NATURAL^HISTORY OF AMIA CALVA LlNNiEXTS. 99 



At this time the larvae do not try to escape the net, and the greater part of a school 

 may often be taken with a single sweep. 



At precisely what stage in their development the larvae begin to react to mechan- 

 ical shock in the water I do not know. Such reaction is present in larvae about 40 

 millimetres long, but is perhaps also present in younger larvae. 



d. Second period of larval history (35 to 100 millimetres). — The movements of 

 schools of larvae of this size are very rapid and increase in rapidity with age. In 

 one case a school of larvae 40 millimetres long was found crossing from one side of 

 the pond to the other through water 3 or 4 metres deep. The male could not be 

 seen, but was probably at the bottom. The larvae were in a vertical column formed 

 by the large numbers which came frequently to the surface for air and then returned 

 again toward the bottom. In a few minutes the school had reached the shore a 

 distance of about 150 metres. Schools of larvae of 70 to 100 millimetres are not 

 often seen, but when seen are moving with great rapidity, sometimes, it seemed to 

 me, as rapidly as .5 metre to 1 metre per second. 



The reaction toward light of the larvae at this period is the same as in the 

 younger larvae, but more pronounced. The schools are now rarely seen in the open, 

 but rather moving rapidly in the dense shade of the aquatic vegetation or beneath 

 overhanging bushes. The schools consist of fewer individuals, and these are less 

 conspicuous and farther apart. This, added to their rapid movements and habit 

 of keeping in the shade, makes them very hard to find. To the practised eye they 

 often reveal themselves by their habit of coming to the surface for air. 



During the second period the larvae show another light reaction, which is not 

 present in the first period. Larvae of 70 millimetres which are kept in a darkened 

 dish have the tail-spot and stripes in the head and fin black and the green of the 

 upper part of the body and head almost black (PL VII, Fig. 3). Upon exposure 

 to strong light all this black becomes very much less intense. All the stripes on the 

 head and fins fade and the whole upper part of the body and head becomes a light 

 green. The whole color impression made by the fish is then changed so that the 

 light larva seen under strong illumination is much less conspicuous than the dark 

 larva. 



I have not attempted to determine the length of time required for this change 

 to take place. It is complete at the end of an hour, but is probably completed in 

 a much shorter time. Its protective character is obvious since it adapts the color 

 of the larvae to the amount of light falling on them, and consequently in a measure 

 to their environment. 



Larvae of 40 millimetres flee from a stick thrust into the school, and at 60 milli- 



