THE WINGS. 
27 
The two upper wings are considerably larger than 
the two lower, and have no coriaceous portion as in 
the Homoptera proper. An approximation to this 
structurd7“^vertheless, may be seen in the wings of 
Chermes and other allied genera. The wings are 
attached to the mesothoracc by an enlargement of the 
basal portion of the main veins. 
In describing species I allude to these swellings by 
the term wing insertions . They often are brighter 
in colour than the rest of the insect, and are distinc- 
tive. 
The lower wings are united by similar attachments 
to the metathorax. A compound hoohlet is fixed on the 
costal margin of eachlower wing, which works in a strong 
fold or thickening of the posterior margin of the upper 
wing, so that a continuous vibrating surface is formed 
by the two membranes during flight.^ 
The nervation or venation of these organs is of great 
value for subdividing the family into suitable genera. 
Frich in Germany, Harris in England in 1782, and 
Jurin in Switzerland in 1807, appear to have been the 
first to propose constructing genera of insects upon 
the basis of a distribution of their wing veins. Later, 
Shuckard, in this country, adopted the same method 
for arranging the Hymenoptera; the nervation of which, 
however, is far more complex than what we find in 
these Homoptera. 
The late Mr. Francis Walker discussed the same 
subject in the ^ Entomologist ’ for February, 1874, 
p. 36 et seq., where he embodies some posthumous 
papers on wing nomenclature, by the late Mr. Haliday. 
Amongst Aphides caution is necessary, lest the not 
infrequent abnormal characters of nervation should 
mislead us. Nervation sometimes accidentally varies 
on each wing. Sometimes we find supernumerary 
veins, sometimes they abnormally anastomose, and 
occasionally even they are absent altogether. Refer- 
ence, therefore, to several specimens should, if pos- 
* Vide Plate XXII, fig. 6. 
