Caterpillars of this family of moths, with very few exceptions, have only
two pairs of claspers or prolegs; when there are more than four claspers,
the extra ones are only rudimentary and therefore useless. In moving from
place to place the caterpillar stretches out to its full length, first to
one side and then to the other, as though measuring the distance. When a
hold is secured with the true legs the body is arched and the claspers are
brought up almost to the point held by the true legs; the latter are then
thrust forward and the measuring business proceeds as before. Some kinds
perform the looping manoeuvre very deliberately, but others at a quick
rate. In common parlance among British entomologists the caterpillars are
called {100} "geometers" or "loopers," but to our American confreres they
are known as "measuring-worms" or "span-worms."
Most of the caterpillars feed openly on the foliage of trees, shrubs, or
low-growing herbs, and the majority remain upon their respective plants
during the day.
A large proportion of the moths may be obtained in the daytime, either by
beating or otherwise disturbing the foliage or herbage among which they
hide; several kinds rest on tree trunks, palings, rocks, walls, etc., where
they are sometimes conspicuous, but more frequently not easy to distinguish
from their surroundings. On the whole, members of this family are more
available to the day collector than are those of the Noctuidae. Although
several species occasionally visit the sugar patch, such species are, as a
rule, obtained more readily and in larger numbers by other methods.
Brilliant light has a great attraction for many of the moths, some are more
often captured at gas or electric lamps than in any other way, and among
these are the migratory species.