The role of the subelytral spiracles in respiration in the flightless dung beetle Circellium bacchus

Abstract

SUMMARY The role of the subelytral cavity in flightless beetle species as an adaptation to water saving in arid habitats is still in dispute. We found that relatively little CO2 was released from the subelytral cavity of a large apterous beetle Circellium bacchus during simultaneous measurements of CO2 emission from the anterior mesothoracic spiracles and posterior body, which included the subelytral spiracles. However, when we sampled air directly from inside the subelytral cavity, we discovered that this pattern was reversed. A discontinuous gas exchange cycle(DGC) was recorded from the posterior body half, revealing a flutter phase that had been absent from the anterior mesothoracic DGC. The anterior mesothoracic and posterior subelytral spiracles act in synchrony to maintain high CO2 and water vapour levels inside the subelytral cavity. In addition, the O2 concentration of the air within the subelytral cavity is lower than the air around the elytral case, irrespective of the time of sampling. These findings lead us to conclude that the subelytral spiracles work in a coordinated fashion with the anterior spiracles to create a DGC,which allows us to extend the hypothesis of the function of the subelytral cavity as a respiratory water-saving device.

The opening-closing rhythms of the subelytral cavity associated with gas exchange patterns in diapausing Colorado potato beetle, Leptinotarsa decemlineata

“…This has previously been demonstrated in several other insects (Kestler, 1980(Kestler, , 1985Sláma, 1988;Wobschall and Hetz, 2004;Jõgar et al, 2011Jõgar et al, , 2014. The SEC also plays an important role in insect respiration and as a waterconserving device (Byrne and Duncan, 2003;Schilman et al, 2007;Duncan et al, 2010;Chown, 2011).…”

“…Therefore, we suggest from our whole-body measurements in L. decemlineata beetles, as representative of insects with flying ability, that most of the CO 2 was expelled from the body via the mesothoracic spiracles, and not from the SEC. That these spiracles are the main outlet for CO 2 emission was also demonstrated in flightless dung beetles by Duncan (2002Duncan ( , 2003 and Byrne and Duncan (2003).…”

“…Nicolson et al (1984) suggested that after air has been expelled via the spiracles, CO 2 accumulated in the SEC is eliminated to the atmosphere by lifting of the elytra. However, when CO 2 release was measured separately and simultaneously from the mesothoracic spiracles and from the SEC in the flightless dung beetles Circellium bacchus Byrne, 2002, 2005;Byrne and Duncan, 2003;Duncan, 2003), there was a predominantly anterograde or tidal airflow in the tracheal system. According to Byrne and Duncan (2003), the air within the SEC in these beetles was high in water vapour and in CO 2 but the concentration of O 2 was below atmospheric.…”

“…However, when CO 2 release was measured separately and simultaneously from the mesothoracic spiracles and from the SEC in the flightless dung beetles Circellium bacchus Byrne, 2002, 2005;Byrne and Duncan, 2003;Duncan, 2003), there was a predominantly anterograde or tidal airflow in the tracheal system. According to Byrne and Duncan (2003), the air within the SEC in these beetles was high in water vapour and in CO 2 but the concentration of O 2 was below atmospheric. Dizer (1955) demonstrated that removal of the elytra resulted in a substantial increase in water loss in wingless beetles with fused elytra, but little increase in winged species capable of flight, suggesting that in the latter the cavity was not hermetically sealed.…”

“…Dizer (1955) demonstrated that removal of the elytra resulted in a substantial increase in water loss in wingless beetles with fused elytra, but little increase in winged species capable of flight, suggesting that in the latter the cavity was not hermetically sealed. Byrne and Duncan (2003) showed that if the SEC in a flightless dung beetle was open, there would be a 74% increase of water loss rate.…”