204 Fiftieth Keport ox the State Museum 



stubble, and even under the bark of adjacent posts for the first few days, 

 or they may simply rest at full length along a well-shaded leaf. Their 

 habit of dropping upon the slightest disturbance, renders their detection 

 more difhcult. During their first week, they eat only of the lower 

 epidermis of the leaf in a manner similar to young Crambid larvae — at 

 least such was the habit of those reared the past season. In about a 

 week they begin to eat holes in the sides of the leaves, and thereafter 

 their appetites develop rapidly. There is considerable difference in the 

 growth of the larvae even from the same mass of eggs and under almost 

 identical conditions, some being nearly a molt in advance of the others. 

 This same difference is the more marked in caterpillars growing under 

 natural conditions in the field, where variation is the rule and not the 

 exception. The abundance and the condition of the food has a great 

 influence on the rapidity of their growth, for if abundant and succulent 

 it will be most rapid, while if dry it will be much slower. The parent 

 moth apparently seeks to give her offspring the best conditions when she 

 searches out the thickest and greenest herbage in which to place her 

 eggs, and in most cases it is in just such spots that the destructive 

 armies have their origin. They are really centers of distribution, and 

 should be so regarded. 



Migrations. — The earlier stages of the army-worm escape observation 

 in most instances, and it is only when they are unusually abundant and 

 after they have become half- grown that they attract attention. It is not 

 until then, and after all the food has been devoured in their immediate 

 vicinity, that they are noticed. The caterpillars are now forced to move 

 elsewhere or starve. In ordinary years this rarely occurs, for they 

 are not sufhciently abundant to work any serious injury, unless it be 

 a slight thinning of the crop. It will be seen, therefore, that the 

 " marching" habit is abnormal, although it may be the one most familiar 

 to many. The uniform movement of the caterpillars in the same direc- 

 tion may be explained as the most natural, because it is the easiest when 

 they are abundant, for otherwise their opposing motions would be a 

 hindrance to one another. The determination of the direction of the 

 march is probably the result of chance to a great extent, and is governed 

 largely by the direction taken by the first to move, although some are in- 

 clined to think that the insects march more frequently toward a certain 

 point of the compass, and others believe that they scent a favorite crop 

 in the distance. The food of the caterpillars is so abundant that it hardly 

 seems necessary to suppose that they are guided to it by a special 

 sense, and it is equally difficult to see how a knowledge of the points of 

 the compass would aid materially in such a search. 



