SIGHTINGS
Up Close with a Squash Bug
Felice Frankel
Do you know how it is when you see an image of some sort and it
becomes burned into your synapses? (I hope my neuroscientist
friends forgive my scientifically imprecise language.) Well, it
happened to me when I saw one of the covers of Science
in July 1998. I was stunned by its drama and content. A
scanning electron micrograph of the surface of the pupa of the
ordinary squash bug revealed a startling armature of glandular
hairs that exude an oily chemical cocktail, seen forming
droplets at the tips. Maria Eisner is a research assistant at
Cornell University. Her photographs are featured in For
Love of Insects (Harvard University Press, 2003), by her husband
and long-time collaborator, Thomas Eisner.
F. F. Your material is on a scale within the range
of optical microscopy. Why did you decide to use electron microscopy
to capture the image?
M. E. With electron microscopy you can work at a
much higher magnification, and you have more depth of field. The
pictures almost seem to be three-dimensional.
F. F. It appears that you had no problem with
electron-beam damage. How can that be with what seems to be a very
delicate sample?
M. E. The droplets at the tips of the hairs consist
of a viscous secretion that is not very volatile and therefore can
stand up under the electron beam at high vacuum.
F. F. Your image is captured on film, but most
scanning-electron-microscopy images these days are captured as
digital files. Do you think there is a difference?
M. E. Our scanning electron microscope is an old
instrument that is not connected to a computer, so we have no way of
comparing photographic and digital images.
F. F. Can you explain to our readers how you
prepare your samples?
M. E. The most important thing about an item to be
viewed under an SEM is that it be completely dry, inside and out.
Yet most biological specimens, like pieces of plants or a
caterpillar, contain water. Just letting them dry by air causes them
to lose their shape, in most cases. In order to avoid such
distortions I let my specimen sit in 70-percent alcohol for some
hours or a day and then change it to increasingly higher
concentrations of alcohol, until I end up with 100 percent. Now I
can use a small apparatus, a critical-point dryer, in which the
specimen is dried under special conditions of pressure and heat,
that leave it natural-looking, mostly without distortions.
After I glue it to a microscope stub, the specimen is subjected to a
vacuum in yet another small apparatus and "sputtered" with
gold atoms. The covering layer of metal keeps my specimen from
charging and from burning up while it is exposed to the electron
beam in the microscope.
Another way of dehydrating specimens is by
freeze-drying. The pupa of the squash beetle that I used for this
image was frozen on the stage of the freeze dryer and dried for
several hours under those conditions.
F. F. A number of SEM images are digitally colored
these days. In fact, I add color myself, but of course I always
indicate that I’ve done so in the process. Do you have any
opinions about digitally coloring SEMs?
M. E. We have never done it nor tried to do it,
because we are happy with our black-and-white images. Coloring them
is to us like playing around with the images to achieve special
unnatural effects.
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