148 THE EARLY STAGES OF TABANID^ 



was three days and the maximum five days. Four days is probably 

 the average length of time required for incubation. It was ob- 

 served that the degree oi temperature and moisture influenced the 

 time of hatching. Slight changes in either of these factors can be 

 used to control the time of emergence from the egg. Mitzmain's 

 observations on the hatching of an egg mass are the following: 



"Twenty-two hours previous to the hatching of the embryo, certain unmistak- 

 able activities were discernible in the egg. The first signs of these were seen in 

 the two eggs which formed the nucleus for the egg mass and which are the first 

 eggs laid. These movements, as indicated by either of the dark eye spots, could 

 be seen with a hand lens at intervals of a few seconds; their action was similar 

 so that of the bubble in a spirit level. In about an hour the movement was 

 teen to be rather general in the egg mass, accompanied in the eggs first laid by 

 an alternate collapsing and distending of the exochorion. This action is the 

 result of the torpedo-like movement of the head capsule of the embryo in the di- 

 rection of the micropyle of the egg. The movement is effected by the piston-like 

 action of the apophyses of the cephalopharynx, which appear to work alternately, 

 bringing the saw-toothed mandibles in contact with the micropyle canal. These 

 movements proceeded uninterruptedly during the hours of the night, the only 

 change observable being that the body segments of the embryonic larva became 

 better defined. At 4.25 the next morning the segments of the embryo could 

 easily be counted through the chorion. The dorsal surface of the exochorion 

 was seen to be sHghtly shrivelled." 



"Fully one hour and thirty minutes intervened during which there was no 

 action worth noting. This quiescence was interrupted by a sudden remarkable 

 activity of all of the visible eggs of the mass. At 6.08 there was a general up- 

 heaval of the surface of the egg mass, an agitation within the eggs, and an alter- 

 nate collapsing and distending of the eggshells. At 6.10 the first layer of eggs 

 gave birth to a silvery horde of young larvae, which at 6.12 had crawled from 

 view [Plate 2, Fig. 27]. Then ensued another spasmodic agitation giving birth 

 to another lot of larvae, which crawled from the mass of empty eggshells [Plate 2, 

 Fig. 26]. The emergence which is effected by the head structures is aided by the 

 posterior protuberances, which functioning as prolegs push the body of the larva 

 clear of the eggshells." 



Mitzmain's discussion of the egg stage and of the hatching process 

 is followed by a description of the morphology and habits of the 

 young larvae. 



Immediately after emerging from the egg, the young larvae (Plate 

 5, Fig. 80, a, b) seek concealment. In nature, no doubt, resort would 

 be had to the convenient water course where aquatic plants, drift- 



