AIR-SACS OF THE LOCUST. 
183 
respiratory efforts of the insect in effecting its change, and which, with a tendency to 
enlarge by the natural forces of growth in these structures, results in the dilatation 
first of the trachea) at the base of the abdomen into distinct sacs. This is the man- 
ner in which the air-sacs are formed in all insects. After the main trunks have be- 
come dilated their ramifications also are enlarged in like manner, and this enlarge- 
ment continues from the time when the insect enters its pupa to that of its appear- 
ance in the perfect state. 
There is no more admirable instance in nature of the adaptation of 
organs to the performance of certain functions than this of the air-sacs, 
which render certain insects true aeronauts. This delicate correlation of 
these organs to the aerial habits of the insects which possess them, seems 
to be in the relation of cause to effect. It has been seen that they do 
not arise until the final winged state of the insect — the time when they 
are first brought into use — and this is an indication that they are the 
result of forces acting upon the organism during its adult winged life. 
This special adaptation of the air-tubes to the exigencies of its aerial 
life may have been suddenly induced, the tracheae in some favored race 
of bee, moth, or locust having been distended during the rapid, violent 
respiratory efforts of the insect during flight, and resulted in a perma- 
nent enlargement of the air-tubes. These initiatory sacs being found 
useful were probably transmitted to the offspring, until they became 
permanent improvements in the organization of certain races of differ- 
ent groups of flying insects, and remained wanting in other even closely 
allied groups which did not possess wings. Thus we see that changes 
in the mode of life, the influence of the environment upon the insect, pro- 
voked the variation,! e., the sudden rise of what ultimately proved to be 
useful organs which became further perfected and finally absolutely in- 
dispensable and unfailingly present in the descendants of those forms 
in which they at first originated. Such is the line of thought or argu- 
ment which we are compelled to adopt in endeavoring to trace the origin 
of such organs as those under consideration. In brief, it is the influence 
of external causes upon the animal, certain changes in the environment, 
which become perpetuated by internal causes or inheritance force. 
CHAPTER X. 
HISTOLOGY OF THE LOCUST (CALOPTENUS) AND THE 
CRICKET (ANABRUS). (Plates II-VIII.) 
By Dr. Charles Sedgwick Minot. 
Insects have hitherto been but little studied by histologists. The 
science of general anatomy or histology, which was first established by 
Bichat in France, acquired a fresh importance and new meaning through 
the investigations of German naturalists, and above all through the 
great discovery of Schwann that all animals are composed, like plants, 
of certain minute elements or units, which are now famibar to all natu- 
