THE BAG-WORM. 25 
The larvz are poor travelers during growth, and though, when in 
great numbers, they must often wander from one branch to another, they 
rarely leave the tree upon which they were born unless compelled to do 
so by hunger through the defoliation of the tree. When full-grown, 
however, they develop a greater activity, especially when very numer- 
ous, and, letting themselves down by a fine silken thread, travel fast 
enough across sidewalks or streets and often for a considerable distance 
until they reach another tree, which they ascend. This migratory desire 
is instinctive; for should the worms remain on the same tree they would 
become so numerous as to necessarily perish for want of food. 
Pupation.—The bags of the worms which are to produce male moths 
attain rather more than an inch in length, while those which produce fe- 
males attain nearly doublethissize. When ready to transform, the larva 
firmly secures the anterior end of the bags to a twigor branch, and in- 
stinct leads it to reject for this purpose any deciduous leaf or leaf-stem 
with which it would be blown down by the winds. The inside of the 
bagis then strengthened with an additional lining of silk, and the change 
to chrysalis is made with their heads always downward. The chrysalis 
is of a dark-brown color, that of the male (Fig. 7, 6) being only half the 
size of that of the female (Fig. 7, e and Fig. 9, a). 
The Imago or perfect Insect.—After a lapse of about three weeks from 
pupation a still greater difference between the two sexes becomes ap- 
parent. The male chrysalis works its way to the lower end of the bag 
Fic. 9.—Thyridopteryx ephemerceformis : 
a, Follicle ent open to show the manner in 
which the female works from her puparium 
and reaches the end of the bag, natural 
size; b, female extracted from her case, 
enlarged. 
and half way out of the opening at the extremity. Then its skin bursts 
and the imago appears as a winged moth with a black, hairy body and 
glassy wings (Fig. 7, d). It is swift of flight, and, owing to its small 
