macrophages (large white blood cells) as they migrate
into various tissues. Lymphocytes, which inhabit the
blood, produce antibodies and cells that go after for-
eign substances. Lymphocytes subtypes are B cells, T
cells, NK cells, and null cells.
See alsoBLOOD.200 leukocyte
Silk Degrees: A Tale of Moths and People,
Part One
By James G. (Spider) Barbour
Silk moths are found around the world, in all hemispheres,
mostly in the tropics and temperate zones. Silk moths, a
group containing not only many of the world’s most useful
insects, but also many of the largest and most spectacularly
beautiful, have had a long and positive impact on people’s
lives. The silk moths comprise two families, the Bombycine
silk moths (family Bombycidae) and the Saturniids or giant
silk moths (family Saturniidae).
Collectively, they inhabit nearly every type of ecosys-
tem within their tremendous geographic range, fromforests to prairies to mountains to deserts. Most silk moths
are medium to large species, with a good number of enor-
mous ones and very few small types. Like all lepidoptera
(butterflies and moths), any silk moth has a four-stage life
cycle, or complete metamorphosis, in which it begins life
as an egg; hatches into a hungry, fast-growing caterpillar;
enters a resting stage as a pupa; and transforms at last
into a winged, reproductive adult. Most adult silk moths do
not eat at all, having built up an ample supply of fat
reserves in the caterpillar stage. Silk moth caterpillars are
eating machines, growing from tiny hatchlings smaller than
rice grains into fat sausagelike creatures, the largest the
size of fingers. Most feed on leaves or some other plant
material, tending to move from branch to branch, eating a
leaf here and a leaf there, in order to avoid consuming their
cover and camouflage.
After feeding, some species burrow into the ground or
simply crawl under a sheltering object, molting one last time
to form the sleeping pupa, a limbless ovoid or cylindrical
thing, capable of nothing more than wiggling and twisting.
To further protect this vulnerable life stage, many caterpil-
lars provide a shield for the pupa, a cocoon made of silk,
which is issued from large glands within the caterpillar’s
body and spun out through organs near its mouth called
spinnerettes. Depending on the species and its habitat, a
cocoon may be spun inside a rolled leaf, attached to a twig,
spun among grass blades, placed under overhanging rocks,
or deposited in the grooves of a tree trunk’s rough bark. In
temperate climates, the pupae typically remain dormant
within their cocoons, the moths developing in the warmth of
spring and emerging in summer. Some genera, such as the
buck moths (Hemileuca), overwinter as eggs, deposited by
the autumn-emerging females in thick rings on twigs or
stems of the host plants.
The material of cocoons has been a part of human cul-
ture for thousands of years, probably having been discov-
ered and used before the beginning of recorded history for
purposes we can only guess at. The technology for the man-
ufacture of silk fabric and the farming of silkworms (sericul-
ture) dates back at least 4,000 years to ancient China, where
it was a closely guarded secret until the third century B.C.E.,
when the Japanese acquired it. From there, silk production
and the making of silk garments spread widely.A highly magnified single-scale strutlike pattern from a
butterfly’s wing magnified ×6,127. Note the struts and
perforations of the individual scale. This helps to promote
heightened aerodynamic lift during the insect’s flight
while also reducing the weight of the wing mechanism.
(Courtesy of Janice Carr, Centers for Disease Control and
Prevention)