12 PRINCIPAL HOUSEHOLD INSECTS. 



very firmly. After tliey liave hatched the mass will disintegrate in a 

 few days, even in perfectly still water. 



The individual eggs are 0.7 mm. in length and 0.16 mm. in diameter 

 at the base. They are slender, broader and blunt at bottom, slenderer 

 and somewhat pointed at tip. The tip is always dark grayish brown in 

 color, while the rest of the egg is dirty white. Repeated observations 

 show that the eggs hatch, under advantageous conditions, certainly as 

 soon as sixteen hours. Water buckets containing no egg masses, placed 

 out at night, were found to contain egg masses at 8 o'clock in the morning, 

 which, as above stated, were probably laid in the early morning, before 

 daylight. These eggs, the third week in May, began to hatch quite 

 regularly at 2 o'clock in the afternoon of the same day on m arm days. 

 In cooler weather they sometimes remained unhatched until the second 

 day. If we apply the evidence of European observers to this species, 

 the period of the egg state may be under twelve hours; but there is a 

 possibility that they are laid earlier in the night, which accounts for the 

 fact that sixteen hours is the shortest period which we can definitely 

 mention. 



The larvae issue from the underside of the egg masses, and are ex- 

 tremely active at birth. When first observed it is easy to fiill into an 

 error regarding the length of time which they can remain underwater, 

 or rather without coming to the surface to breathe, since, in striving to 

 come to the surface for air, many of them will strike the underside of 

 the egg mass and remain there for many minutes. It is altogether 

 likely, however, that they get air at this point through the eggs or 

 through the air film by which the egg mass is surrounded, and that 

 they are as readily drowned by continuous immersion as are the older 

 ones, as will be shown later. 



One of the first peculiarities which strikes one on observing these 

 newly hatched larviie under the lens is that the tufts of filaments which 

 are conspicuous at the mouth are in absolutely constant vibration. 

 This peculiarity, and the wriggling of the larvae through the water, and 

 their great activity, render them interesting objects of study. In gen- 

 eral, the larvae, passing through apparently three different stages, reach 

 maturity and transform to pupae in a minimum of seven days. When 

 nearly full grown their movements were studied with more care, as 

 they were easier to observe than when newly hatched. At this time 

 the larva remains near the surface of the water, with its respiratory 

 siphon at the exact surface and its mouth filaments in constant vibra- 

 tion, directing food into the mouth cavity. Occasionally the larva 

 descends to the bottom, but, though repeatedly timed, a healthy indi- 

 vidual was never seen to remain voluntarily below the surface more 

 than a minute. In ascending it comes up with an effort, with a series 

 of jerks and wrigglings with its tail. It descends without effort, but 

 ascends with difficulty; in other words, its specific gravity seems to be 

 greater than that of the water. As soon, however, as the respiratory 



