6 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.Y. 



they commence reproducing, and tlieir i^rogeny, under favorable circum- 

 stances, becomes exceedingly abundant. The groAving points of the tree 

 are affected with larger or smaller colonies, crowding and covering both 

 the surfaces of the leaves, the petioles, and the stem. I have known 

 young Elm trees to be so thoroughly covered with these lice, in the earher 

 part of June, that not a single leaf was unaffected, and upon giving 

 the tree the slightest jar there would be a perfect shower of the liquid 

 globules excreted by the lice. At this season of the year, Avhen the Hce 

 are thus immerous, they may be found during the heat of the day actively 

 craAvling over all portions of the tree — a veritable migration, necessita- 

 ted by the want of sufiicient succulent leaves, but evidently premature, 

 and destined to be the death of the individuals participating in it, exces- 

 sive multiplication here, as in all other cases, obliging the destruction 

 of the excess. While the individuals thus wandering are mostly the 

 younger ones, the migrating instinct seems sometimes to possess indi- 

 viduals of all ages, especially where the tree is badly affected ; and that 

 they perish is proved by the mass of dead lice which in such a case may 

 be found around the base of a tree. So far as I have been able to learn 

 by confining specimens of the fifth generation, which is very simihir to 

 the fourth, but with shorter i>romuscis, the fifth reproduces like the 

 foui'tli mthout acquiring wings. The individuals of the sixth generation, 

 on the contrary, all acquire wings, the pupa being active, with but a 

 small amount of flocculence, confined to the posterior i^art of the body 

 The winged lice of this sixth generation abound during the latter 

 part of June and the early part of July. They resemble those of the 

 third generation, except that they are perhaps on the average somewhat 

 smaller and paler, and less prolific. They instinctively congregate on 

 the bark, and consign to the crevices, and sheltered parts thereof, their 

 young, which, as in the foiu'th generation, are euA^eloped in a sort of 

 pellicle. These young also resemble the young of the fourth generation 

 in general form, but have very short and stout beaks. Instead of being 

 active, they are quite sluggish, congregating in clusters in the sheltered 

 portions of the bark, and being essentially bark-feeders. The color soon 

 inclines strongly to orange or salmon, and, after two or three days of 

 sluggish existence, they shed their skin, and become more active, pene- 

 trating more deeply into the interstices of the bark, and huddling to- 

 gether in groups of various sizes. They are now of a pale buff", or, more 

 correctly, salmon color, the surface at first smooth and polished, but 

 becoming in some instances slightly pulverulent. Simulating closely the 

 color of the bark, and being quite small, thej'^ are not easily detected, 

 unless in great numbers. A careful examination shows that they have 

 entirely lost the beak, and that they consist of both males and females, 

 the females being the larger, and the males showing the genital charac- 

 ters given in the description. They live grouped together for several 

 days Avith little motion, the female (Fig. 1, e) iiicreasing in size by the 

 enlargement of the single egg contained in her body. Both soon perish, 



