702 Rospert MATHESON 
Third instar.— Length 1.28-1.30 mm.; width 0.72 mm. 
In coloring and markings this instar differs but slightly from the pre- 
ceding. The cauda is more prominent and the cornicies are twice as 
long, 0.08 mm. in length. The lateral tubercles now begin to appear 
on the prothoracic and abdominal segments. 
Fourth instar.— Length 1.44-1.50 mm.; width 0.88 mm. 
In this stage the nymph is yellowish green in color, with the legs, the 
distal half of the antennae, and the cauda dusky to black. The cornicles 
are dusky, with black at their tips. The dusky patches on the head are 
apparently absent. The antennae are 5-jointed. The cornicles now 
— 
Fic. 113. cORNICLES OF MATURE STEM MOTHERS 
A, Aphis pomi; B, A. sorbi; C, A. avenae. All drawn to same scale 
measure 0.16 mm. in length. The eyes are black and are now rather 
prominent. The lateral tubercles on the prothoracic and abdominal 
segments show more distinctly than in the preceding instar, and lateral 
tubercles appear also on the last two thoracic segments. 
Fifth instar, mature stem mother (Plate XIX).— Length 1.6-1.7 mm.; 
width 0.95-1 mm. 
The head and the thorax are yellowish green, the head usually having 
a dusky appearance. The abdomen is bright to dark green, frequently 
mottled with yellowish green. The eyes are black and rather prominent. 
The cornicles are black, cylindrical, slightly tapering, 0.24 mm. long 
(fig. 118, a). The cauda is black, setose, prominent; the sub- 
