106 REPOKT UNITED STATES ENTOMOLOGICAL COMMISSION. 
sensation, and hence they continue to move upward until they reach a 
current of air of sufficient strength to bear them onward ; they continue 
to float on this until exhausted, a change in the condition of the atmo- 
sphere causes them to descend, or a suitable place for depositing eggs or 
procuring food is observed. 
That a dry condition of the atmosphere does originate this migrating 
instinct I think is evident not only from the fact that migratory locusts 
are only found in dry and arid regions, but from the additional fact that 
in excessively dry years we see in this country Acridium amcricanum, 
Caloptenus diffcrentialis, C. atlantis, and ft femurrubrum, exhibiting a 
strong tendency to mass together and migrate. Even their forms appear 
to be modified so as to adapt them to this purpose. C. femur-rubrum, 
under such circumstances, I am satisfied from numerous observations 
made in the last twenty years, undergoes certain modifications which 
bring it nearer C. spretus. This, as a matter of course, is only distinctly 
apparent when two or three unusually dry years follow in succession. 
One fact noticed, and for which no satisfactory explanation can so far 
be given, is the tendency (among the Acridians) of the last segment of the 
male to become elongated and pointed, and generally to become notched 
at the tip, or where there is a notch, as in the species of Cyrtacanthacris 
{Acridium americanum and allies) to change from the square or U form to 
the sharp or V form. Whether the same thing is true with reference to 
the CEdipodeans I am unable to say ; so far as my observations extend, 
I have observed nothing of the kind in any species of this group. 209 
Another fact worthy of notice is that as a very general rule — so far as 
I am aware without exception — the elytra are spotted, the spots usually 
more or less quadrate 
A careful comparison of the internal anatomyjof the migratory and 
closely allied non-migratory species would probably reveal some con- 
stant peculiarity which would assist in explaining how the climatic con- 
ditions mentioned bring about this disposition or propensity to migrate. 
I am inclined to think it is largely due to the effect these atmospheric 
conditions have on the air-tubes and air-sacs. The vivi-dissections made 
by Mr. Packard, and recorded in our First Report, tend to confirm this 
view by showing the important bearing the numerous and large air-sacs 
must have on the flight of the insect ; and the large amount of tracheal 
surface brought in contact with the air and thereby rendered sensible to 
its changes. 
Our conclusion, therefore, is that tin- migratory habit or instinct of lo- 
custs, whether in the Eastern or Western Continent, is directly attributa- 
ble to the arid condition of the area iu which they originate ; that the unus- 
ually dry and raritied state of the atmosphere is the chief factor in originat- 
ing this instinct ; that while it affects, to a greater or less extent, all parts 
of the insect its chief influence is produced by its effect on the tracheae and 
air-sacs. It also must have a tendency to harden the integuments and to 
109 I have since noticed the same tendency in (E. atrox. 
