1916] Biology of Aquatic Lepidoptera 173 



The effect of case-making upon the food plant is frequently 

 serious. Some of the yellow water-lily beds {N. americana) 

 about Douglas Lake are heavily infested at times with the 

 larvae of N. maculalis and suffer greatly (Figs. 12-19). The 

 total effect of the larvae on the food plant includes the amount 

 of plant tissue consumed as food and the plant tissue utilized in 

 case construction. According to the observations of the writer, 

 the plant suffers much more from the case-making than from 

 the removal of tissue for food. Case construction results in a 

 reduction of the leaf surface which may be extensive enough to 

 leave only the midrib. The writer observed beds of N. amer- 

 icana, in August, which, as nearly as could be estimated, had 

 lost 40 per cent, of the total leaf surface by the case-making 

 activities of these larvae. 



Food. — Very young larvae, reared in shallow aquaria, fed on 

 the lower side of the yellow water-lily leaf, feeding and case- 

 making being accomplished at the same time. The translucency 

 ■of the body made it possible to observe the first occurrence of 

 green plant tissue in the digestive tract. After the case was 

 made, the tiny larva fed to some extent upon the tissue of the 

 case. However, the normal field habits of the larva in the first 

 instar were not determined. In rearing young larvae in aquaria, 

 some difficulty was experienced in securing the second instar 

 and in preventing a very high mortahty among those which 

 hatched from egg masses. The following circumstantial evi- 

 dence suggests that possibly the larvae, after hatching, settle to 

 the bottom and after one or two ecdyses, approach the surface 

 of the water on the petioles of the water-Hly : 



(1) Recently hatched larvae have a tendency, in the aquaria, to drop to the 



bottom and to wander about. 



(2) The first and second instars were not found on the floating leaves of the 



yellow water-lily where the egg masses occurred. 



(3) It was frequently observed that in water-lily beds in which the floating 



leaves had petioles two or more feet long, the submerged leaves bore 

 young larvag (third instar or a little later) while the floating leaves 

 bore only the more advanced larvae. Some of the submerged leaves 

 had petioles only about one inch long, so that the leaf was practically on 

 the bottom. 



(4) Very rarely did pupse occur on the submerged leaves. 



Larvce, hatching in very shallow aquaria, ate the yellow 

 water-lily leaf tissue and some developed into the second instar, 

 but it is possible that it was not the normal reaction and that 

 the first instar may be passed on the bottom. In later instars, 



