of the 

 issuing eggs. But this pressure is not exerted along the axis of the 

 sack, as in the Caligidae, and hence the eggs are not flattened at right 

 ano;les to that axis. The reason for this is a mechanical one and is 

 to be found in the structure and arrangement of the ovary and ovi- 

 duct, and in the muscular control of the external openings of the 

 latter. 



