*Egg pods of overwintering embryos oi D. pulex and D. middendorffiana were 



collected on 15 June 1971 and were incubated in glass tubes (25 x 200 mm), 1 



five pods per tube. Racks of tubes were held at ambient temperature and i 

 natural light levels of Near Ditch. 



hatching interval and 95% complete in the first 36 hours. Under the same ! 



conditions, the D. middendorffiana hatch was more spread out with about | 



50% completion in the first 24 hours and near 100% by 72 hours. The daily | 



patterns of hatching were also different for the two species. Thus, D. pulex i 



hatching took place mostly in the warm half of the first day (the pond j 

 water reached maximum temperature at 1800 hours) and the pattern 



repeated on the second day. In contrast, D. middendorffiana emerged at i 



approximately a constant rate over the entire 24 hours. The same pattern j 



for D. middendorffiana was seen in the embryos held under constant light I 



and temperature. , 



Later in the same summer, there was a similar synchrony in the time j 



of appearance of non-diapausing embryos in the brood pouches of the ] 



females. Hatching of the embryos was likewise synchronous (Figure 6-11). ] 



For example, hatching was mostly complete within 48 hours in Pond B and I 



