By Ed Ricciuti
Researchers employing near infrared still photographs and time-lapse video have peered into the pupa of the living tsetse fly and for the first time have watched its development unfold from its start as a larva to its completion and emergence as an adult. The research is described in the Journal of Insect Science.
The fact that the pupae are transparent to near infrared imaging not only promises improved observation of other pupal insects but, importantly, more effective ways to engineer the release of sterile male adult tsetse flies used to control sleeping sickness and nagana. The tsetse fly is the vector of both diseases, forms of trypanosomiasis that infect people and livestock in many areas of Africa.
Releasing sterilized male tsetse flies from captive colonies to mate with females of an isolated wild population can minimize and even eliminate reproduction of the vector in areas subject to control. Female tsetse flies typically mate only once in a lifetime, so mating with sterile males produces no offspring. As more sterile males are released, the numbers of offspring continues to shrink, hopefully to zero. If the population that was eliminated was sufficiently isolated, chances of renewed infestation are minimal.
Only males are packaged and shipped to release points. Due to the tsetse fly’s slow rate of reproduction, females must be retained to breed and thus maintain captive colonies. When a new crop of flies develops, therefore, lab workers must separate males from females. At present, the flies can only be sorted when they emerge as adults. The process is far from ideal because at this stage, within the first few hours after emergence, the males are most vulnerable to damage from vibration, an issue when handling and irradiating them during sterilization and shipping. Presently, males are chilled, which weakens and often kills many of them.
Any technique that enables sorting of the sexes before the new newly formed adults emerge would make the whole process easier for lab workers and less stressful on the males. At the same time, females could be effectively culled and added to the colony as pupae, making them much easier to handle. Near infrared photography does just that, enabling users to see sexual differences several days before emergence.
On the electromagnetic spectrum, near infrared straddles the boundary between the visible spectrum and the longer, lower frequency wavelengths of infrared itself. The term “near infrared” is somewhat subjective, scientifically verging on the visible range while in other applications, including photography, considered just beyond it. Near infrared photography can produce extremely sharp images, with outstanding contrast.
The research was carried on at the Insect Pest Control Laboratory, Joint FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture at the International Atomic Energy Agency in Vienna, Austria. It is considered a breakthrough because it demonstrates that near infrared light can penetrate the hard, rigid case of the pupa, which shields the developing insect from ordinary photography.
Video started when the pupae were two hours old and continued until all adults had emerged. Structures and appendages began to appear by day six of development.
“NIR imaging permits observation of living pupae, allowing the entire development process to be observed without disruption,” the authors wrote.
Never before has the entire development of structures and the changes that occur inside the pupa been studied in living samples. Previous studies on pupal development have relied on external observation or dissection that partly destroys the tissue under examination.
NIR allows viewers to see full structures long before they are visible under dissection.
“Pupae dissected at 10 days of age had no discernable structures and consisted only of loose, white tissue. NIR photographs taken at this age, however, clearly show legs and the outline of eyes,” the authors wrote.
A key to distinguishing between the sexes may be “a clear difference in the timing of wing pigmentation” in male and female flies.
“On day 25, females all showed some pigmentation in wings, and the beginning of pigmentation in legs and proboscis, while males still appeared white,” they wrote. “On day 26, wings on many males had started to darken, but females showed far more pigmentation, both in wings and legs. On day 27 the difference became less obvious, with females still appearing to have significantly darker wings, but at less of a contrast to males.”
The authors express hopes for wide application of the infrared technique they have used, writing that “It is our hope that this technique will be adapted for many purposes in the field of entomology and beyond.”
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Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.