4 RICE-PEST OF BRITISH BURMA. 



PyralidcB. Other Pyralidce and Crambidce, it is true, have 

 similar aquatic habits, as, for example, those of the genera 

 Hydrocampa, Palustra, Pliilampelits, Aceiitropus, but in 

 none of them are tracheal-gills present ; the explanation 

 of the absence of these structures probably being that 

 the species of the genera mentioned have only compara- 

 tively recently passed from an aerial to an aquatic mode 

 of life, and that the action of the new environment 

 upon them has not been sufficiently prolonged to work 

 those adaptive structural changes it has brought about 

 in the European, Brazilian, and Burmese insects. 



The caterpillars of the European and Brazilian forms 

 reside, the one beneath a rude shelter formed by itself 

 out of a small piece of the food-plant fastened by silk 

 threads to a leaf, the other beneath a much more com- 

 plex tent-like silken structure of its own spinning. Both, 

 in correlation with this habit, have the body more or less 

 depressed, with the tracheal-gill tufts more or less 

 lateral and horizontally extended and less numerous ; the 

 Brazilian form having the body much the more depressed 

 of the two, and the tracheal-gill tufts much the more lateral 

 and horizontal, much the less numerous, and all reduced 

 to simple unbranched filaments. In the Burmese form, 

 on the contrary, the body of the caterpillar has the typi- 

 cal (subcylindrical) shape of non-aquatic members of the 

 same family, and the innermost series of tufts higher 

 up on the back and more vertical than in the European 

 species, to which in form and structure, as in distributional 

 area, it comes nearest, and from which it differs in matters 

 of detail so trifling as to be at most of specific value ; 

 whence it may, with tolerable confidence, be inferred that 

 the Burmese form does not possess the shelter-building 

 instinct (which, by the way, is not yet quite perfectly 

 fixed in the European form), but that it crawls free and 

 uncovered in the midst of the water over the submerged 



