Encyclopedia of Biology

them to the optic nerve by using a set of nerve cells—
horizontal, bipolar, amacine, and retinal ganglion
cells—finally leading to the brain for interpretation.
Two types of sensory cells, called photoreceptors,
allow the visualization of color, contrast (sharp cen-
tral vision), and night vision. Cone cells, which are
moredense in the center of the retina, are responsible
for seeing color and contrast during daylight, while
the rods, which are more numerous around the
peripheral areas of the retina, are thousands of times
more sensitive than cones and allow vision in the
dark and allow side or peripheral vision. The choroid
is alayer of the retina filled with blood vessels that
nourish it.

retinue The group of workers that surrounds a
queen in insect societies; attendants or followers.
Queen bees release a pheromone, queen mandibular
pheromone (QMP), that is both a releaser and a
primer. As a releaser it attracts worker bees to their
queen, and as a primer it inhibits queen rearing and
suppresses the ontogeny of worker foraging by dimin-
ishing the release of juvenile hormone in worker bees.
QMP also stimulates the retinue response in honeybees
by way of licking and antennating behavior that signals
the presence of a dominant reproductive queen and
establishes and stabilizes the social structure of the bee

colony. QMP is spread throughout the colony by ret-

inue behavior. When worker bees turn toward the

queen and form a retinue around her, several bees con-

currently touch the queen (using their antennae,

forelegs, or mouthparts) and then proceed through the

hive passing on the pheromone.

retrovirus An RNA virus that encodes an enzyme

called REVERSE TRANSCRIPTASEto convert into DNA

when inserted into a host’s cell DNA, thereby repro-

ducing the virus. HIV viruses are retroviruses, and they

also are a class of cancer-causing viruses. Retrois short

for REverse TRanscriptase Onko.

reverse transcriptase An enzyme that retroviruses

use to encode RNA to convert to DNA.

revision In taxonomy, the reclassification of a group

of species as the result of additional study.

Rh blood types Ablood group, which involves 45

different antigens on the surface of red cells, that is

Rhblood types 293

Transmission electron micrograph of the Ebola virus, an RNA

virus that causes hemorrhagic fever.(Courtesy of Centers for

Disease Control and Prevention)

The retina converts light energy into nerve impulses and sends
them to the optic nerve by using a set of nerve cells.