260 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 
above described. The Bilk itself is extremely tough aud with difficulty pulled 
asunder. 
The larva 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 migratory desire is instinctive ; for 
should the worms remain on the same tree they would become so uumerous 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 trausform, the larvae 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 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. 96, &) being only half the size of that of the female 
(Fig. 96, e and Fig. 99, a). 
The imago or perfect insect. — After a lapse of about three weeks from pupation a still 
greater difference between the two sexes becomes apparant. The male chrysalis works 
its way to the lower end of the bag and half way out of the opening at the extremity. 
Ylo.98.—Thyridopteryxephemer(eformig: FlG. 99. — Thyridopteryx ephemer&formis : b, The 
a, Follicle cut open to show the manner in end of male ahdomen from the side, showing gen- 
\\ bich the female works from herpnpari urn italia extended; c, genitalia in repose, ventral 
and reaches the end of the hag, natural view ; (/, do., dorsal view enlarged. (After Riley) 
size; 6, female extracted from her case, 
enlarged. (After Riley). 
Then its skin bursts and the imago appears as a winged moth with a black, hairy body 
aud glassy wings (Fig. 96, d). It is swift of flight, and owing to its small size and 
transparent wings, is rarely observed in nature. The life-duration of this sex is also 
very short. The female imago is naked (save a ring of pubescence near the eud of 
the body of yellowish-white color), and entirely destitute of legs and wings (Fig. 96, 
o, and Fig. 98, 6). She pushes her way partly out of the chrysalis, her head reaching 
to the lower end of the bag, where, without leaving the same, she awaits the approach 
of the male. The manner in which the chrysalis shell is elongated aud reaches to the 
end of the bag is shown in Fig. 98, a, and an enlarged side view of the female showing 
