48 SEVENTEENTH Report STATE ENTOMOLOGIST OF MINNESOoTA—1918 
transverse folds of the flagellum. Second, the primary sensoria, usually 
fringed with a row of hairs, one on the terminal segment close to the 
base of the spur; and a second on the subterminal segment near the 
apex. The primary sensoria are the first to appear and are usually 
present in the larvae in the earliest stage as well as in the adult; they 
are the most constant of all the sensoria. Third, the secondary sen- 
soria, variable in number and position on the segments of the flagellum, 
are characteristic of the adult but may be present in the larva as well. 
The secondary sensoria are usually circular like the primary, but in 
the Pemphiginae and forms with shortened antennae they become 
crowded and may be transverse or annulated, extending around the 
segment like a ring. Fourth, the marginal sensoria, a group of five or 
six small sensoria close to the margin of the primary sensorium of the 
terminal segment. The arrangement of the marginal sensoria shows 
two distinct types: first, where they are scattered over some area, as 
in the Lachnina and related tribes, which may be considered as the 
primitive condition of the family; second, where they are grouped 
close together in a row or in pyramidal form as in the Aphidina. Sen- 
soria may also be present on other parts of the body, on the coxa, the 
hind tibiae of oviparous females, on the wing-veins, and on the spur 
of the antennae (Mordwilkoja). The rostrum is typically composed 
of five segments, which is undoubtedly the primitive condition of the 
family from which the more common four-segmented condition has 
come by fusion of the two terminal segments. The five-jointed con- 
dition is clearly seen in the Lachnini.. In some aphids the prothorax 
is provided with lateral protuberances or tubercles, the significance 
of which is obscure. Similar tubercles may also be found on the 
segments of the abdomen, which indicates an original segmental 
condition of these bodies. They are seen at their best in the Pterocom- 
mini and certain Aphidini. 
The long and slender legs of the Lachnina and the Macrosiphini 
show undoubtedly the primitive condition of the family; the reduction 
of the legs reaches its furthest in the gall and underground forms of the 
Pemphiginae; atrophied and greatly modified tarsi are known only 
for some of the more specialized genera of the Aphidini. The simple 
venation of a Pemphigus is not to be considered as the primitive type, 
but rather the complete venation of the Aphidinae from which the 
secondary types are derived by reduction or loss of certain veins. Too 
great value has been given to venation by most writers in establish- 
ing the larger groups of the family. Modifications of the type form 
(Lachnina) by reduction or loss of certain veins are found to have 
taken place in several of the tribes, producing parallel and convergent 
