HEMIPTERA. 1 65 



Three genera occur in this country. These can be distinguished 

 as follows : 



A. Hind wings with two discoidal veins. 1. PEMPHIGUS. 

 A A. Hind wings with only one discoidal vein. 



B. Antennae six-jointed. 2. TETRANEURA. 



BB. Antennae five-jointed. 3. Hor.maphis. 



Among the common representatives of this section are certain 

 species of Pemphigus, which make galls upon various trees of the 

 genus Populus. One of these, the Poplar-leaf Gall-louse, Pemphi- 

 gus populicaulis, is common on the leaves of cotton-wood and of 

 aspen. It makes a swelling the size of a small marble on the leaf at 

 the junction of the petiole with the blade. This gall is of a reddish 

 tint, and has on one side a slit-like opening. In the early part of 

 the season each gall is occupied by a single, wingless female, prob- 

 ably the agamic stem-mother, which, by midsummer, becomes the 

 mother of numerous progeny; these often amount to one hundred 

 and fifty in number. 



A closely allied species occurs throughout the Western and South- 

 western States, which makes a similar gall near the base of the leaf 

 of Populus monolifera and P. balsamifera. This gall has a trans- 

 verse slit-like opening, which probably suggested the name of the 

 insect. Pemphigus populi-transversus. As in the preceding species, 

 the gall is started in the spring by a single, wingless stem-mother; 

 by the latter part of June the stem mother is surrounded with young 

 of various sizes, all covered with the usual white secretion, and 

 mixed with liquid globules. A generation of winged individuals is 

 produced in the autumn, sometimes not until the leaves have fallen. 



Another of these species is known as the Vagabond Gall-louse, 

 Pemphigus va.gabii.ndus. It infests the tips of the twigs of certain 

 cotton-woods and the balsam poplar; here it makes large corrugated 

 galls, which somewhat resemble the flower of the double cockscomb 

 of our gardens. These galls turn black, and remain on the trees 

 during the winter. On opening the galls in midwinter I have found 

 many remains of winged lice in them. 



Section II.— SCHIZONEURINI. 



This section includes genera in which the third discoidal vein of 

 the front wings is forked ; the antennae are six-jointed. Two genera 



