THE BAG-WORM. 25 
pieces of twigs. When kept in captivity the worms are very fond of 
using bits of cork, straw, or paper, if such are offered to them. When 
the bags, with the growth of the larva, get large and heavy, they are no 
longer carried, but allowed to hang down (Fig. 7f). The worms un 
dergo four molts, and at each of these periods they close up the mouth 
of their bags to remain within until they have cast their skin and re- 
covered from this effort. The old skin, as well as the excrement, is 
pushed out through a passage which is kept open by the worms at the 
extremity of the bag. ° 
The young larva is of a nearly uniform brown color, but when more 
full-grown that portion of the body which is covered by the bag is soft, 
of light-brown color and reddish on the sides, while the head and tho- 
racic joints are horny and mottled with dark-brown and white (Fig. 7a). 
The numerous hooks with which the small, fleshy prolegs on the middle 
and posterior part of the body are furnished, enable the worm to firmly 
cling to the silken lining of the bag, so that it can with difficulty be 
pulled out. ° 
The bag of the full-grown worm (Fig. 7/) is elongate-oval in shape, 
its outlines being more or less irregular on account of the irregularities 
in the ornamentation above described. The silk itself is extremely 
tough and with difficulty pulled asunder. j 
The larvee 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 
numerous, 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 mi- 
gratory 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 
females attain nearly double this size. When ready to transform, the 
larva firmly secures the anterior end of the bags to a twig or branch, 
and instinct 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 bag is then strengthened with an additional iining 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. 7b) being 
only half the size of that of the female (Fig. 7 e and Fig. 9 «). 
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 
and half way out of the opening at the extremity. ‘Then its skin bursts 
