MEMOIHS OF THE :N^ATI0^^AL ACADEMY OF SCIEXCES. 
19 
living ill trees or shrubs several or many feet above the ground are certainly exposed to a more 
even temperature, as it is colder at niglit even in midsummer within a few inches of the ground, 
say abou t a foot, the usual height to which grasses and herbs grow. The changes, therefore, by day 
and night are greater at the surface of the ground than among the leaves and branches of a tree. 
^Moreover, forests, not too dense for insect life, with glades and paths to admit the sunlight and 
heat, must necessarily have a more even teinjierature and be less exposed to coo! winds, and less 
subject to periods of drouglit than grassy fields. There is also a less free circulation of air among 
grasses and herbs, which may be more oi* less matted and lodged after heavy rains, than amoug the 
separate and coarser leaves of trees, such as the different species of oak, which in North America, 
at least north of Mexico, harbors a far greater iniinber of species of insects (over 500) than any 
other plant known. On the whole, forest trees support a far larger number of kinds of iiliytojilia- 
gous insects than grasses or herbs, and may this not be due to better air and a freer circulation, 
to a more equable temperature, perhaj)s of a higher average, aud thus lead insects to eat moref 
May not the jduinp bodies of the larger silkworms, as the larval Attaci, the Cevatocaiupids, and 
especially the Cochliopodidie (Liinacodes), be in some way due to their strictly arboreal enviT'on- 
ment?^ 
When the ancestors of the present groui)s became fairly established under these changed 
conditions, becoming high feeders, aud rarely wandering to low herbaceous plants, we should 
have a coiiditioii of things akin to geographical isolation. The species would gradually tend to 
become segregated. The females would more and more tend to deposit their eggs on the bark or 
leaves of trees, gradually deserting annual herbs. 
For example, the females of the Attaci and their allies, as well as the Cochliopodidm, may have 
at first had larger wings aud smaller bodies, or been more active during flight than their descendants. 
Their present heavy, thick bodies and sluggish habits are evidently secoudary and adaptive, aud 
these features were induced perhaps by the habit of the females ovipositing directly u])Ou leaving 
their cocoon, and cocoon-spinning moths are perhaps as a rule more sluggish and heavy-bodied 
than those which enter the earth to transform, as witness the Ceratocampidie compared with 
the cocoon-spinning silkworm (i>. mori) and the Attaci. Spinning their cocoons among the 
leaves at a period in the eartli’s history when there was no alternation of winter and summer- 
and probably only times of drought, as in the dry season of the Tropics at the present day, the 
females may have gradually formed the habit of depositing their eggs immediately after exclusion 
and on the leaves of the trees forming their larval abode. The females thus scarcely used their 
wings, while (as iu Callosmnia promethea) tlie males, with their larger wiugs, lighter bodies, 
broadly pectinated auteuiue, and coUvSequently far keener sense of smell, could fly to a greater 
or less distance iu search of their mates.^ The principal of segregation^ so well work<Kl out by 
Mr. Gulick, to which Mr. Eomaiies’ theory of physiological selection is a closely allied factor, if 
not covering the same ground, would soon be iu operation, aud the teudeucy to breed only amoug 
themselves, rather than with the low feeders, would more aud more assert itself, until, as at present, 
arboreal moths, as a rule almost, if not wholly, oviposit exclusively on the leaves or bark of trees. 
’ The fjit, overgrown slngworms (Limacodea) may ho com])ared to the overfed, high-hred pig, which eats 
voraciously, has little need of rooting, and takes hut little exercise. Where, as amoug cave animals, there is a 
deficiency of food, we have a constant tendency to slimness, to an attenuation of the body. This is seen iu the 
blind eavo arthropods, such as the blind crayfish, blind beetles, hliml Ciccidotjea, etc., compared with their allies 
which live under normal conditions. (See the author's memoir on the Cave Fauna of North America, etc., Mem. 
Nat. Acad. Sciences, iv, 24. 1889.) 
-The secondary sexual characters so marked in liombyces are perhaps the result of their peculiar arboreal 
habits; so also the apterous tendency of Orgyia and a few other forms, especially the arboreal I'sychida^ ((P^cvticua 
and Tk\jr\dopteryx)j as well as Anlsoptcryx and Jfibernia. The larvte of the Ntjsaia feed on trees or low ]»laut8. It 
may he questioned whether any wingless female Lepidoptera live on herbaceous ])lant8. Contrast with them the 
grass-feeding species of Noctuidse, at) tliose of AgrolU, LcHcania, etc. 
®Iu fact nearly the whole group of insects is an example on a vast scale of the principles of segregation, 
geographical isolation, and physiological selection. As soon as the ancestors of inserts acejuired wings their vuUeii 
was changed. The air rather than the earth became their habitat; the acquisition of wings introduced them to a 
new world of existence, and free from the attacks of creeping enemies and other adverse conditions to which the 
terrestrial Myriopods aud Arachnids were subjected; the winged insects living apart of their lives, and the most, 
important part, above the surface of the soil, multiplied prodigiouslj’, the number of species being estimated by 
millions when we take into account the fossil as well as the living forms. 
