helium An inert gas; an element with atomic
number 2. Helium is produced in stars and is the
second most abundant element in the universe. Its
atom contains two protons, two neutrons, and two
electrons.
See alsoELEMENT.
helix A particular rigid left- or right-handed arrange-
ment of a polymeric chain, characterized by the num-
ber of strands, the number (n)of units per turn, and its
pitch (p), the distance the helix rises along its axis per
full turn. Examples of single-stranded helices are the
protein helices: α-helix: n= 3.6, p= 540 picometer;
310 -helix: n= 3.0, p= 600 picometer; π-helix: n= 4.4,
p= 520 picometer.
See alsoDOUBLE HELIX.
helminth A worm or wormlike organism. Three
major helminths exist that affect humans: the nema-
todes (roundworm), trematodes (flukes), and cestodes
(tapeworms).
See alsoPLATYHELMINTHES.
helper T cell A type of T cell needed to turn on anti-
body production by activating cytotoxic T cells and
causing other immune responses in the body. They aid
in helping B cells to make antibodies against thymus-
dependent antigens. TH1 and TH2 helper T cells
secrete materials (interleukins and gamma interferon)
that help cell-mediated immune response.heme A near-planar COORDINATIONcomplex obtained
from iron and the dianionic form of PORPHYRIN. Deriva-
tives are known with substitutes at various positions on
the ring named a, b, c, d, etc. Heme b, derived from PRO-
TOPORPHYRIN IX, is the most frequently occurring heme.hemerythrin A dioxygen-carrying protein from
marine invertebrates, containing an oxo-bridged dinu-
clear iron center.
See alsoNUCLEARITY.hemichordates Consisting of only a few hundred
species, the hemichordates include the acorn wormshemichordates 159(Coccinella novemnotata), the “official” New York State
insect, was eradicated from New York by the seven-spotted
ladybug (Coccinella septempunctata), an intentionally intro-
duced European species. No nine-spotted ladybug has
been collected in New York State since the 1940s, yet it had
been a dominant species before the introduction of this
competitor. Another exotic introduction, the Asian ladybug
(Harmonia axyridis), made its way to the Northeast in the
1990s and has partially displaced the previously introduced
seven-spotted ladybug.
It is now somewhat of a rarity to see a native ladybug
in New York State. The level at which the various ladybugs
compete is not that well understood. The nine-spotted and
the seven-spotted are very close relatives, so the fact that
they compete directly should not have surprised scientists.
What is more surprising is that the distantly related Asian
ladybug displaced the nine-spotted and many others,
despite the lack of any close relationship. Parasites and
diseases of introduced insects are some of the baggage
that comes with these introductions. The introductions
have evolved a resistance or at least a tolerance to thesecoevolved agents to which our native species have never
been exposed. It is smallpox at an insect level.
All the aforementioned ladybugs eat aphids, but they
can be particular about which species and in what habitats.
The diversity of native ladybug species was achieved
through natural selection. Introductions upset the balance.
We can never take back the dozens of introduced ladybugs.
If all life (not just ladybugs) from all of the earth’s conti-
nents were mixed together tomorrow, it would result in a
die-off of diversity that would parallel that of the Devonian
extinction that occurred millions of years ago. This is what
we are doing, albeit slowly, with accidental and purposeful
introductions. Once an undesirable exotic has gained a
foothold, we are caught up in a cycle of introducing preda-
tors, parasitoids, and diseases in an attempt to suppress or
eradicate the original introduction. As shown by these
examples, it often backfires.—Timothy L. McCabe, Ph.D.,is curator of
entomology at New York State Museum in
Albany, New York.