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Mermis nigrescens
''Mermis nigrescens'' is a species of nematode known commonly as the grasshopper nematode.〔Capinera, J. (Grasshopper nematode, ''Mermis nigrescens''. ) EENY-500. University of Florida, IFAS. 2011.〕〔Cranshaw, W. (Weird Worms: Horsehair Worm and the Grasshopper Nematode. ) Colorado State University Extension. 2008. Revised 2013.〕 It is distributed in the Americas, Europe, and Asia. It occurs in Tasmania, but it has not been observed on mainland Australia. It has been rarely observed in Africa.〔 It is a parasite of insects, especially grasshoppers.〔
==Description==
This is a very large nematode, the male about 4 to 6 centimeters long〔 and the female known to exceed 20 centimeters.〔(''Mermis nigrescens''. ) Nematology. University of Nebraska, Lincoln.〕 The size is unusual for entomopathogenic nematodes, which are generally almost microscopic.〔 The body is pale brown,〔 and the gravid female has a dark stripe down the length of its body due to the presence of up to 14,000 eggs.〔 The body surface is smooth. It tapers at the front end, and the head and tail are rounded.〔
The adult female has a bright red〔Burr, A. H. and F. I. Harosi. (1985). (Naturally crystalline hemoglobin of the nematode ''Mermis nigrescens''. An ''in situ'' microspectrophotometric study of chemical properties and dichroism. ) ''Biophysical Journal'' 47(4), 527–36.〕 or orange-red〔 spot on its head. Early in studies of the nematode, this was called the "chromatrope", because it apparently had a function in the animal's response to light.〔 On closer examination it had the form of a hollow cylinder,〔Burr, A. H. J., et al. (2000). (A hemoglobin with an optical function. ) ''Journal of Biological Chemistry'' 275(7), 4810-15.〕 and the red pigment inside was determined to be hemoglobin.〔 This hemoglobin, generally as oxyhemoglobin, is densely concentrated in crystalline form.〔 The cylinder was confirmed to be an ocellus, a form of eye.〔Burr, A. H. J., et al. (2000). (Ocellar pigmentation and phototaxis in the nematode ''Mermis nigrescens'': changes during development. ) ''Journal of Experimental Biology'' 203(8), 1341-50.〕 Hemoglobin takes the place of melanin as the shadowing pigment of the eye, and plays a role in the nematode's sensation of light, a function not observed in any other organism.〔 Some other nematodes have eye structures, but that of the female ''M. nigrescens'' is unique. It has a single eye, where other nematodes have two. Only the female has an eye, where eyes are present in both sexes of other eyed nematodes. Its eye takes up the entire front end of the body, the cylinder filling the entire body cavity. It has a cornea, or a structure that acts as a cornea. All other eyed nematodes have melanin as the shadowing pigment. The unique eye of ''M. nigrescens'' probably evolved independently among nematode taxa.〔Mohamed, A. K. (2007). (Unique two-photoreceptor scanning eye of the nematode ''Mermis nigrescens''. ) ''The Biological Bulletin'' 212(3), 206-21.〕
This and other ''Mermis'' species are sometimes mistaken for horsehair worms, but the latter are longer and uniformly dark in color, and their bodies do not taper.〔
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