1916] Biology of Aquatic Lepidoptera 175 



Possibly, cutaneous respiration continues to be operative in the 

 later instars. 



Dissemination. — The larvae have several methods of dispersal ; 



(1) Since N. americana grows in beds in which the leaves are very frequently- 



contiguous or overlapping, the larvae hatching from a single egg mass 

 may, by crawling, scatter over a number of leaves. The older larvae 

 are more efficient at crawling and may ultimately get a considerable 

 distance from the original food plant. 



(2) Larvae may propel themselves in detached cases from one food plant 



to another as already described. 



(3) Wind, wave action, and currents are sometimes very effective in scatter- 



ing detached cases containing larvae. 



(4) Water-lily leaves, broken from the petioles by wave action or loosened 



by the attacks of certain species of insects, frequently bear the attached 

 cases of N. mactilalis and such leaves float about from place to place 

 at the will of the waves and currents. This form of dispersal was very 

 common in some of the water-lily beds about Douglas Lake, where the 

 plants were badly affected by the larvae of Hydromyza confluens (Welch, 

 '14a, pp. 139-140) which cause the petiole to break under very slight 

 side to side strains, and by the larvae of Belliira melanopyga (Welch, 

 '14b, p. 104) which sometimes sever the leaf from the petiole near the 

 upper end of the latter. 



Pupa (PI. VIII, Figs. 6-7). 



Forbes ('10, p. 222) gives the following brief description of 

 the pupa: ''Pupa similar in general form to that of obscuralis, 

 as described by Dyar, but of the seven ridges near the tip of the 

 abdomen beneath, only the central one remains, and the anal 

 opening is not distinctly Y-shaped. The case for the hind legs 

 varies considerably in length." Measurements of a large num- 

 ber of pupae showed that there is some variation in size. The 

 length, exclusive of the spike-like setae on the vertex, varies 

 from 10 mm. to 14.5 mm., average 12.6 mm. The maximum 

 diameter is in the region of the second to the fourth abdominal 

 segments and, in the specimens examined, the average is 3.4 

 mm., the extremes being 2.0 mm. and 4.0 mm. The color is 

 uniformly light yellow, except in specimens about ready to 

 transform in which the developing colors of the adult begin to 

 show through the pupal integument. Pupae preserved in alcohol 

 lose the yellowish appearance and become whitish. The body 

 is rather soft, smooth throughout, naked, and semi-opaque. 

 The anterior region tapers slightly cephalad while the posterior 

 region tapers distinctly caudad. The abdomen is bent so that 

 the ventral surface of the body is nearly plane while the dorsal 

 surface is distinctly convex. 



