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Mosquitoes transmit a diverse range of pathogens6–10, and thus, an improved understanding of mosquito physiology and the development of novel control strategies are critically important. AL has afforded us new insights into mosquito larval respiration. The application of AL resulted in the expulsion of gas bubbles that originated from within the tracheal system. When the impinging acoustic frequency is equal to the resonant frequency of the gas within the tracheal system, that gas maximally absorbs acoustic energy and begins to pulsate in synchrony with the impinging frequency. As energy continues to be applied, the amplitude of the pulsation increases to the point of rupturing the DTTs. By adjusting the amplitude and pulse length of our acoustic signal, we observed the earliest manifestations resulting from AL, the severing of the DTTs, with minimal collateral tissue trauma.

This novel technique allowed us to reveal several new aspects of the larval tracheal system. Our study has five major outcomes. First, we improved our understanding of the mosquito tracheal system, including the possible isolation of the tracheal system from the atmosphere. Second, we presented a potential mechanism for the maintenance of pressure during impingement, which damages the DTTs (as opposed to gas venting through the siphon). Third, we provided new insights into the morphology of the siphon (the identification of the TO). Fourth, we confirmed that the damage-inducing mechanism of action of AL is the acoustic resonance of the gas within the tracheal system. Finally, we observed that the siphon does not play an obligate role in respiration for the following reasons:  The TO appears to isolate and maintain the tracheal system at an elevated pressure thus making it at best an inefficient port for the two-way exchange of metabolic gasses. Larvae with completely blocked spiracles or severed DTTs continued to live for long periods of time. After acoustic exposure we did not observe any hemolymph (liquid, solids or gasses) pass by the TO as would have been expected if the siphon was open to the atmosphere.

Margaret L. Keister reported “… a survey of the literature (see Wigglesworth,’31) shows that there are numerous gaps and contradictions in our knowledge of insect tracheal systems”11. Our anatomical findings complement a recent revival of interest in mosquito respiratory physiology. However, some conflicts still exist12–16.  Accordingly, it is important to define some terms used herein. The “FC” is identified by Keilin, Tate and Vincent as the terminal chamber between the spiracles and DTT17. An appreciation of the physiology of the DTTs is important in analyzing gas volumes. Regarding “active DTTs”, during the development of a given instar between molts, the DTTs are comprised of a chitinous partially gas-filled trunk; the active DTTs are enclosed in the large and fluid-filled living-tissue trunks of the future-instar DTTs (Supplemental 1a). However, the composition of this fluid is unknown18,19 and needs further investigation. The active DTTs are withdrawn during molting. While resting on the water surface with the five PLs extended, it appeared that mosquito larvae were in an ideal position to freely exchange metabolic gases, intake oxygen and expel carbon dioxide. Our results indicated that there was not an obligate need for this, and beyond the incidental cuticular exchange of gas with the atmosphere in the atrium, the direct exchange of tracheal gases with the atmosphere (breathing) is unlikely.

By comparing various anatomical and physical characteristics before and after acoustic exposure we identified the tracheal system to be at an elevated pressure. Mosquito larvae are nearly neutrally buoyant20. They are composed of solids, liquids and gasses. In order to maintain their buoyant condition, the volume proportions of the gas filled tracheal system to body volume must fall within a precise range. As reported by Ha, this percentage for Anopheles sinensis larvae was.34%16. The results of our observations and calculations using 37 A. aegypti samples was.33%. This is expected as most of the body is liquid therefore only a small percentage of body volume could be gas. We calculated that the mean post acoustic exposure proportion of gas bubbles to body volume is 2.02%.

This represents and expansion of 5.9 times meaning the initial pressure in the tracheal system was high. The mean direct expansion of the gas bubble was 5.0 times that of as original tracheal size. The function of pressurization in the DTT is unknown, and its potential relationship with tracheal filling or emergence should be further studied. A pressurized tracheal system makes the inhalation of oxygen difficult if not impossible. The TO is quite strong because it involves acoustically induced pressure oscillations that exceed the ability of the DTTs to contain them. The dimensions of the TO between surface-resting or submerged larvae do not change, suggesting that the restriction prohibits the exhaust of carbon dioxide.

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