509 
[Packard. 
than among 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 number of species of insects (over 
five hundred) than any other plant known. On the whole, forest 
trees support afar larger number of kinds of phytophagous insects 
than grasses or herbs, and may this not be due to better air and a 
freer circulation, to a more equable temperature, perhaps of a higher 
average, and thus lead insects to eat more? May not the plump 
bodies of the larger silkworms, as the larval Attaci, the Cerato- 
campids, and especially the Cochlidise (Limacodes), be in some way 
due to their strictly arboreal environment ? 1 
When the ancestors of the present groups became fairly estab- 
lished under these changed conditions, becoming high-feeders, and 
rarely wandering to low herbaceous plants, we should have a con- 
dition 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 Cochlidiae, may have at first had larger wings and smaller 
bodies, or been more active during flight than their descendants. 
Their present heavy, thick bodies and sluggish habits are evi- 
dently secondary and adaptive, and these features were induced 
perhaps by the habit of the females ovipositing directly upon leav- 
ing 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 Ceratocampidae compared with the co- 
coon-spinning silkworm ( B . mon) and the Attaci. Spinning their 
cocoons among the leaves, at a period in the earth’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 
1 The fat overgrown slug-worms (Limaeodes) may be compared to the over-fed, 
high-bred pig, which eats voraciously, has little need of rooting, and takes but little ex- 
ercise. Where, as in cave Arthropods, there is a deficiency of food, we have a con- 
stant tendency to slimness, to an attenuation of the body. This is seen in the blind 
cave animals, such as the blind crayfish, blind beetles, blind Cracidotaea, etc., com- 
pared 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.) 
