H. M. PARSHLEY 
5 
ill species which are normally fully winged (such as acrpialis and 
quadriUneatufi) , and very often there is considerable individual 
variation in hemielytral length. We may suppose that the true 
polymorphic species have become established as such when un- 
usually iironounced modifications of this kind appeared as muta- 
tions, that is, in connection with corresponding germinal changes. 
None of the species arc known to produce normally forms having 
various distinct degrees of brachyptery, such as occur in the 
Xaliidae and Clerridae. From the following talde, which shows 
the known morphological features of pterygo-polymorphism in 
the American species, we may note that stenoptery is found 
only in male individuals, while lirachyptcry is, with one excep- 
tion, confined to females, suggesting that the iihenomenon is 
connected with hereditary processes of a Mcndelian nature, 
which invite experimentation. At present the ethological sig- 
nificance of this polymorphism seems finite beyf)nd conjecture, 
although Reuter (1875) and Kirkaldy (1809) have thought to 
trace here the workings of natural selection. 
Table of Pterygo-polijmorphic Species 
Bracliyptc'i’ou.s 
Species 
Macrop- 
tcrous 
Stenop- 
terous 
AIemi)rano 
vestigial 
Membrane 
alisent 
iiitcclus 
— 
— 
9 
— 
curticollis 
d' 
— 
9 
— 
proboscidens 
cf 9 
— 
9 
— 
orhiculus 
— 
d 
9 
— 
cinnnmonieus 
9 
d 
9 
9 
niger 
9 
— 
— 
d’ 9 
Iicidemnuni 
9 
— 
9 
Species Formation and Variation 
The species of Aradus are, with very few exceptions, distinctly 
differentiated, although the group as a whole is remarkably homo- 
geneous. This condition, with its wide distribution over the 
earth, would seem to mark the genus as of great geological aii- 
tifiuity, among insects, and as one which has reached a state of 
relative stability, though still far from the stage when signs of 
approaching extinction begin to appear. It is evidently enjoy- 
ing the optimum period of its evolutionary career, after the 
more profitable adjustments have lieen accomplished and l)efoi'e 
TRANS. AM. ENT. SOf'., XLVII. 
