<4 APHIDS OF THE ROCKY MOUNTAIN REGION 
as a projection from the tip of the abdomen, varying from incon- 
spicuous, rounded, half-moon-shape or short-tapering or knobbed to 
elongate-tapering or spatulate. The latter two organs are peculiar 
to this family. 
LIFE CYCLE 
The life cycle (Fig. 1) is quite characteristic of a type known as 
heterogamy or cyclic reproduction. There occurs an alternation of 
asexual generations with a sexual generation. The typical life cycle 
forms are as follows: 
Stem Mother or Fundatrix is the form hatching in the spring 
from the over-wintering egg and reproducing parthenogenetically 
(without mating with a male) and viviparously (bringing forth the 
young alive). 
Summer Viviparae* consist of apterous (wingless) and alate 
(winged) summer viviparous females, or viviparae, also called 
agamic females, and develop from the young produced by the stem 
mother and by succeeding generations throughout the sumnier. 
They reproduce in the same way as the stem mother. The individuals 
born from the fundatrix are called fundatrigeniae. 
Sexuparae, either apterous or alate viviparae, typically develop 
in the last generation of the summer viviparae and appear in late 
summer or fall. They reproduce in the same manner as their parents 
but their young develop into the sexuales. 
Sexuales typically appear in the last generation of the cycle and 
consist of the true female, usually apterous, and the male, either 
alate or apterous. This female is called an oviparous female or ovi- 
para because she lays eggs instead of bringing forth her young alive. 
She is also called gamic because she mates with the male. 
Eggs remain dormant on the plant where they were laid until 
spring when they hatch into the fundatrices thus completing the 
cycle. 
LIFE HISTORY 
The life history is also very characteristic but varies greatly 
with the different species. Most species are very restricted as to 
host plants. Many feed on only one species of plant. Others accept 
related plants and a relatively small number have a primary or 
winter host, a perennial, usually woody plant, and one or more 
*Occasionally adult wingless individuals show abortive wing pads or on the venter of 
the thorax show indications of the sutures of the alate thorax. Such an individual is 
called an intermediate or alatiform female. This form may show some antennal char- 
acters of the alate form such as increased number of sensoria. The nymph of the alate 
form in the later instars when the wing pads appear is often called a pupa but more 
properly perhaps an alatoid nymph. A dimorph is an immature aestivating form which 
occurs in certain genera. The individual remains small, in a state of arrested growth 
for several weeks; is usually flattened and often bearing flabellate hairs on the vertex 
and along the lateral margins of the body. It later resumes growth, attains maturity as 
a normal vivipara called a dimolt, (Davis, 1908b:130) and reproduces either viviparae 
or oviparae. 
