1863.] 
213 
throughout Odonata the % abdominal appendages are most remarkably 
constant; and it is expressly remarked of the genus Hetaerina in the 
Monographie that “ the species resemble one another greatly, so that 
many of them are distinguishable with difficulty, especially 9 9 ,” and 
that in % , besides the characters drawn from the wings, “we should 
pay special attention to the organization of the abdominal appendages, 
which are fortunately a little different almost in every species.” (p. 87.) 
I have carefully collated 36 specimens of 'S riipamn.emU n. sp. and 
can find no appreciable variation whatever in the structure of these 
organs. Hence we may infer, I think, that they are generally con¬ 
stant in other species of the genus, and as the characters in which our 
insect differs from americana are by no means constant, while that of 
the % abdominal appendages, in which it agrees with americana and 
differs from basalis, may be assumed to be constant, we should place 
more dependance upon a single constant character than upon several 
inconstant ones. 
The principle above referred to, which though not universally is, I 
think, pretty generally true, may be called the Law of Equable Va¬ 
riability, and may be thus expressed :—If any given character is 
very variable in one species of a group, it will tend to be variable in 
allied species; and if any given character is perfectly constant in one 
species of a group, it will tend to be constant in allied species. For 
example, almost all the spotted Ooccuiella(Jsc are, as is well known, more 
or less variable in the characters drawn from their spots ; Anom.ala 
varkms Fabr. is exceedingly variable in its spottings. and so is A. Juci- 
cola Fabr. and A. hinofata, 8chbnh.; the European Doiuicia Proteus 
has the ground-color remarkably variable, and most of our N. A. spe¬ 
cies vary similarly more or less; Orsodacna vittata Say, is most remark¬ 
ably variable in its vittae, and so is another vittate species 0. chil- 
dreni Kirby, from Huclson’s Bay; the different species of Meldsea and 
Argjjuuis are notoriously variable in the markings of their wings; the 
spotted species of Tettigoniadse are more or less variable in their spots 
and some astonishingly so; and throughout Perlina the neuration, 
which in most families of insects is constant, is more or less inconstant. 
On the other hand, in the allied family Odonata the neuration is won¬ 
derfully constant. Finally, in the tribes uPisclmina and Libelhdina 
the coloration is pretty constant; in the tribe Agrionina it is as incon- 
