968 
Journal of Agricultural Research 
Vol. V, No. si 
Table I .—Lengths of third antennal segment and of proximal and distal portions of the 
fourth segment in Aphis pomi, A. avenae, and A. malifoliae 
Species. 
Segment III. 
Segment IV base. 
Unguis IV. 
Aphis pomi . 
Mm. 
0. 08 to 0. 09 
. 08 to . 096 
. 128 
Mm. 
O. 048 to 0. 056 
.03 to . 048 
. 048 
Mm. 
0.048 to 0. 064 
.08 to . 104 
. 048 to . 16 
Aphis avenae . 
Aphis malifoliae . 
It will be noted that A. pomi has a much shorter unguis than either of 
the other species and this at once distinguishes it. 
STEM MOTHER 
DESCRIPTION 
First instar. —Morphological characters: Antennal segments (PL LXXIV, fig. 16) 
as follows: I, 0.032 mm.; II, 0.032 mm.; Ill, 0.08 to 0.09 mm.; IV, base 0.048 to 0.056 
mm., unguis 0.048 to 0.064 mm.; segments III and IV imbricated and covered with a 
few stout spines, III armed with a distal sensorium, and IV with a sensory group at 
the base of the unguis composed of one large sensorium and several small ones. Eyes 
with 8 to 10 facets. Rostrum long. Cornicles short, thick, and rounded at the distal 
extremities. Cauda and anal plate rounded and densely setose. Legs with spinelike 
hairs. 
Color characters: Body dark green (sometimes very dark) with appendages dusky; 
crown dusky to black with a median longitudinal uncolored suture-like stripe. Entire 
insect often slightly pruinose. 
Second instar. —Morphological characters: Antennal segments as follows: I, 
0.032 mm.; II, 0.033 mm.; Ill, 0.08 mm.; IV, 0.048 to 0.064 mm.; V, base 0.048 to 
0.064 mm, unguis 0.048 to 0.088 mm., usually about 0.075 mm.; segments III to V 
imbricated and with a few stout, spinelike hairs, IV with a distal sensorium similar 
to that on III of first instar, and V with the usual sensory group. Eyes with about 
10 facets. Rostrum comparatively shorter than in first instar. Cornicles short and 
thick and rounded distad. Cauda and anal plate rounded and setose. Legs more 
slender than in preceding instar. 
Color characters: Similar to the first instar, but lighter in color. 
Third instar. —Morphological characters: Antennae much more slender than in 
the previous instars, with lengths as follows: I, 0.045 mm.; II, 0.042 to 0.045 mm.; 
Ill, 0.112 to 0.144 mm.; IV, 0.064 to 0.096 mm.; V, base 0.064 to 0.08 mm., unguis 
0.088 to o. 112 mm., usually about 0.1 mm.; sensory characters as in previous instars. 
Eyes with over 20 facets; cornicles more elongate than in the other instars and not 
so rounded distad; cauda and anal plate rounded and setose. 
Color characters: Similar to those of the previous instar. 
Fourth instar. —Morphological characters: Antennae fairly long and slender, with 
lengths as follows: I, 0.053 mm.; II, 0.048 mm.; Ill, 0.192 to 0.224 mm.; IV, 0.088 to 
0.128 mm.; V, base 0.08 to 0.096 mm., unguis 0.112 to 0.128 mm.; segments III to V 
strongly imbricated, sensory characters as in other instars. Eyes with about 40 
facets. Cornicles 0.153 mm. in length and imbricated. Cauda somewhat conical. 
Legs slender, tibiae somewhat curved, 0.571 mm. long. 
Color characters: Approaching those of the adult form. 
Fifth instar (adult). —Morphological characters: Antennae (PI. LXXIV, fig. 5) 
rather long and slender, with lengths as follows: I, 0.06 mm.; II, 0.056 mm.; Ill, 
