194 



TRANSACTIONS OF THE ILLINOIS 



less obsolete, especially toward the base; the mid vein of the hind wings, 

 instead of sending off two discoidal branches obliquely toward the hind 

 border, as in nearly all of the preceding species, usually divides about the 

 middle of the wing into three nearly equal diverging branches ; but Mr. 

 Walsh represents it (see No. VI., Fig. 3,) as sending off two oblique 

 branches on the posterior side. It is possible that there is some variation 

 in this respect in the different species. They produce various kinds of 

 galls on the leaves, leaf-petioles, buds and twigs of plants in which they 

 pass their entire existence until the winged individuals come forth to seek 

 new homes for future colonies. The agamic females, so far as observed, 

 appear to be viviparous. The species are doubtless quite numerous, 

 although, as yet, comparatively few have been studied and described. 

 As these insects have no jaws with which to escape from the galls which 

 they form, when the winged individuals wish to wander away to form 

 new calonies, nature has wisely arranged it that the galls, in forming, 

 always leave an opening on one side where the parts meet, but do not 

 combine. In the galls formed by Hymenopterous insects which possess 

 biting jaws, this is not needed, and hence no such opening is provided. 



74. Pemphigus vagabundiis , Walsh. The Vagabond Gall Plant-louse. 



This species produces a rather 

 large, irregular gall on the tips 

 of the twigs of certain cotton- 

 woods, and also occasionally on 

 balsam poplars, which somewhat 

 resemble the flower-head of the 

 double cockscomb of our gar- 

 dens. These galls turn black in 

 the winter, giving the tree a 

 singular and rather unsightly 

 appearance after the leaves have 

 fallen off. The winged insect 

 generally makes its appearance 

 in September ; the body is black, 

 and about one-tenth of an inch 

 long ; the expanded wings meas- 

 ure rather more than one-third 

 ^'s- 5- of an inch from tip to tip ; an- 



tennse, six-jointed. The name vagalnindus, or vagabond, has been given 

 to it on account of its habit of wandering to a great distance from its 

 place of birth. It is possible, as has been suggested, that Dr. Fitch's 

 winged specimens, described as P. pyri, belonged to this species. A gall 

 now before me, just plucked from a tree where it has been hanging for 

 two seasons, contains not even a remnant of the lice, but in some of the 

 convolutions I find the larvaj of a species oi Psocus, a genus of minute 

 Neuropterous insects. 



