Another Hummingbird Moth - Manduca spp.

Also known as a tomato or tobacco hornworm, in its larval stage.  Like the clearwings in Kathy's post below, they belong to the Sphingidae family and according to that University of Missouri fact sheet, are referred to as "hornworms" because they have a conspicuous horn or spine on the top of the last abdominal segment. The bodies of these caterpillars are usually free of hairlike setae and smooth except for shallow wrinkles in each segment.

Adult Tomato Hornworm
(Manduca quinquemaculata)

Adults are referred to as "sphinx," "hawk," or "hummingbird" moths. They are fast, strong fliers with a rapid wing beat and often hover in front of a flower to feed on the nectar in much the same manner as a hummingbird (and superficially they look like a hummingbird too!). The name "sphinx" is probably in reference to the sphinx-like position that some of the caterpillars assume when disturbed.

This one has defoliated part of my tomato plant (I have plenty, so I can share), a genetically ancient one, a currant tomato, Lycopersicon esculentum (Pimpinellifolium group), which still grows wild in Peru and Brazil. 

Parasitized Hornworm

You'll also notice that this caterpillar has been parasitized, probably by a small braconid wasp, Cotesia congregatus.

Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate. The cocoons appear as white projections protruding from the hornworms body (see photos above and below). If such projections are observed, the hornworms should be left in the garden to conserve the beneficial parasitoids. The wasps will kill the hornworms when they emerge from the cocoons and will seek out other hornworms to parasitize.

Parasitized Hornworm

The main difference between the currant tomato (aside from the size), and our common garden tomato, Lycopersicon esculentum (Cerasiforme group) is that the flowers of the currant tomato do not fully enclose the stamens and their pollen, so it's much easier for cross pollination to occur. Since I like to save seeds, these were planted well away from my main crop.

Currant Tomato - About the size of a pea

There are many other groups of the tomato plant with untapped genetic material which will provide a source to keep scientists continually working to bring us even more varieties of flavorful, healthful, and colorful fruits.

The parasitized caterpillar in the pictures above is probably a tobacco hornworm, Manduca sexta (hard to tell given the wasp cocoons, but the barely visible white stripe points there), in which case an unparasitized caterpillar and corresponding adult looks like this:

Tobacco Hornworm Caterpillar
(Manduca sexta)

Tobacco Hornworm Adult
(Manduca sexta)

Tobacco Hornworm - picture courtesy of Gary Ankney

Update:  Monday, August 23.  Gary Ankney, Fulton County (and honorary Franklin County) Master Gardener sends these pictures of a tobacco hornworm from his garden.  Although I said above that I have plenty and am willing to share with a few caterpillars, I would not do that for a full blown infestation, which can destroy your tomato crop.  They are fairly easy to hand pick and drop in soapy water as a mechanical method for control, without resorting to pesticides, but are notoriously difficult to spot.  An enterprising gardener, however, recommended using an Ultra Violet light, or a Black Light, at night to get them to light up and make hand removal easy.  Gary includes a couple of pictures of what they look like under Black Light. 

Hornworm under Black Light - picture courtesy of Gary Ankney

Hornworm under Black Light - Picture courtesy of Gary Ankney

Load some "Iron Butterfly" into your Ipod, go out in the garden after dark with a battery operated black light atop a spelunker helmet, and hunt for hornworms. And people wonder why we gardeners are sometimes considered a little eccentric ...