Biological Clocks in Mosquitoes

Biological Clocks in Mosquitoes - Section 2
Comparative analysis of the patterns of flight activity under various photoperiods - Fully night active species

(i) Culex pipiens quinquefasciatus

A cosmotropical species, found in the USA, up to 40-42°N, but adults do not appear in large numbers until July or August at these northern limits, and they disappear in September or early October, except for hibernating adults. In the southern USA adults and larvae can be found throughout the year. Figure 19 shows the photoperiodogram. In all five LD regimes there is a major N peak, with greatest activity some 6h after light-off; this is seen well after light-on in LD 20:4. In the mid-length regimes, LD 8:16 to LD 16:8, a sharp M reaches the highest level of the whole 24h-cycle. The broad M running well into the light in LD 20:4 may well be a combination of N and M. There is E activity in LD 16:8 and LD 20:4, which suggests that N also may be entrained by light-on some 16h before, thus appearing as a pseudo-E in the long L regimes.

Figure 19
Culex quinquefasciatus

(ii) Culex torrentium

A species of northern Europe, ranging from a limit of 67°N, in Norway, to about 45°N, in the Crimea. The females used in the experiments were reared from larvae collected in July (natural LD 15:9). Figure 20 shows the photoperiodogram. It is strongly night-active, with almost no activity in light. E and M peaks are clear in all three LD regimes but perhaps reflect responses to the abrupt changes in light intensity by a rhythm which is unimodally nocturnal.

Figure 20
Culex torrentium

Discussion

The pattern in Cx. p. quinquefasciatus (see Figure 19) has a clear N peak when L < 16h, with the commencement of activity apparently entrained by light-off some 4-6h before, and a peak some 8h after light-off. The N appears to be manifested as a broad M in LD 16:8 and as a late M in LD 20:4. The M response shown in LD 12:12 and LD 8:16 could be a startle response when the broad N permits activity, as it does not happen in LD 4:20. Taylor (1977) pointed out that this pattern differs from that reported by Jones (1976), where a strong E occurred in LD 12:12. Some aspects of this anomaly can be found in the appropriate Appendix but the report by Jones (1982) may shed some light on the situation. Jones (1982) used a selection of constant light (LL) regimes with differing intensities of light. In light of less than 0.5 lux, there was a unimodal pattern, with E disappearing and N being broad enough to facilitate M if light-on came at an appropriate time. He also studied activity in LD 18:6, LD 12:12 and LD 6:18. Apart from the anomaly of E, the pattern was similar in all three LD regimes with an N and activity ceasing no more than 12h after light-off. The results shown in Figure 19 are for what probably were inseminated females, and the pattern does resemble the majority of field reports of activity. What may be true is that for virgin females, as mainly used by Jones and his colleagues, the E is important to ensure activity when the males are swarming, a process which requires at least low light to enable the swarm to stabilise over a visual marker.

The results for Cx. torrentium (see Figure 20) differ in there being a pattern of activity throughout the night in LD 12:12 and LD 18:6. The drop in activity in LD 6:18 suggests a maximum duration of the potential activity to be some 12-14h after light-off.

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Š1998, 2010 - Brian Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.
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