260 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



above described. The silk itself is extremely tough and with difficult}' pulled 

 asunder. 



The larvjB 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 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 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, h) being only half the size of that of the female 

 (Fig. 96, e and Fig. 99, o). 



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. 



Fia.SS.—Thyridopteryxephemercefor'mis: Fig. 99. — Thpridopteryx ephemerceformis : b, The 



o, Follicle cut open to show the manner in end of male abdomen from the side, showing gen- 



which thefcmaleworksfiomherpuparium italia extended; c, genitalia in repose, ventral 



and reaches the end of the bag, natural view; d, do., dorsal view enlarged. (After Riley) 

 size; b, 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 

 and 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 end of 

 the body of yellowish-white color), and entirely destitute of legs and wings (Fig. 96, 

 0, and Fig. 98, b~). 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 and reaches to the 

 end of the bag is shown in Fig. 98, a, and an enlarged side view of the female showing 



