Figure 30.2 A Diffusible Substance Triggers Molting
Sir Vincent Wigglesworth investigated the substance responsible for molting in the South American kissing bug, Rhodnius prolixus. Wigglesworth proposed that this substance diffused from the insect’s head into the body in order to control molting. To test this hypothesis, he decapitated juvenile insects at different times after a blood meal and determined the effect on molting. Results showed that if the insect was decapitated one hour after a blood meal the insect would not molt, but remain a juvenile. By contrast, if the insect was decapitated one week after a blood meal, it would molt into an adult. These findings suggested that whether or not decapitation had an effect on molting was determined by the interval between a blood meal and decapitation. Further, this result supported his hypothesis that a diffusible substance responsible for molting passes from the head to the body. To test this idea directly, Wigglesworth again decapitated insects as above, but this time joined the decapitated insects with a glass tube to allow for substances to diffuse between the insects. Under these circumstances, both insects molted into adults, suggesting that a diffusible substance is necessary for molting. These results were consistent with Wigglesworth’s hypothesis. Of note, R. prolixus is one of the main carriers of a small, parasitic protozoan, Trypanosoma cruzi, which causes Chagas’ disease. This incurable human tropical disease affects the nervous system, digestive system, and the heart. Currently, approximately 16–18 million people, mostly in Central and South America, are affected by Chagas’ disease. Sequence data are now available for the entire genome of R. prolixus, with hopes that these genomic data lead to the development of more efficient insecticides to control R. prolixus populations in affected areas.
Original Paper
Wigglesworth, V. B. 1934. The Physiology of Ecdysis in Rhodnius Prolixus (Hemiptera). II. Factors Controlling Moulting and “Metamorphosis.” Quarterly Journal of Microscopical Science 77: 191–222.
http://jcs.biologists.org/cgi/reprint/s2-77/306/191
Links
Edwards, J. S. 1998. Sir Vincent Wigglesworth and the coming of age of insect development. International Journal of Developmental Biology 42: 471–473.
http://www.ijdb.ehu.es/web/paper.php?doi=9654033
Developmental Biology 8e Online: Hormonal Control of Insect Metamorphosis
http://10e.devbio.com/article.php?ch=18&id=179
Kimball’s Biology Pages: Insect Hormones
http://home.comcast.net/~john.kimball1/BiologyPages/I/InsectHormones.html
University of Arizona: Rhodnius prolixus
http://www.u.arizona.edu/~weichsel/txt/rhodnius.html
Truman, J. W. 2006. Steroid hormone secretion in insects comes of age. Proceedings of the National Academy of Sciences 103: 8909–8910.
http://dx.doi.org/10.1073/pnas.0603252103
NCBI: Genome Project: Rhodnius prolixus
http://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list_uids=13645
Wikipedia: Chagas Disease
http://en.wikipedia.org/wiki/Chagas_disease
World Health Organization: TDR: Chagas disease
http://www.who.int/tdr/diseases-topics/chagas/en/index.html