n not to be
adult, in some cases feel the sexual impulse, while a number of species
in each of the families represented by these two insects never acquire
wings.

[Illustration: 205. Larva. 206. Semi-pupa.

207. Advanced Semi-pupa. 208. Pupa.

EARLY STAGES OF THE HUMBLE BEE.]

Still how did the perfect metamorphosis arise? We can only answer this
indirectly by pointing to the Panorpa and Caddis flies, with their
nearly perfect metamorphosis, though more nearly allied otherwise to
those Neuroptera with an incomplete metamorphosis, as the lace-winged
fly, than the insects of any other suborder. If, among a group of
insects such as the Neuroptera, we find different families with all
grades of perfection in metamorphosis, it is possible that larger and
higher groups may exist in which these modes of metamorphosis may be
fixed and characteristic of each. Had we more space for the exposition
of many known facts, the sceptic might perceive that by observing how
arbitrary and dependent on the habits of the insects are the
metamorphoses of some groups, the fixed modes of other and more general
groups may be seen to be probably due to biological causes, or in other
words have been acquired through changes of habits or of the temperature
of the seasons and of climates. Many facts crowd upon us, which might
serve as illustrations and proofs of the position we have taken. For
instance, though we have in tropics rainy and dry seasons when, in the
latter, insects remain quiescent in the chrysalis state as in the
temperate and frigid zones, yet did not the change from the earlier ages
of the globe, when the temperature of the earth was nearly the same the
world over, to the times of the present distribution of heat and cold in
zones, possibly have its influence on the metamorphoses of insects and
other animals? It is a fact that the remains of those insects with a
complete metamorphosis (the bees, butterflies and moths, flies and
beetles) abound most in the later deposits, while those with an
incomplete metamorphosis are fewer in number and the earliest to appear.
Again, certain groups of insects are not found in the polar regions.
Their absence is evidently due to the adverse climatic conditions of
those regions. The development of the same groups is striking in the
tropics, where the sum of environing conditions all tend to favor the
multiplication of insect forms.

It should be observed that some insects, as the grasshopper,